Longwave DXing

"Longwave" refers to all frequencies below the lower end of the AM broadcasting band at 540 kHz. The lower limit of what frequencies constitute "radio" is not precisely defined, but 5 kHz is a widely accepted starting point for the radio spectrum.

For many years, radio hobbyists ignored longwave because most commonly available communications receivers only tuned down to 540 kHz. However, most new receivers today tune down to at least 150 kHz and longwave DXing is enjoy new popularity.

One big problem when tuning longwave is electrical noise from power lines, electrical devices, motors, etc. Longwave is far more susceptible to such noise than higher frequencies, and you might hear only a loud "buzz" when you tune across longwave from your location. Also, static crashes from thunderstorms can be severe, especially in summer. To combat noise, many longwave DXers use an indoor "loop" antenna that allows rejection of nearby electrical noise sources. Other longwave DXers use special phasing units to reduce noise levels.

Reception distance on longwave is similar to that on the AM broadcast band, as are reception patterns. Greater range is possible when the signal is reaching you over a water path, as is often the case in coastal regions. At night, reception of stations from hundreds or even thousands of miles away is possible. Night reception on longwave is better in winter than in summer, and the equinoxes often give the best propagation.

Unlike the shortwave frequencies above 1700 kHz, the longwave spectrum is allocated on a more "ad hoc" basis, with different users and services frequently sharing the same frequency range. Here is a general description of the world below 540 kHz:

Below 155 kHz: Signals below 155 kHz don't propagate very well via the ionosphere; the absorption is too great even at night during winter. These signals can travel for thousands of miles via ground wave, but high transmitter powers are required. Signals at very low frequencies, about 50 kHz and lower, can penetrate sea water very well. As a result, these frequencies are used by military forces of the major powers, especially for communication with submarines. The U.S. Navy's "Omega" navigation system is found on 10.2, 12, and 13.6 kHz. The Russian navy operates a similar system on 15.62 kHz. The U.S. Air Force has a FSK-based communications system on 29.5 and 37.2 kHz. This system was established to provide a backup in case nuclear explosions rendered the ionosphere useless for propagation. Miscellaneous FSK-based stations are found here for direct communications with submarines and naval forces.

150 to 175 kHz: In the United States, this range is used by the U.S. Air Force's ground wave emergency network (GWEN), a packet-based network to provide communications during a nuclear war. Transmitters are kept continuously operational here on a "standby" basis, and it's easy to hear their loud, "raspy" signal bursts.

155 to 281 kHz: This is another AM broadcasting band in Europe and parts of Asia. In Europe, there are numerous high powered (1,000,000 watts or more) stations here. These stations are capable of covering an entire European nation like France or Germany with reliable signals around the clock. Although ionospheric propagation is not good at these frequencies, the high powers used means that many of these broadcasters can be heard along the Atlantic seaboard during the fall and winter. Best reception is usually from local sunset to about 0600 UTC. A few longwave stations in Asiatic Russia can be heard on the Pacific Coast beginning an hour or so before local sunrise.

160 to 190 kHz: In the United States, this range is open to unlicensed experimental transmissions. Transmitter power is restricted to one watt, and the maximum antenna length (including feedline) can be no more than 50 feet. Any mode can be used. Some of these "lowfer" (as they are known) unlicensed stations have been heard several hundreds of miles away under favorable conditions.

200 to 430 kHz: This range is used mainly by navigation beacons, which continuously repeat their call signs in Morse code. Call signs do not follow the international allocations given elsewhere on this site. Instead, the call signs usually give an idea of the location of the beacon. For example, beacon "FT" on 365 kHz is located at Fort Worth, Texas.

430 to 500 kHz: This range is used for two-way Morse code communications between ships at sea and shore stations. Shore stations use three-letter callsigns, while ship station callsigns consist of four letters. All callsigns are from international allocations. The number of stations you can hear in this range is rapidly declining due to a shift in maritime communications to satellites and shortwave frequencies. After February, 1999, radio operators skilled in Morse code were no longer required on ships sailing in international waters.

500 kHz: This was an international ship calling and distress frequency for maritime communications in Morse code. It is no longer used, and after February, 1999, ship stations and shore stations were no longer required to monitor this frequency for calls.

500 to 540 kHz: This segment is populated by miscellaneous beacons and stations. Perhaps the most interesting frequency here is 518 kHz, used for transmission of maritime safety and navigation information via FSK. This system is known as NAVTEX, and includes weather bulletins as well as notices of missing and overdue vessels. 530 kHz is used in the United States and Canada for low powered road and traffic information broadcasts.

A good source of information about longwave reception techniques, stations currently being heard, and experimental stations currently being heard is the Longwave Club of America (LWCA). A good directory of beacon stations active in North America is the Aero/Marine Beacon Guide published by Ken Stryker. Information on the latest edition can be obtained by sending a self-addressed stamped envelope to Ken Stryker, 2856-G West Touhy Ave., Chicago, IL, 60645.

Selecting a Scanner

Scanners are much different than other consumer-level radios----or even shortwave radios, for that matter. If you're looking to buy your first scanner radio, you probably feel a bit confused and overwhelmed by the features and specifications of the models you're considering!

As with most consumer items, there is no one "best" scanner radio for everyone. For example, if you want to simply listen to your local police and fire departments, a basic, low cost scanner will do fine. On the other hand, you can easily spent over $1000 for a scanner capable of high performance over a broad frequency range in a variety of modes.

General Considerations

The first thing you need to consider about any scanner is what frequency ranges you're interested in monitoring. To get a better idea of what can be heard on different ranges, click here to visit The World About 30 MHz section of this site.

Portable scanners have become popular recently. Some are small enough to fit into a shirt pocket and let you follow the action at sporting events, exhibitions, shows, accident scenes, etc. However, a portable scanner will usually cost more than a home (or "base") unit of comparable features and performance. And remember that having a scanner visible at certain places and events can result in a quick escort out the door! Many avid scanner fans have both a home scanner and a portable unit.

Scanners really differ in the number of channels you can program in them. Some low cost scanners only have a couple of dozen channels available, while some deluxe scanners have 1000 or more channels you can program. The best advice here is to buy a scanner with more channels than you think you will need, as you'll probably run across interesting new frequencies you want to monitor. Maybe the most common wish of scanner fans is that their radios had more channels!

Make sure you understand how new frequencies can be programmed into a scanner. Some scanners will let you enter new frequencies only in specific increments, such as at 5 kHz intervals. Others force you to use the standard spacing between channels commonly used on a given band. More advanced scanners let you enter frequencies down to a single kilohertz. A scanner that tunes only in fixed increments means you may miss hearing some interesting things.

Most scanners automatically tune narrow band (that is, deviation of 5 kHz or less) FM on all frequencies except for the 108 to 136 and 225 to 400 MHz aviation bands, where AM is used. Some scanners allow you to receive wide band FM (deviation of 10 kHz or more) as well. This will let you monitor the FM broadcast band, television audio, and some government transmissions. However, use of wide band FM outside of the FM broadcast band and television channels is rare. A few scanners, such as the Icom R10 let you receive SSB as well, but SSB is seldom used above 30 MHz outside the ham radio bands, and even there narrow band FM heavily dominates. For most listening, a scanner that tunes narrow band FM (and AM on the aviation bands) should be more than adequate.

If you would like to monitor scanner frequencies and AM and shortwave, then consider a wideband receiver. Such radios offer exceptional frequency coverage and models such as the Icom R5 are quite affordable.

Understanding Specifications

The importance of the specifications indicating a scanner's performances largely depends on where you live. If you live in a large urban area, you will need a high degree of selectivity (the ability to reject interfering signals) because of the large number of radio signals found in urban areas. If you live in a rural area with few stations, then greater sensitivity (the ability to detect weak radio signals) will be more important.

Sensitivity is measured in microvolts, abbreviated mV. The lower the number of microvolts, the weaker the signal that the scanner can detect and produce intelligible audio from.

Selectivity is measured in kHz for a certain level of interference rejection. This rejection is measured in decibels (dB), usually at 50 dB. A "50 dB" rejection means an interfering signal is reduced to a level 100,000 times weaker than its actual strength. If a scanner has a selectivity specification of "40 kHz at 50 dB," this means signals 40 kHz or more away from the signal you want to hear are reduced in strength 100,000 times.

If you live in a rural area, good sensitivity is more important than good selectivity. With fewer stations to hear, you need to be able to catch weak signals and don't have to worry as much about interference. In an urban area, the opposite is true; your main concern is in rejecting interference from stations on adjacent channels, not catching weak signals. In a rural area, narrow band FM selectivity of 40 kHz at 50 dB will usually be adequate, while in an urban environment you will usually need selectivity of 30 kHz at 50 dB or better.

Signals can also "mix" in a scanner's internal circuits, producing false signals known as images. Images are an unavoidable by-product of a scanner's circuitry, but the better scanners can reject most of these phantom signals and reduce their strength. Image rejection is how this is measured, and a good scanner should have image rejection of 50 dB or greater.

While there are some exceptions, as a general rule you do get what you pay for in scanner performance. More expensive models will have better sensitivity, selectivity, and image rejection than less expensive units.

Some municipalities use trunking systems whereby a group or block of frequencies are used on a rotating basis. To properly copy such transmissions, you will need a Trunk Tracking scanner such as the Bearcat BC245XLT with Trunk Tracking that can "follow" the various channels as they are used and changed.

Some municipalities are now transmitting in APCO25 digital voice mode. Traditional scanners cannot "decode" these voice transmissions. Some of the newer scanners such as the Bearcat BC296D can handle both Trunk Tracking and Digital transmissions.

Scanner Features and Controls

Here are explanations for features and controls commonly found on scanners:

  • Attenuator. This reduces the sensitivity of a scanner in order to reduce images and other effects of strong nearby signals.
  • Audio squelch. This resumes scanning if a signal has no audio on a channel after pausing on the channel for a few seconds.
  • Autoload. This automatically stores new frequencies found during a search into the scanner's memories.
  • Bank. This is a way of dividing a scanner's channels into smaller, manageable blocks for specific purposes.
  • Delay. This determines how long a scanner pauses on a channel for another transmission before resuming scanning.
  • Hold. This lets you stop scanning on a channel so you can monitor it continuously.
  • Lockout. This causes the scanner to skip over a channel during its scanning sequence.
  • Priority channel. When a signal is present on a priority channel, the scanner switches to it regardless of whether signals are present on other channels being scanned.
  • Search. With this, the scanner tunes through a range and stops when an active frequency is found. This is very handy for finding new stations and users not listed in frequency directories.
  • Squelch. This silences the scanner's audio until a signal of a certain strength is received. The squelch level can be manually set.

The World Above 30 MHz

Since VHF and UHF propagation is usually "line of sight," frequency allocations and usage are far more "localized" on frequencies above 30 MHz. However, there are some broad allocations for different purposes used in the United States and most of the rest of the Americas. The following is a summary of the main frequency bands found above 30 MHz. Please remember that listening to cellular phones, cordless phones and wireless intercoms is illegal in the United States.

30 to 50 MHz: This is known as the "VHF low" band. Most transmissions will be in narrow band FM with channels spaced at 20 kHz intervals. A wide variety of stations can be heard on this range, including businesses, federal, state, and local governments, law enforcement agencies, and various industrial radio services.

50 to 54 MHz: This is the six-meter ham radio band. The first megahertz is mainly used for USB, AM, CW, FSK modes, digital modes. The remainder of the band is used for narrow band FM, both simplex and through repeaters. 52.525 MHz is widely used as a simplex and calling frequency.

54 to 72 MHz: Television channels 2, 3, and 4 are located in this range. The video portions will sound like distorted noise on a scanner. The audio portions are in FM, but will sound "clipped" and "tinny" unless your scanner can tune this range in wide band.

72 to 76 MHz: This range is used for remote control signals for model airplanes and garage door openers, wireless microphones (including those used by law enforcement agencies), and two-way communications inside factories, warehouses, and other industrial facilities. Most channels are spaced at 20 kHz intervals.

76 to 88 MHz: This range is used for television channels 5 and 6.

88 to 108 MHz: This is where the FM broadcasting band is located.

108 to 136 MHz: This band is used for civilian aeronautical communications and all transmissions are in AM. Aeronautical beacons occupy 108 to 118 MHz; these continuously transmit a station identification and are used for navigation. The rest of the band is used for traffic between aircraft and air traffic control towers on channels spaced at 25 kHz intervals.

136 to 138 MHz: This segment is mainly used by weather satellites to transmit photographic images.

138 to 144 MHz: The various military services are the biggest users of this segment in the United States, with most transmissions in narrow band FM and spaced at 5 kHz intervals. You can also hear ham radio operators who are members of the military affiliate radio service (MARS).

144 to 148 MHz: This is the two-meter ham radio band. This is the most heavily used ham radio band in the United States. USB and various FSK modes are mainly used in the first 500 kHz, and the rest of the band is FM. Most activity is through repeaters, although simplex activity is found on frequencies like 146.52 MHz. For more information about this band, visit the ham radio section of this site.

148 to 150.8 MHz: The usage here is similar to the 138 to 144 MHz range.

150.8 to 174 MHz: This is known as the "VHF high" band, and it is used by the same wide spectrum of users as the 30 to 50 MHz band.

174 to 216 MHz: This range is used for television channels 7 through 13.

216 to 220 MHz: In the United States, this band is used by the automated maritime telecommunication system (AMTS) used on major inland waterways such as the Great Lakes and the Mississippi river. Communications are in FM on channels spaced at 12.5 kHz intervals. However, the 219 to 220 MHz range is shared with ham radio. On this range, ham stations can be used to relay digital messages to other hams, subject to a maximum power of 50 watts. Hams must first register to use their shared allocation, and cannot use it within range of maritime users.

220 to 222 MHz: This range was reallocated a few years ago from ham radio to land mobile radio. Frequency usage and modulation have not yet been finalized, although new narrow bandwidth modes are expected to be used.

222 to 225 MHz: This is the 1.25-meter ham radio band. It is mainly used for FM communication through repeaters, although it is much less heavily used than the two-meter band.

225 to 400 MHz: This very wide band is used for military aviation communications in AM. Most channels are 100 kHz apart.

400 to 406 MHz: This range is used primarily by government and military stations in FM.

406 to 420 MHz: In the United States, this band is used exclusively by the federal government. All transmissions are in FM, with most channels spaced at 25 kHz intervals.

420 to 450 MHz: This is the 70-centimeter ham radio band, second in popularity to the two-meter band on VHF/UHF. The 420 to 444 MHz range is used for USB, digital modes, ham television, and ham communications satellites. The 444 to 450 MHz range is used for FM, mainly in conjunction with repeaters.

450 to 470 MHz: This is the "UHF" band on most scanners, used for many of the same purposes as the 30 to 50 and 150.8 to 174 MHz bands.

470 to 512 MHz: This is known as the "UHF-T" band, and covers the same frequency range as television channels 14 to 20. This band is used for many of the same purposes as the "UHF" band in areas of the country without television stations on those channels.

512 to 825 MHz: This range is where television channels 21 through 72 are located.

825 to 849 MHz: This range is used for cellular telephone service, with cellular units transmitting here. Listening in this range is prohibited.

849 to 851 MHz: This band is used to provide telephone service from aircraft in flight. SSB is generally used here. Listening in this range is prohibited.

851 to 866 MHz: This is used by many of the same users as the 450 to 470 MHz band, with channels spaced at 25 kHz intervals.

866 to 869 MHz: This allocation is used by public safety and law enforcement agencies.

869 to 894 MHz: This range is used for cellular telephone service, with cells transmitting here. Listening in this range is prohibited.

894 MHz and above: These higher frequencies are where new communications technologies, such as wireless local area networks, spread spectrum telephony, and direct satellite broadcasting are being implemented.

AM Band DXing

Each year, dedicated listeners manage to snag stations from thousands of miles away on the AM broadcast band (540 to 1700 kHz). In fact, the AM band is where DXing began.

Back in the 1920s, the first radio stations were anxious to know how far away they were being heard. They asked for reception reports from listeners, and promised to reply to reports with souvenir postcards confirming that the listener indeed heard the station. The entire hobby of "SWLing" grew from those beginnings!

Getting start in AM band DXing is easy—just tune across the AM band from your local sunset to your local sunrise! If you mainly keep your AM radios set to local stations, you may be surprised at how well you can hear stations from hundreds and even thousands of miles away at night using an ordinary AM radio.

In North and South America, AM stations are spaced on channels at 10 kHz intervals (540, 550, 560, etc.). Most AM stations are located from 540 to 1600, with new stations soon to take to the air in the 1610 to 1700 kHz. When you tune the AM band at night, you will soon discover that there are a lot of stations active on the AM band! Despite the seeming cacophony, AM band frequencies are carefully allocated into three categories: local, regional, and clear channel.

Local channels are 1230, 1240, 1340, 1400, 1450, and 1490. Stations are limited here to a maximum transmitter power of 1000 watts and must use a non-directional antenna. These are very congested frequencies, with maximum reliable reception range at night usually restricted to less than 30 miles. (If you have no nearby stations on these frequencies, you will usually hear only a "rumble" at night on them.) However, reception at greater distances is possible with patience and good equipment. Local channels are often referred to as "graveyard" frequencies.

Stations on regional channels can use higher transmitter powers, typically up to about 20,000 watts, and directional antennas. As you might expect from the term "regional," these stations are intended to serve specific geographic areas. Regional stations often use different power levels and directional antennas for day and night operation; since AM band signals travel further at night, regional stations will reduce transmitter power and use a "tighter" directional antenna between their local sunset and local sunrise.

QSL from station KLZ KLZ, 560 kHz, in Denver is a regional station. The "5 KW DA-U" notation on this QSL card means it operates with 5000 watts with a directional antenna for an "unlimited" (i.e., 24-hours per day) amount of time. This card was received for a special "DX test" (explained below).

The term "clear channel" is a misnomer today. Clear channel stations can use 50,000 watts of power and many use non-directional antennas. In the early days of radio, no other stations could operate on a clear channel station's frequency between sunset and sunrise. Because the channel wasliterallyclear and high transmitter powers were used, clear channel stations could be heard over much of the country at night.

Beginning in the early 1980s, additional stations were authorized to operate on clear channel frequencies at night, often with greatly reduced power and directional antennas. Many of the stations so authorized had previously been allowed only to operate during their local daytime, and lost listeners when they had to sign off at sunset. While the "breaking up" of clear channels may have been economically necessary for daytime-only stations, it did result in many clear channel frequencies sounding much like regional channels at night.

One practice that has continued since the early days of radio is the "DX test." FCC rules allow stations which must reduce power or change antennas at night to briefly test using daytime power and antennas during an "experimental period" from midnight to sunrise. A DX test is a special program transmitted after local midnight using higher transmitter power or different antennas than the station normally uses. Often, station identifications in Morse code are used; the Morse code will often make it through interference better than voice announcements. Most DX tests are arranged in conjunction with one or both clubs for AM band DXers to assure a large listening audience.

Outside of North and South America, AM stations operate on channels spaced 9 kHz apart (765, 774, 783, etc.). These so-called "split" frequencies means it is possible to hear AM stations from Europe, Asia, and Africa between the 10 kHz channels used in North and South America. Listeners along the east coast can hear European and African stations from their local sunset to about 0600 UTC, while Pacific Coast listeners can catch Asian stations from about an hour before sunrise to actual sunrise.

To hear such foreign stations, you will need a receiver with excellent selectivity and a high performance antenna. Many AM DXers use an indoor rotatable loop antenna with a preamplifier. A loop antenna will reject signals coming from right angles to it, and this helps reduce interference. Other AM DXers use "Beverage" antennas, which are wires in straight lines running for hundreds or thousands of feet.

The best time for long distance AM band reception is during the fall and winter months, with the period around the equinoxes being especially good. Stations located to the east of you will start fading in about an hour before your sunset, while stations to your west may remain audible up to an hour after your local sunrise.

There are currently two clubs specializing in AM band DXing, the National Radio Club and the International Radio Club of America. Both publish weekly bulletins during the fall and winter "DX season" giving news about what's being heard and upcoming DX tests. Both clubs also offer station directories and other publications of interest to AM band DXing specialists.

Take a spin across the AM dial tonight. You might be pleasantly surprised at what you can hear!

Introducing the "Action Bands"

Listening to the "action bands"—that is, those frequencies above 30 MHz—on a scanner radio may well be the most popular of all hobby radio activities. And it's a relatively new activity, as the first scanner radios weren't introduced until about 1970.

A scanner radio is one that automatically tunes through a set of frequencies, usually called "channels," at a predetermined rate. When the scanner finds a signal on a channel, it pauses there to let you hear the communications. When the signals end on a channel, the scanner resumes tuning through its channels until it finds another "active" channel.

Scanners were a real boon to listeners because most transmissions above 30 MHz are brief, and operating frequencies are quiet for long periods between transmissions. Older radios that tuned above 30 MHz had to be manually retuned to change frequencies. If you were tuned to the frequency used by your local police department, for example, you would miss a call on the frequency used by your local fire department. Scanners made it possible to keep track of several different channels simultaneously.

The first programmable scanners were introduced in the late 1970s, and this really boosted the popularity of scanner listening. The first scanners required to you to install new frequency-controlling crystals each time you want to receive a new frequency. Not only was this expensive, there was often a delay of weeks before new crystals "cut" to the desired frequencies arrived. Programmable scanners make changing frequencies as easy as tuning a new station on an AM or FM radio. Most scanners today also have a search function that lets you seek out active frequencies that you're not aware of.

Here is a sampling of what can be heard on a typical scanner:

* Police, fire, and emergency services. Few things are as gripping as listening to the police in pursuit of criminals, firefighters attempting a rescue inside a burning building, or an ambulance rushing to the hospital!

* Aviation. Civilian aircraft and airports can be heard from 108 to 136 MHz, while military aircraft are found from 225 to 400 MHz.

* Marine communications. 156.80 MHz is the ship calling and emergency channel, with several other channels near it. This frequency is used on rivers, lakes, etc., in addition to the oceans.

* Government. Federal, state, and local governments are heavy users of the bands above 30 MHz. Listening can range from law enforcement agencies to your local sanitation and road maintenance services. This is a great way to keep track of how your tax dollars are being spent!

* Ham radio. Ham radio operators are found at 50 to 54 MHz, 144 to 148 MHz, and several other bands.

* Private businesses. You can hear the activities of businesses ranging from taxicab companies to motion picture crews on your scanner.

* Miscellaneous. Wireless microphones, weather bulletins, and even garage door openers can be received on most scanners.

Most communications heard above 30 MHz will be in FM, with the exception of AM on the aeronautical bands. Propagation on the bands above 30 MHz is usually restricted to "line of sight." This is defined as the optical horizon as viewed from the receiving antenna plus about another 15% due to radio signal "bending" caused by the Earth's curvature. While receiving range can be increased by using an outdoor antenna mounted high in the air, most signals heard above 30 MHz will be within 100 miles or less. However, under unusual propagation conditions, stations on the 30 to 50 MHz band can be heard from hundreds or even thousands of miles away.

To improve coverage, many users of the frequencies above 30 MHz employ repeater stations. A repeater station is located on top of a mountain or tall building, and automatically re-transmits a signal received on one frequency (the input frequency) on a second frequency (the output frequency). Some favorably located repeater stations can be reliably heard at distances of over 150 miles.

For listening to radio services within a radius of about 30 miles or so, an indoor scanner antenna, like the telescoping "whip" built into many scanners, is adequate. However, reception range and signal strength will be greatly improved if you use an external outdoor antenna.

While you're generally free to listen to anything tuned by a scanner (except cellular telephone calls, wireless intercoms and cordless phones), Section 705 of the federal Communications Act prohibits divulging or using the contents of any message you hear not intended for the general public. In practice, this is widely ignored; many wrecker and towing services have scanners in their offices to keep up with traffic accidents where their services might be needed, for example. Nonetheless, the law is on the books and could be enforced. It is prudent to not divulge the contents of anything you hear on a scanner.

In 1992, Congress went further and outlawed the sale and manufacture of scanners that can tune the cellular telephone bands. There have been attempts since then at the federal level to restrict scanning listening, but all have been unsuccessful.

Various states and localities have tried to restrict the use of portable or mobile scanners in an attempt to cut down on "ambulance chasing" and similar activities. States like New York and New Jersey have criminalized the use of a scanner to hamper police, fire, and emergency services or in the commission of a crime. A local scanner dealer can give you details about any restrictions in your area.

Interest in scanning listening continues to grow.

Frequency vs. Wavelength

There are different ways to indicate where to find a certain station on a radio dial. For example, we could say that a station is operating on 9680 kiloHertz (kHz), 9.68 megahertz (MHz), or on 31 meters. And all three ways would be correct!

Radio waves are transmitted as a series of cycles, one after the other. The hertz (abbreviated Hz) is equal to one cycle per second. Hertz was named after Heinrich Hertz, a German physicist [1857-1894] who experimentally proved the existence of electromagnetic waves. You may have noticed that the electric power supplied to your home is rated at 60 Hz. Electric power is distributed as alternating current (AC), meaning it goes through a cycle of changing directions of flow. When we say that electric power is "60 Hz," we mean 60 cycles per second (in which time the direction of flow changes 120 times).

Radio waves go through far more cycles in a second than electric current, and we need to use bigger units to measure them. One is the kilohertz (kHz), which is equal to 1000 cycles per second. Another common one is the megahertz (MHz), which is equal to 1,000,000 cycles per second----or 1000 kHz. The relationship between these units is like this:

1,000,000 Hertz = 1000 kilohertz = 1 megahertz

Radio is usually thought of "beginning" at frequencies of approximately 5 kHz, although most available receivers can only tune down to about 150 kHz.

The term "wavelength" is left over from the early days of radio. Back then, frequencies were measured in terms of the distance between the peaks of two consecutive cycles of a radio wave instead of the number of cycles per second. Even though radio waves are invisible, there is a measurable distance between the cycles of electromagnetic fields making up a radio wave. The distance between the peaks of two consecutive cycles is measured in meters. The relationship between a radio signal's frequency and its wavelength can be found by the following formula:

wavelength = 300 / frequency in MHz

According to this formula, a frequency of 9680 kHz would be equivalent to a wavelength of 30.99 meters, which we would round to 31 meters. Thus, 9680 kHz, 9.68 MHz, and 31 meters all refer to the same operating frequency!

As the formula indicates, the wavelength of a radio signal decreases as its frequency increases. This is important because the length or height of various types of antennas must often be a fraction (usually one-quarter or one-half) of the wavelength of the signal to be transmitted or received. This means that most antennas designed for frequencies near 4000 kHz will be physically much larger than antennas designed for frequencies near 30 MHz.

Frequencies are seldom given in terms of wavelength anymore. However, certain segments of the shortwave bands are referred to in terms of "meter bands" as a convenient form of shorthand. For example, the term "10-meter band" is used to refer to the ham radio band that extends from 28000 to 29700 kHz. The following is a table of the most common ham radio and shortwave broadcasting "meter bands" found on frequencies below 30 MHz:

Meter Band Frequency Range and Use
160 meters 1800-2000 kHz ham radio
120 meters 2300-2498 kHz broadcasting
90 meters 3200 to 3400 kHz broadcasting
80 meters 3500 to 4000 kHz ham radio
60 meters 4750 to 4995 kHz broadcasting
49 meters 5950 to 6250 kHz broadcasting
41 meters 7100 to 7300 kHz broadcasting
40 meters 7000 to 7300 kHz ham radio
31 meters 9500 to 9900 kHz broadcasting
30 meters 10100 to 10150 kHz ham radio
25 meters 11650 to 11975 kHz broadcasting
22 meters 13600 to 13800 kHz broadcasting
20 meters 14000 to 14350 kHz ham radio
19 meters 15100 to 15600 kHz broadcasting
17 meters 18068 to 18168 kHz ham radio
16 meters 17550 to 17900 kHz broadcasting
15 meters 21000 to 21450 kHz ham radio
13 meters 21450 to 21850 kHz broadcasting
12 meters 24890 to 24990 ham radio
11 meters 25670 to 26100 kHz broadcasting
10 meters 28000 to 29700 kHz ham radio

You'll notice some inconsistencies in the table above. For example, the 17-meter ham radio band is actually higher in frequency than the 16-meter broadcasting band. These inconsistencies have come about from years of use (misuse?) of a particular "meter band" to refer to a certain range of frequencies.

Secret Radio Frequencies

 Sandwiched into the gap between the AM and FM dials are
hundreds of secret communications frequencies - some so
secret that no one owns up to them. The usual consumer gear -
AM/FM radios, TVs, CB radios - brings in only a small
portion of the electromagnetic spectrum. To pick up the
secret signals, you need a shortwave receiver - and you need
to know the unlisted frequencies.
Allocation of radio frequencies is quirky. When you flip
the TV dial from channel 6 to channel 7, you unknowingly jump
over the entire FM radio band as well as such exotia as
secret service communications and a special frequency
designated for emergency use during prison riots. The U.S.
government will provide information on unclassified
allocations (those for the Coast Guard, Forestry Service,
weather reports, etc.). But it is quiet about secret
government frequencies and those of mysterious illegal
broadcasters here and abroad.
Many shortwave-radio hobbyists keep track of the secret
frequiences, however. Their findings appear in such
publications as the "Confidential Frequency List" by Oliver
P. Ferrell (Park Ridge, N.J.: Gilfer Associates, 1982
[periodically updated]), "How to Tune in the Secret Shortwave
Spectrum" by Harry L. Helms (Blue Ridge Summit, Pa.: TAB
Books, 1981), and "The 'Top Secret' Registry of U.S.
Government Radio Frequencies" by Tom Kneitel (Commack, N.Y.:
CRB Research, 1981 [periodically updated]). These and similar
publications should be consulted for the most up-to-date
listings. The selection below includes only the most
noteworthy or inexplicable broadcasts.

Air Force One

Many of the in-flight phone calls from Air Force One are
not scrambled and can be picked up by anyone with a shortwave
radio. You just have to watch the newspapers for information
on the presidents travels and listen to the right frequencies
shortly before landing or after takeoff at Andrews Air Force
Base (when calls are less likely to be scrambled
electronically). A presidential phone call is usually
prefaced by a request for "Crown", the White House
communications center.
Air Force One uses several frequencies including those
assigned to Andrews Air Force Base. Transmissions are on
single, usually upper, sideband. These transmissions are
usually secret, but the frequency numbers have long since
leaked out or have been discovered independently. It is
suspected that wire services and TV news operations monitor
them for leads. The reported frequencies (in kilohertz) are:


6731 13201
6756 13215
8967 13247
9018 15048
11180 18027


In addition, 162.685 MHz and 171.235 MHz are secret service
frequencies used for Air Force One communications. The White
House staff uses 162.850 MHz and 167.825 MHz. Secret Service
channel "Oscar", 164.885 MHz, is used for the Presidents
limousine. Air Force Two uses the same Frequencies as Air
Force One.
Although everyone concerned must know that outsiders may
be eavesdropping, conversations are often surprisingly
candid. (shortwave listeners heard the White House staff
urging Air Force Two back to Washington after the 1981
attempt on President Regan's life, complete with reports that
then-secretary of state Alexander Haig was confusing
everybody with his claim of being "in control.") No law seems
to forbid such eavesdropping. Ironically, it is illegal
(section 605 of the communications act of 1934) to reveal
intercepted conversations to anyone else - that being regarded
as the wireless equivalent to wiretapping. Even so, The New
York Times has run snippets of Air Force One conversations.

The Central Intelligence Agency

The CIA and Other Government agencies with clandestine
operations are believed to have dozens of authorized
frequencies, which may be rotated as needed to throw off
eavesdroppers off the track. Call letters are rarely used and
several government agencies may share the same frequencies. A
further, rather thin veneer of security comes from the use of
code words. Government surveillance opperations use a common
code: "Our friend" or "Our boy" is, of course, the person
being followed. "O" is his office. "R" is his residence. A
"Boat" is his car. Once apprehended a suspect is a "Package"
and may be taken away to the "Kennel", the agents'
headquarters. Does this fool anyone? Probably not. Some are
so obvious that it's questionable if they're code words at
all.
Not all U.S. government broadcasts can be identified as
to agency. Conversations are cryptic; letters to the Federal
Communications Commission and Commerce Department bring form
replys. These frequencies (in megahertz) have been identified
with the CIA:

163.81
165.01
165.11
165.385
408.60


Note: I am only going to list a few of the many
frequencies known. More can be obtained from the sources
listed earlier or from the EXCHANGE [904] 878-4413 via
modem.

DEA - Drug Enforcement Administration (MHz)
FBI - Federal Bureau of Investigation (MHz)
SS - Secret Service (MHz)

DEA FBI SS
--- --- --
163.185 120.425 162.375 (note that
163.535 149.375 162.685 the frequencys are
165.235 163.835 164.885 usually in bands.
172.00 163.875 165.025 Search each band
172.20 163.985 165.085 for more.)
418.625 167.675 166.405
418.675 168.885 169.625
418.725 406.275 168.45
418.825 408.925 169.925
418.975 419.525 171.235


Morse Code Letter Beacons

Dozens of low-power stations transmit only a letter of
Morse code endlessly. No one, including government agencies
and the International Telecommunications Union, admits to
knowing where the signals are coming from, who is sending
them, or why.
"K" (dash-dot-dash) is the most common letter. Letters
are repeated every two to five seconds, depending on the
station. The stations never identify themselves. The
frequency used for the broadcast shifts slowly with time, so
this list is only an approximate guide:

Frequency (KHz) Letter
--------------- ------

4,005 K
4,466 U
5,306 D and W
5,307 F
5,795 K
5,890 K
5,920 K
6,203 P
6,770 A and N
6,800 F and K
6,806 Q
7,590 W
7,656 W
7,954 K
8,137 U
8,144 K
8,647 F
8,703 E
8,752 K
9,043 K
9,058 U
10,211 U
10,442 E
10,570 K
10,614 F
10,638 K
10,644 D
10,645 F
10,646 R and K
11,156 K
12,151 K
12,185 U
12,329 U
13,328 U
13,637 F
14,478 K
14,587 K
14,967 K
15,656 U
15,700 U
15,705 U
17,015 D
17,016 C
17,017 F
17,018 UE and TA
18,343 K
20,456 E
20,992 O and C


These stations broadcast mostly during the night hours of
North America. They are most often picked up in North
America, Australia, and the Orient. But because of the easy
propagation of shortwave signals, no one is sure where they
are coming from.
An analysis in the Confidential Frequency List holds
that the signals are coming from 25- to 100-watt unattended
transmitters somewhere in the South Pacific. An alternate
theory places the Morse code "beacons" in Cuba. It is known
that there used to be a "W" station operating at 3,584 KHz, a
frequency supposedly reserved for amateur use. When the
American amateurs protested to the Federal Communications
Commission about the interference, the FCC complained to the
Cuban government. The staion disappeared shortly thereafter.
Actually, all of the beacons must be presumed to be
illegal. Shortwave stations are supposed to be registered
with the International Telecommuncations Union; none of those
listed above are. The purpose of the stations is as unclear
as their location. A single letter conveys no information.
There are legitimate navigational beacon stations, which
broadcast their call letters. But such stations are
registered and operate on fixed frequencies from known
locations. Keeping location and frequency information secret
would defeat their purpose.
Maybe, then, the letter beacons are navigational
stations operated for the benefit of a select few. Some think
they are operated by the Soviet Union, in Cuba, for some
military purpose. Still, the globe is crosshatched with
legitimate navigational beacons. It is hard to see what
further navigational aid the Soviets could expect to derive
from their own secret network of beacons.
It has also been suggested that the beacon stations are
really teletype or other data transmission stations and that
the Morse code letters are just a way of keeping the channel
free between transmissions. A few of the stations started
transmitting some sort of data - audible as a characteristic
high-speed typewriterlike sound - in 1980. There are other
ways of keeping a data channel open, though. Most
radioteletype stations transmit the code for space between
transmissions. (The radioteletype code is different from
Morse code.)
Finally, still others think the letter transmissions are
themselves some sort of code. Granted, the letter can't mean
anything, but some wonder if the precise length of the
interval between the letters means somthing. Or the frequency
shifts may hold the message.
The number of Morse code letter stations seems to be
increasing.

Numbers Stations

Well over a hundred "numbers" or "spy" stations have
been reported, all rather closely following a pattern. On the
typical numbers station, the announcer is - or seems to be -
a woman. No one knows who the woman is or where she is
broadcasting from. She speaks Spanish, German, or Korean.
Save for a few words at the begining and the end of the
transmission, the message consists of reandom numbers,
announced in groups of five, four, or, rarely, three digits.
As with the Morse code stations, the numbers stations are all
on unauthorized frequencies. No government or organization
owns up to the broadcasts; offically, at least, the FCC
claims no knowledge of them.
Many of those who have listened to the broadcasts
carefully are convinced that the woman is in fact a robot.
The voice has a mechanical ring, somtimes a click between
each digit. It seems to be the same type of device used by
the telephone company to give the time or to forward phone
numbers.
The exact format of the messages varies with the
language and number of digits per group. With Spanish, five
digit groups, for example, a typical transmission might be:

Atencion 290 22...Atencion 290 22...Atencion 290 22
...65438...34742...23453...23454...29584...24836...
22334...34635...10202...19375...34653...23457...
12345...94532...24643...27543...14795...24568...
75744...74755...87194...63549...Final,final.

Broadcasts are during the night hours of North America
and seem to start shortly after the hour. After the
"Final,final," the transmission stops. It is claimed that a
given transmission is repeated a few minutes later on a
slightly different frequency.
There seems to be no escaping the conclusion that the
messages are numerical code. The second number (22 in the
example) - is the number of digit groups in the message.
There dosen't seem to be any demonstrable significance to the
first number although it probably has some signifigance. Some
think it is an identifying number for the sender or the
receiver. It may also indentify the code used if there is
more than one. Note that the numbers above are only random
(except for 22) and were never really broadcast.
The four-digit transmissions in Spanish are different. A
three-digit number (perhaps that of the sender or receiver)
is repeated several times, followed by the digits 1 through
10. ("uno, dos, tres...") and a string of Morse code dashes.
the word "grupo" is followed by the number of four-digit
groups to come and repeated once - for example, "Grupo 22,
grupo 22." The message - groups of four Spanish numbers -
follows. At the end the voice says, "Repito grupo 22," and
the message repeats. The station goes off the air after the
repeat.
Any attempt to explain these broadcasts is complicated
by numbers broadcasts in other languages. There are also
broadcasts in German, Korean, and English. Occasional
transmissions in Russian, French, Portuguese, and even
Serbo-Croatian are reported. Somtimes a male (mechanical?)
voice reads the numbers. The female robot voice doing English
language broadcasts is often described as having an Oriental
or German accent. Typical of the uncertainty surrounding
numbers stations are the reported English messages prefaced
with a female voice saying "Groups disinformation" and ending
with "End of disinformation." Perhaps the voice machine has a
bad rendering of "This information."
Still other stations transmit messages consisting of
letters from the phonetic alphabet (Alpha, Bravo,
Charlie...). Some spice their broadcasts with music, which
ranges from ethnic tunes to wierd tones that may or may not
conceal a message. Reported frequencies for numbers and
phonetic-alphabet stations include:

F/M = Female/Male
S = Spanish R = Russian
F = French E = English
P = Portuguese C = Czech
SC= Serbo-Croatian G = German


Frequency Male language
(KHz) Female
--------- ------ --------

3060 F S (All are numbers stations
3090 F S unless otherwise noted)
3365 M SC
4640 M S
4642 F F
4670 F S&E Numbers & phonetic
4740 M S&P Interlude from Aida
4770 F G
5020 F S
5075 F S
5110 M C Slavic musical interlude
5812 F S
6770 F S
6790 F S
8875 F S
9040 F S&E
9345 F S
9450 F E + Musical tones
9463 F S
9950 F S
10450 F K
10500 F G
10532 F S
11545 F G
11618 F G
11635 F S
13320 M R
14947 F G
14970 F E + Beep tones
23120 F G
30050 E
30250 E
30420 E
30470 E


Whatever is going on, it's a big operation. Harry L.
Helms' "How to tune in the shortwave spectrum" has a list of
sixty-two stations that includes only those with a female
voice reading five digit codes in Spanish. Much time and
effort are going into the broadcasts. Some numbers stations
transmit on the upper sideband rather than using amplitude
modulation (AM). Signals are usually strong. Because of
ionospheric reflection, they can be picked up over most of
the globe. This makes direction finding difficult.
Two explanations are offered for the numbers stations.
It is rumored that some of the stations are communications
links in the drug traffic between the United States and
Latin America. If so, Spanish is the logical language. The
numerically coded messages could tell where drops are to be
made, how much to expect, and other minutiae that would
change from day to day. Weak support for this comes from some
amateur direction finding, which seems to place many of the
Spanish broadcasts Somewhere south of the United States.
But even those who subscribe to this explanation agree
that other numbers stations, probably most of them worldwide,
are engaged in espionage - governmental or organizational
communication with agents in the field.
Which government? The Spanish stations are usually heard
between 7:00 PM and 6:00 AM Eastern Standard Time. The night
hours are best for clandestine broadcasting as weak signals
propagate farther. So the spanish language broadcasts are
probably coming from a time zone not far removed from Eastern
Standard Time (the EST time zone includes the central
Caribbean, Columbia, Ecuador, and Peru.)
On the basis of signal strengths and broadcast times, it
can be similarly be postulated that the German Stations are
coming from Europe, or maybe Africa, and the Korean stations
are coming from the Orient - oddly enough.
As far as the Spanish stations are concerned, suspision
points to Cuba. In 1975 U.S. listeners reported muffled radio
Havana broadcasts in the background of the Spanish stations.
A station at 9920KHz is said to have used the same theme
music as radio Havana.
But then there are American ham radio operators who
swear that the spanish stations must be in the United States.
"How to Tune the Secret Shortwave Spectrum" tells of
listeners in Ohio who reported four digit numbers stations
coming in stronger than anything else on the dial execpt for
a 50 kilowatt broadcast band station a few miles distant.
Similar reports come from the Washingtom, D.C., area.
Probably the simplest of all the many possible
explanitions is that the Spanish stations are opperated by
Cuba for the benefit of Cuban agents in the United States.
The Radio Havana Broadcasts in the background would have been
a mistake. The engineer was listening to radio Havana and
forgot the mike was on, or maybe radio Havana and some of the
numbers stations share facilities and the signals got mixed.
The local quality broadcasts heard in the U.S. could be Cuban
agents reporting back to Havana. Each agent would have his
own mechanical voice setup. Not that you can carry around a
50000 watt transmitter in your pocket.
The actual explanation may not be the simplest, though.
According to Helms, some shortwave listeners believe that the
four and five digit number transmissions are totally differnt
opperations. The four digit transmissions, at least some of
which seem to originate in the United States, may be the work
of the U.S. government. Only the five-digit transmissions may
come from Latin America - and may be associated with local
governments or U.S. foreign agents. Harry L. Helms
speculates that the United States may have faked the radio
Havana background just to divert suspission from an American
espionage operation.
Any glib explanation of the numbers stations is further
challenged by another incident Helms cites. An unnamed
listener was receiving a five digit numbers broadcast in
Spanish. At the end of the broadcast, the station
accidentally (?) stayed on the air, and faint female voices
were heard reading numbers in German and English. If the
report was accurate, then the numbers stations could be the
work of one worldwide operation. Choice of language could be
arbitrary. Whatever his or her native tounge, an agent need
only need learn ten words of, say, Korean in order to receive
a numerical broadcast in Korean.
No one willing to talk has broken the code or codes used
for the transmissions. If the codes are sophisticated enough
it may be pointless to even try. A random four or five digit
number added to each number in the group will scramble the
code. The numbers would have to be agreed upon before
transmission. If a different number is used for each number
block and if they are not repeated it is mathematically
impossible for outsiders to break the code.
At 3820KHz there is a four-note electronic tune. At
12700KHz there is a plaintive, twenty-one-note, flutelike
melody. At 15507 KHz there are beeps.
The EXCHANGE serves as a message base for exchanging
information dealing with radio frequencies. If you wish to
post the frequencies from your area (confidential or not),
get frequencies for other areas, post sample broadcasts,
reveal the coding method or purpose of these broadcasts, or
just talk to a friendly bunch of guys and gals feel free to
call.

The EXCHANGE : (904) 878 - 4413..24HRS..300/1200/2400 baud
(Modem only, of course)


Special thanks to William Poundstone








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"Raw Data for Raw Nerves"

Modes and Modulation

Modulation is the process by which voice, music, and other "intelligence" is added to the radio waves produced by a transmitter. The different methods of modulating a radio signal are called modes. An unmodulated radio signal is known as a carrier. When you hear "dead air" between songs or announcements on a radio station, you're "hearing" the carrier. While a carrier contains no intelligence, you can tell it is being transmitted because of the way it quiets the background noise on your radio.

The different modes of modulation have their advantages and disadvantages. Here is a summary:

Continuous Wave (CW)

CW is the simplest form of modulation. The output of the transmitter is switched on and off, typically to form the characters of the Morse code.

CW transmitters are simple and inexpensive, and the transmitted CW signal doesn't occupy much frequency space (usually less than 500 Hz). However, the CW signals will be difficult to hear on a normal receiver; you'll just hear the faint quieting of the background noise as the CW signals are transmitted. To overcome this problem, shortwave and ham radio receivers include a beat frequency oscillator (BFO) circuit. The BFO circuit produces an internally-generated second carrier that "beats" against the received CW signal, producing a tone that turns on and off in step with the received CW signal. This is how Morse code signals are received on shortwave.

Amplitude Modulation (AM)

In amplitude modulation, the strength (amplitude) of the carrier from a transmitter is varied according to how a modulating signal varies.

When you speak into the microphone of an AM transmitter, the microphone converts your voice into a varying voltage. This voltage is amplified and then used to vary the strength of the transmitter's output. Amplitude modulation adds power to the carrier, with the amount added depending on the strength of the modulating voltage. Amplitude modulation results in three separate frequencies being transmitted: the original carrier frequency, a lower sideband (LSB) below the carrier frequency, and an upper sideband (USB) above the carrier frequency. The sidebands are "mirror images" of each other and contain the same intelligence. When an AM signal is received, these frequencies are combined to produce the sounds you hear.

Each sideband occupies as much frequency space as the highest audio frequency being transmitted. If the highest audio frequency being transmitted is 5 kHz, then the total frequency space occupied by an AM signal will be 10 kHz (the carrier occupies negligible frequency space).

AM has the advantages of being easy to produce in a transmitter and AM receivers are simple in design. Its main disadvantage is its inefficiency. About two-thirds of an AM signal's power is concentrated in the carrier, which contains no intelligence. One-third of the power is in the sidebands, which contain the signal's intelligence. Since the sidebands contain the same intelligence, however, one is essentially "wasted." Of the total power output of an AM transmitter, only about one-sixth is actually productive, useful output!

Other disadvantages of AM include the relatively wide amount of frequency space an AM signal occupies and its susceptibility to static and other forms of electrical noise. Despite this, AM is simple to tune on ordinary receivers, and that is why it is used for almost all shortwave broadcasting.

Single Sideband (SSB)

Since so much power is wasted in AM, radio engineers devised a method to transmit just one sideband and put all of the transmitter's power into sending useful intelligence. This method is known as single sideband (SSB). In SSB transmitters, the carrier and one sideband are removed before the signal is amplified. Either the upper sideband (USB) or lower sideband (LSB) of the original AM signal can be transmitted.

SSB is a much more efficient mode than AM since all of the transmitter's power goes into transmitting useful intelligence. A SSB signal also occupies only about half the frequency space of a comparable AM signal. However, SSB transmitters and receivers are far more complicated than those for AM. In fact, a SSB signal cannot be received intelligibly on an AM receiver; the SSB signal will have a badly distorted "Donald Duck" sound. This is because the carrier of an AM signal does play a major role in demodulating (that is, recovering the transmitted audio) the sidebands of an AM signal. To successfully demodulate a SSB signal, you need a "substitute carrier."

A substitute carrier can be supplied by the beat frequency oscillator (BFO) circuit used when receiving CW signals. However, this means that a SSB signal must be carefully tuned to precise "beat" it against the replacement carrier from the BFO. For best performance, a SSB receiver needs more precise tuning and stability than an AM receiver, and it must be tuned more carefully than an AM receiver. Even when precisely tuned, the audio quality of a SSB signal is less than that of an AM signal.

SSB is used mainly by ham radio operators, military services, maritime and aeronautical radio services, and other situations where skilled operators and quality receiving equipment are common. There have been a few experiments in using SSB for shortwave broadcasting, but AM remains the preferred mode for broadcasting because of its simplicity.

Frequency Modulation (FM)

In CW, AM, and SSB, the carrier of the signal will not change in a normally operating transmitter. However, it is possible to modulate a signal by changing its frequency in accordance with a modulating signal. This is the idea behind frequency modulation (FM).

The unmodulated frequency of a FM signal is called its center frequency. When a modulating signal is applied, the FM transmitter's frequency will swing above and below the center frequency according to the modulating signal. The amount of "swing" in the transmitter's frequency in any direction above or below the center frequency is called its deviation. The total frequency space occupied by a FM signal is twice its deviation.

As you might suspect, FM signals occupy a great deal of frequency space. The deviation of a FM broadcast station is 75 kHz, for a total frequency space of 150 kHz. Most other users of FM (police and fire departments, business radio services, etc.) use a deviation of 5 kHz, for a total frequency space occupied of 10 kHz. For these reasons, FM is mainly used on frequency above 30 MHz, where adequate frequency space is available. This is why most scanner radios can only receive FM signals, since most signals found above 30 MHz are FM.

The big advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and a FM receiver will not respond to AM signals. FM receivers also exhibit a characteristic known as the capture effect. If two or more FM signals are on the same frequency, the FM receiver will respond to the strongest of the signals and ignore the rest. The audio quality of a FM signal increases as its deviation increases, which is why FM broadcast stations use such large deviation. The main disadvantage of FM is the amount of frequency space a signal requires.

Frequency-Shift Keying (FSK)

Like FM, frequency-shift keying (FSK) shifts the carrier frequency of the transmitter. Unlike FM, however, FSK shifts the frequency between just two separate, fixed points. The higher frequency is called the mark frequency while the lower of the two frequencies is called the space frequency. (By contrast, an FM signal can swing to any frequency within its deviation range.)

FSK was originally developed to send text via radioteleprinter devices, like those used by the TeleType Corporation. The shifting of the carrier between the mark and space was used to generate characters in the Baudot code, which can be thought of as a more elaborate version of the Morse code. At the receiver, the Baudot signals were used to produce printed text on printers and, later, video screens.

As technology improved, FSK was used to transmit messages in the ASCII code used by computers; this permitted the use of upper and lower case letters and special symbols. The introduction of microprocessors made it possible to use FSK to send messages with automatic error detection and correction capabilities. This is done by including error checking codes into messages and allowing the receiving station to request a retransmission of a message if the message and its error checking code are in conflict (or if the code is not received.) Among the most common such FSK modes are amateur teleprinting over radio (AMTOR) and forward error correction (FEC).

FSK is the fastest way to send text by radio, and the error-correcting modes offer high accuracy and reliability. The frequency space occupied depends on the amount of shifting, but typical FSK signals occupy less than 1.5 kHz of space. The big disadvantage of FSK is the more elaborate receiving gear required.

Special receiving terminals and adapters are available to let you "see" FSK modes. Many of these work in conjunction with personal computers.

Digital Modes

The same technology that makes it possible for you to view this Web site is also being used on the air. Digital modes can organize information into packets that contain address fields, information about the transmission protocol being used, error detection code, a few hundred bytes of data, and bits to indicate where each packet begins and ends.

Instead of transmitting messages in continuous streams, packet modes break them into packets. At the receiving end, the different packets are re-assembled to form the original message. If a packet is missing or received with errors, the receiving station can request a retransmission of the packet. Packets can be received out of sequence or even from multiple sources (such as different relaying stations) and still be assembled into the original message by the receiving station.

While packet modes have mainly been used to send text, any information that can be converted into digital form---sound, graphics, video, etc.---can be transmitted by digital modes.

Another advantage of packet modes is that packets can be addressed to specific stations in the address field of each packet. Other stations will ignore packets not addressed to them.

The big disadvantage of packet modes is the complexity of the necessary receiving and transmitting gear. The frequency space occupied is directly proportional to the speed at which messages are transmitted, and radio digital modes are very slow compared to their Internet equivalents. The slowest Web connection via the Internet is 14,400 baud (14.4K), while the maximum practical digital mode rate via radio is 9600 baud (9.6K). On frequencies below 30 MHz, it is even slower; rates are usually restricted to just 300 baud (0.3K)! As a result, digital modes via radio today deliver performance far short of their potential.

Special receiving adapters for packet modes are available, and these usually work in conjunction with personal computers. Most offer FSK receiving capabilities as well.

Another form of digital modulation is known as spread spectrum. Most other modulation methods pack all of the transmitter's output power into a bandwidth of only a few kHz. (Even in FM, the carrier doesn't occupy much bandwidth, although its frequency may be deviated over a wide range.) Spread spectrum literally "spreads" the carrier over a frequency range that may be as much as 10 kHz on frequencies below 30 MHz. (Spreading over 100 kHz or more is common on the VHF and UHF bands.) This spreading is usually done via a "spreading code" contained in an internal microcontroller chip.

When heard on a conventional receiver, spread sprectrum sounds like random noise or "gurgling" water. A receiver equipped with a microcontroller having the matching "spreading code" is necessary to properly receive the spread spectrum transmission. Advantages of spread spectrum include a high degree of privacy and freedom from intereference, since the spread spectrum receiver will reject any signal not having the proper spreading code. Almost all users of spread spectrum below 30 MHz are various military and government services.

Would you like to actually hear what some of these modes sound like?
Click here to visit the Digital Sounds Page of the World Utility News.

VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters


A workshop given at the 1997 TAPR/ARRL Digitial Communications Conference


Barry McLarnon, VE3JF
2696 Regina St. Ottawa, ON K2B 6Y1

Abstract

This paper attempts to provide some insight into the nature of radio propagation in that part of the spectrum (upper VHF to microwave) used by experimenters for high-speed digital transmission. It begins with the basics of free space path loss calculations, and then considers the effects of refraction, diffraction and reflections on the path loss of Line of Sight (LOS) links. The nature of non-LOS radio links is then examined, and propagation effects other than path loss which are important in digital transmission are also described.

Introduction

The nature of packet radio is changing. As access to the Internet becomes cheaper and faster, and the applications offered on the "net" more and more enticing, interest in the amateur packet radio network which grew up in the 1980s steadily wanes. To be sure, there are still pockets of interest in some places, particularly where some infrastructure to support speeds of 9600 bps or more has been built up, but this has not reversed the trend of declining interest and participation. Nevertheless, there is still lots of interest in packet radio out there - it is simply becoming re-focused in different areas. Some applications which do not require high speed, and can take advantage of the mobility that packet radio can provide, have found a secure niche - APRS is a good example. Interest is also high in high-speed wireless transmission which can match, or preferably exceed, landline modem rates. With a wireless link, you can have a 24-hour network connection without the need for a dedicated line, and you may also have the possibility of portable or mobile operation. Until recently, most people have considered it to be just too difficult to do high-speed digital. For example, the WA4DSY 56 Kbps RF modem has been available for ten years now, and yet only a few hundred people at most have put one on the air. With the new version of the modem introduced last year, 56 Kbps packet radio has edged closer to plug 'n play, but in the meantime, landline modem data rates have moved into the same territory. What has really sparked interest in high-speed packet radio lately is not the amateur packet equipment, but the boom in spread spectrum (SS) wireless LAN (WLAN) hardware which can be used without a licence in some of the ISM bands. The new WLAN units are typically integrated radio/modem/computer interfaces which mimic either ethernet interfaces or landline modems, and are just as easy to set up. Many of them offer speeds which landline modem users can only dream of. TAPR and others are working on bringing this type of SS technology into the amateur service, where it can be used on different bands, and without the Effective Radiated Power (ERP) restrictions which exist for the unlicenced service. This technology will be the ticket to developing high-speed wireless LANs and MANs which, using the Internet as a backbone, could finally realize the dream of a high-performance wide-area AMPRnet which can support the applications (WWW, audio and video conferencing, etc.) that get people excited about computer networking these days.

Although the dream as stated above is somewhat controversial, the author believes it represents the best hope of attracting new people to the hobby, providing a basis for experimentation and training in state-of-the-art wireless techniques and networking, and, ultimately, retaining spectrum for the amateur radio service. One problem is that most of the people attracted to using digital wireless techniques have little or no background in RF. When it comes to setting up wireless links which will work over some distance, they tend to lack the necessary knowledge about antennas, transmission lines and, especially, the subtleties of radio propagation. This paper deals with the latter area, in the hopes of providing this new crop of digital experimenters with some tools to help them build wireless links which work.

The main emphasis of this paper is on predicting the path loss of a link, so that one can approach the installation of the antennas and other RF equipment with some degree of confidence that the link will work. The focus is on acquiring a feel for radio propagation, and pointing the way towards recognizing the alternatives that may exist and the instances in which experimentation may be fruitful. We'll also look at some propagation aspects which are of particular relevance to digital signaling.

Estimating Path Loss

The fundamental aim of a radio link is to deliver sufficient signal power to the receiver at the far end of the link to achieve some performance objective. For a data transmission system, this objective is usually specified as a minimum bit error rate (BER). In the receiver demodulator, the BER is a function of the signal to noise ratio (SNR). At the frequencies under consideration here, the noise power is often dominated by the internal receiver noise; however, this is not always the case, especially at the lower (VHF) end of the range. In addition, the "noise" may also include significant power from interfering signals, necessitating the delivery of higher signal power to the receiver than would be the case under more ideal circumstances (i.e., back-to-back through an attenuator). If the channel contains multipath, this may also have a major impact on the BER. We will consider multipath in more detail later - for now, we will focus on predicting the signal power which will be available to the receiver.

Free Space Propagation

The benchmark by which we measure the loss in a transmission link is the loss that would be expected in free space - in other words, the loss that would occur in a region which is free of all objects that might absorb or reflect radio energy. This represents the ideal case which we hope to approach in our real world radio link (in fact, it is possible to have path loss which is less than the "free space" case, as we shall see later, but it is far more common to fall short of this goal).

Calculating free space transmission loss is quite simple. Consider a transmitter with power Pt coupled to an antenna which radiates equally in all directions (everyone's favorite mythical antenna, the isotropic antenna). At a distance d from the transmitter, the radiated power is distributed uniformly over an area of 4d2 (i.e. the surface area of a sphere of radius d), so that the power flux density is:

(1)

The transmission loss then depends on how much of this power is captured by the receiving antenna. If the capture area, or effective aperture of this antenna is Ar, then the power which can be delivered to the receiver (assuming no mismatch or feedline losses) is simply

(2)

For the hypothetical isotropic receiving antenna, we have

(3)

Combining equations (1) and (3) into (2), we have

(4)

The free space path loss between isotropic antennas is Pt / Pr. Since we usually are dealing with frequency rather than wavelength, we can make the substitution = c/f (where c, of course, is the speed of light) to get

(5)

This shows the classic square-law dependence of signal level versus distance. What troubles some people when they see this equation is that the path loss also increases as the square of the frequency. Does this mean that the transmission medium is inherently more lossy at higher frequencies? While it is true that absorption of RF by various materials (buildings, trees, water vapor, etc.) tends to increase with frequency, remember we are talking about "free space" here. The frequency dependence in this case is solely due to the decreasing effective aperture of the receiving antenna as the frequency increases. This is intuitively reasonable, since the physical size of a given antenna type is inversely proportional to frequency. If we double the frequency, the linear dimensions of the antenna decrease by a factor of one-half, and the capture area by a factor of one-quarter. The antenna therefore captures only one-quarter of the power flux density at the higher frequency versus the lower one, and delivers 6 dB less signal to the receiver. However, in most cases we can easily get this 6 dB back by increasing the effective aperture, and hence the gain, of the receiving antenna. For example, suppose we are using a parabolic dish antenna at the lower frequency. When we double the frequency, instead of allowing the dish to be scaled down in size so as to produce the same gain as before, we can maintain the same reflector size. This gives us the same effective aperture as before (assuming that the feed is properly redesigned for the new frequency, etc.), and 6 dB more gain (remembering that the gain is with respect to an isotropic or dipole reference antenna at the same frequency). Thus the free space path loss is now the same at both frequencies; moreover, if we maintained the same physical aperture at both ends of the link, we would actually have 6 dB less path loss at the higher frequency. You can picture this in terms of being able to focus the energy more tightly at the frequency with the shorter wavelength. It has the added benefit of providing greater discrimination against multipath - more about this later.

The free space path loss equation is more usefully expressed logarithmically:

(f in MHz, d in km) (6a)

or

(f in MHz, d in miles) (6b)

This shows more clearly the relationship between path loss and distance: path loss increases by 20 dB/decade or 6 dB/octave, so each time you double the distance, you lose another 6 dB of signal under free space conditions.

Of course, in looking at a real system, we must consider the actual antenna gains and cable losses in calculating the signal power Pr which is available at the receiver input:

(7)

where

Pt = transmitter power output (dBm or dBW, same units as Pr)

Lp = free space path loss between isotropic antennas (dB)

Gt = transmit antenna gain (dBi)

Gr = receive antenna gain (dBi)

Lt = transmission line loss between transmitter and transmit antenna (dB)

Lr = transmission line loss between receive antenna and receiver input (dB)

A table of transmission line losses for various bands and popular cable types can be found in the Appendix.

Example 1. Suppose you have a pair of 915 MHz WaveLAN cards, and want to use them on a 10 km link on which you believe free space path loss conditions will apply. The transmitter power is 0.25 W, or +24 dBm. You also have a pair of yagi antennas with 10 dBi gain, and at each end of the link, you need about 50 ft (15 m) of transmission line to the antenna. Let's say you're using LMR-400 coaxial cable, which will give you about 2 dB loss at 915 MHz for each run. Finally, the path loss from equation (6a) is calculated, and this gives 111.6 dB, which we'll round off to 112 dB. The expected signal power at the receiver is then, from (7):


According to the WaveLAN specifications, the receivers require -78 dBm signal level in order to deliver a low bit error rate (BER). So, we should be in good shape, as we have 6 dB of margin over the minimum requirement. However, this will only be true if the path really is equivalent to the free space case, and this is a big if! We'll look at means of predicting whether the free space assumption holds in the next section.

Path Loss on Line of Sight Links

The term Line of Sight (LOS) as applied to radio links has a pretty obvious meaning: the antennas at the ends of the link can "see" each other, at least in a radio sense. In many cases, radio LOS equates to optical LOS: you're at the location of the antenna at one end of the link, and with the unaided eye or binoculars, you can see the antenna (or its future site) at the other end of the link. In other cases, we may still have an LOS path even though we can't see the other end visually. This is because the radio horizon extends beyond the optical horizon. Radio waves follow slightly curved paths in the atmosphere, but if there is a direct path between the antennas which doesn't pass through any obstacles, then we still have radio LOS. Does having LOS mean that the path loss will be equal to the free space case which we have just considered? In some cases, the answer is yes, but it is definitely not a sure thing. There are three mechanisms which may cause the path loss to differ from the free space case:

  • refraction in the earth's atmosphere, which alters the trajectory of radio waves, and which can change with time.

  • diffraction effects resulting from objects near the direct path.

  • reflections from objects, which may be either near or far from the direct path.

We examine these mechanisms in the next three sections.

Atmospheric Refraction

As mentioned previously, radio waves near the earth's surface do not usually propagate in precisely straight lines, but follow slightly curved paths. The reason is well-known to VHF/UHF DXers: refraction in the earth's atmosphere. Under normal circumstances, the index of refraction decreases monotonically with increasing height, which causes the radio waves emanating from the transmitter to bend slightly downwards towards the earth's surface instead of following a straight line. The effect is more pronounced at radio frequencies than at the wavelength of visible light, and the result is that the radio waves can propagate beyond the optical horizon, with no additional loss other than the free space distance loss. There is a convenient artifice which is used to account for this phenomenon: when the path profile is plotted, we reduce the curvature of the earth's surface. If we choose the curvature properly, the paths of the radio waves can be plotted as straight lines. Under normal conditions, the gradient in refractivity index is such that real world propagation is equivalent to straight-line propagation over an earth whose radius is greater than the real one by a factor of 4/3 - thus the often-heard term "4/3 earth radius" in discussions of terrestrial propagation. However, this is just an approximation that applies under typical conditions - as VHF/UHF experimenters well know, unusual weather conditions can change the refractivity profile dramatically. This can lead to several different conditions. In superrefraction, the rays bend more than normal and the radio horizon is extended; in extreme cases, it leads to the phenomenon known as ducting, where the signal can propagate over enormous distances beyond the normal horizon. This is exciting for DXers, but of little practical use for people who want to run data links. The main consequence for digital experimenters is that they may occasionally experience interference from unexpected sources. A more serious concern is subrefraction, in which the bending of the rays is less than normal, thus shortening the radio horizon and reducing the clearance over obstacles along the path. This may lead to increased path loss, and possibly even an outage. In commercial radio link planning, the statistical probability of these events is calculated and allowed for in the link design (distance, path clearance, fading margin, etc.). We won't get into all of the details here; suffice it to say that reliability of your link will tend to be higher if you back off the distance from the maximum which is dictated by the normal radio horizon. Not that you shouldn't try and stretch the limits when the need arises, but a link which has optical clearance is preferable to one which doesn't. It's also a good idea to build in some margin to allow for fading due to unusual propagation situations, and to allow as much clearance from obstacles along the path as possible. For short-range links, the effects of refraction can usually be ignored.


Figure 1 Shadowing of Radio Waves by an Object
Diffraction and Fresnel Zones

Refraction and reflection of radio waves are mechanisms which are fairly easy to picture, but diffraction is much less intuitive. To understand diffraction, and radio propagation in general, it is very helpful to have some feeling for how radio waves behave in an environment which is not strictly "free space". Consider Fig. 1, in which a wavefront is traveling from left to right, and encountering an obstacle which absorbs or reflects all of the incident radio energy. Assume that the incident wavefront is uniform; i.e., if we measure the field strength along the line A-A', it is the same at all points. Now, what will be the field strength along a line B-B' on the other side of the obstacle? To quantify this, we provide an axis in which zero coincides with the top of the obstacle, and negative and positive numbers denote positions above and below this, respectively (we'll define the parameter used on this axis a bit later).



Figure 2 Signal Levels on the Far Side of the Shadowing Object

Intuition may lead one to expect the field strength along B-B' to look like the dashed line in Fig. 2, with complete shadowing and zero signal below the top of the obstacle, and no effect at all above it. The solid line shows the reality: not only does energy "leak" into the shadowed area, but the field strength above the top of the obstacle is also disturbed. At a position which is level with the top of the obstacle, the signal power density is down by some 6 dB, despite the fact that this point is in "line of sight" of the source. This effect is less surprising when one considers other familiar instances of wave motion. Picture, for example, tossing a rock in a pond and watching the ripples propagate outward. When they encounter an object such as a boat or a pier, you will see that the water behind the object is also disturbed, and that the waves traveling past, but close to, the object are also affected somewhat. Similarly, consider a distant source of sound waves: if the sound level is well above the ambient level, then moving behind an object which absorbs the incident sound energy completely does not result in the sound disappearing completely - it is still audible at a lower level, due to diffraction (as an aside, it is interesting to note that the wavelength of a 1 KHz sound wave is nearly the same as a 1 GHz radio wave). So much for analogies - let's get back to radio waves.

The explanation for the non-intuitive behavior of radio waves in the presence of obstacles which appear in their path is found in something called Huygens' Principle. Huygens showed that propagation occurs as follows: each point on a wavefront acts as a source of a secondary wavefront known as a wavelet, and a new wavefront is then built up from the combination of the contributions from all of the wavelets on the preceding wavefront. The secondary wavelets do not radiate equally in all directions - their amplitude in a given direction is proportional to (1 + cos a), where a is the angle between that direction and the direction of propagation of the wavefront. The amplitude is therefore maximum in the direction of propagation (i.e., normal to the wavefront), and zero in the reverse direction. The representation of a wavefront as a collection of wavelets is shown in Fig. 3.



Figure 3 Representation of Radio Waves as Wavelets



Figure 4 Building of a New Wavefront by Vector Summation

At a given point on the new wavefront (point B), the signal vector (phasor) is determined by vector addition of the contributions from the wavelets on the preceding wavefront, as shown in Fig. 4. The largest component is from the nearest wavelet, and we then get symmetrical contributions from the points above and below it. These latter vectors are shorter, due to the angular reduction of amplitude mentioned above, and also the greater distance traveled. The greater distance also introduces more time delay, and hence the rotation of the vectors as shown in the figure. As we include contributions from points farther and farther away, the corresponding vectors continue to rotate and diminish in length, and they trace out a double-sided spiral path, known as the Cornu spiral.



Figure 5 The Cornu Spiral

The Cornu spiral, shown in Fig. 5, provides the tool we need to visualize what happens when radio waves encounter an obstacle. In free space, at every point on a new wavefront, all contributions from the wavelets on the preceding wavefront are present and unattenuated, so the resultant vector corresponds to the complete spiral (i.e., the endpoints of the vector are X and Y). Now, consider again the situation shown in Fig. 1, and for each location on the wavefront B-B', visualize the makeup of the Cornu spiral (note that the top of the obstacle is assumed to be sufficiently narrow that no significant reflections can occur from it). At position 0, level with the top of the obstacle, we will have only contributions from the positive half of the preceding wavefront at A-A', since all of the others are blocked by the obstacle. Therefore, the received components form only the upper half of the spiral, and the resultant vector is exactly half the length of the free space case, corresponding to a 6 dB reduction in amplitude. As we go lower on the line B-B', we start to get blockage of components from the positive side of the A-A' wavefront, removing more and more of the vectors as we go, and leaving only the tight upper spiral. The resulting amplitude diminishes monotonically towards zero as we move down the new wavefront, but there is still signal present at all points behind the obstacle, as shown in the graph in Fig. 2. How about the points along line B-B' above the obstacle, where the graph shows those mysterious ripples? Again, look at the Cornu spiral: as we move up the line, we begin to add contributions from the negative side of the A-A' wavefront (vectors -1, -2, etc.). Note what happens to the resultant vector - as we make the first turn around the bottom of the spiral, it reaches its maximum length, corresponding to the highest peak in the graph of Fig. 2. As we continue to move up B-B' and add more components, we swing around the spiral and reach the minimum length for the resultant vector (minimum distance from point Y). Further progression up B-B' results in further motion around the spiral, and the amplitude of the resultant oscillates back and forth, with the amplitude of the oscillation steadily decreasing as the resultant converges on the free space value, given by the complete Cornu spiral (vector X-Y).

So, in a nutshell, to visualize what happens to radio waves when they encounter an obstacle, we have to develop a picture of the wavefront after the obstacle as a function of the wavefront just before it (as opposed to simply tracing rays from the distant source). Now we're in a position to talk about Fresnel zones. A Fresnel zone is a simpler concept once you have some understanding of diffraction: it is the volume of space enclosed by an ellipsoid which has the two antennas at the ends of a radio link at its foci. The two-dimensional representation of a Fresnel zone is shown in Fig. 6. The surface of the ellipsoid is defined by the path ACB, which exceeds the length of the direct path AB by some fixed amount. This amount is n/2, where n is a positive integer. For the first Fresnel zone, n = 1 and the path length differs by /2 (i.e., a 180 phase reversal with respect to the direct path). For most practical purposes, only the first Fresnel zone need be considered. A radio path has first Fresnel zone clearance if, as shown in Fig. 6, no objects capable of causing significant diffraction penetrate the corresponding ellipsoid. What does this mean in terms of path loss? Recall how we constructed the wavefront behind an object by vector addition of the wavelets comprising the wavefront in front of the object, and apply this to the case where we have exactly first Fresnel zone clearance. We wish to find the strength of the direct path signal after it passes the object. Assuming there is only one such object near the Fresnel zone, we can look at the resultant wavefront at the destination point B. In terms of the Cornu spiral, the upper half of the spiral is intact, but part of the lower half is absent, due to blockage by the object. Since we have exactly first Fresnel clearance, the final vector which we add to the bottom of the spiral is 180 degrees out of phase with the direct-path vector - i.e., it is pointing downwards. This means that we have passed the bottom of the spiral and are on the way back up, and the resultant vector is near the free space magnitude (a line between X and Y in Fig. 5). In fact, it is sufficient to have 60% of the first Fresnel clearance, since this will still give a resultant which is very close to the free space value.



Figure 6 Fresnel Zone for a Radio Link

In order to quantify diffraction losses, they are usually expressed in terms of a dimensionless parameter , given by:

(8)

where d is the difference in lengths of the straight-line path between the endpoints of the link and the path which just touches the tip of the diffracting object (see Fig. 7, where d = d1 + d2 - d). By convention, is positive when the direct path is blocked (i.e., the obstacle has positive height), and negative when the direct path has some clearance ("negative height"). When the direct path just grazes the object, = 0. This is the parameter shown in Figures 1 and 2. Since in this section we are considering LOS paths, this corresponds to specifying that is negative (or zero). For first Fresnel zone clearance, we have d = /2, so from equation (8), = -1.4. From Fig. 2, we can see that this is more clearance than necessary - in fact, we get slightly higher signal level (and path loss less than the free space value) if we reduce the clearance to = -1, which corresponds to d = /4. The = -1 point is also shown on the Cornu spiral in Fig. 5. Since d= /4, the last vector added to the summation is rotated 90 from the direct-path vector, which brings us to the lowest point on the spiral. The resultant vector then runs from this point to the upper end of the spiral at point Y. It's easy to see that this vector is a bit longer than the distance from X to Y, so we have a slight gain (about 1.2 dB) over the free space case. We can also see how we can back off to 60% of first Fresnel zone clearance ( = -0.85) without suffering significant loss.

But how do we calculate whether we have the required clearance? The geometry for Fresnel zone calculations is shown in Fig. 7. Keep in mind that this is only a two-dimensional representation, but Fresnel zones are three-dimensional. The same considerations apply when the objects limiting path clearance are to the side or even above the radio path. Since we are considering LOS paths in this section, we are dealing only with the "negative height" case, shown in the lower part of the figure. We will look at the case where h is positive later, when we consider non-LOS paths.

For first Fresnel zone clearance, the distance h from the nearest point of the obstacle to the direct path must be at least

(9)

where d1 and d2 are the distances from the tip of the obstacle to the two ends of the radio circuit. This formula is an approximation which is not valid very close to the endpoints of the circuit. For convenience, the clearance can be expressed in terms of frequency:

(10a)

where f is the frequency in GHz, d1 and d2 are in km, and h is in meters. Or:

(10b)

where f is in GHz, d1 and d2 in statute miles, and h is in feet.



Figure 7 Fresnel Zone Geometry

Example 2. We have a 10 km LOS path over which we wish to establish a link in the 915 MHz band. The path profile indicates that the high point on the path is 3 km from one end, and the direct path clears it by about 18 meters (60 ft.) - do we have adequate Fresnel zone clearance? From equation (10a), with d1 = 3 km, d2 = 7 km, and f = 0.915 GHz, we have h = 26.2 m for first Fresnel zone clearance (strictly speaking, h = -26.2 m). A clearance of 18 m is about 70% of this, so it is sufficient to allow negligible diffraction loss.

Fresnel zone clearance may not seem all that important - after all, we said previously that for the zero clearance (grazing) case, we have 6 dB of additional path loss. If necessary, this could be overcome with, for example, an additional 3 dB of antenna gain at each end of the circuit. Now it's time to confess that the situation depicted in Figures 1 and 2 is a special case, known as "knife edge" diffraction. Basically, this means that the top of the obstacle is small in terms of wavelengths. This is sometimes a reasonable approximation of an object in the real world, but more often than not, the obstacle will be rounded (such as a hilltop) or have a large flat surface (like the top of a building), or otherwise depart from the knife edge assumption. In such cases, the path loss for the grazing case can be considerably more than 6 dB - in fact, 20 dB would be a better estimate in many cases. So, Fresnel zone clearance can be pretty important on real-world paths. And, again, keep in mind that the Fresnel zone is three-dimensional, so clearance must also be maintained from the sides of buildings, etc. if path loss is to be minimized. Another point to consider is the effect on Fresnel zone clearance of changes in atmospheric refraction, as discussed in the last section. We may have adequate clearance on a longer path under normal conditions (i.e., 4/3 earth radius), but lose the clearance when unusual refraction conditions prevail. On longer paths, therefore, it is common in commercial radio links to do the Fresnel zone analysis on something close to "worst case" rather than typical refraction conditions, but this may be less of a concern in amateur applications.

Most of the material in this section was based on Ref. [2], which is highly recommended for further reading.

Ground Reflections

An LOS path may have adequate Fresnel zone clearance, and yet still have a path loss which differs significantly from free space under normal refraction conditions. If this is the case, the cause is probably multipath propagation resulting from reflections (multipath also poses particular problems for digital transmission systems - we'll look at this a bit later, but here we are only considering path loss).

One common source of reflections is the ground. It tends to be more of a factor on paths in rural areas; in urban settings, the ground reflection path will often be blocked by the clutter of buildings, trees, etc. In paths over relatively smooth ground or bodies of water, however, ground reflections can be a major determinant of path loss. For any radio link, it is worthwhile to look at the path profile and see if the ground reflection has the potential to be significant. It should also be kept in mind that the reflection point is not at the midpoint of the path unless the antennas are at the same height and the ground is not sloped in the reflection region - just the remember the old maxim from optics that the angle of incidence equals the angle of reflection.

Ground reflections can be good news or bad news, but are more often the latter. In a radio path consisting of a direct path plus a ground-reflected path, the path loss depends on the relative amplitude and phase relationship of the signals propagated by the two paths. In extreme cases, where the ground-reflected path has Fresnel clearance and suffers little loss from the reflection itself (or attenuation from trees, etc.), then its amplitude may approach that of the direct path. Then, depending on the relative phase shift of the two paths, we may have an enhancement of up to 6 dB over the direct path alone, or cancellation resulting in additional path loss of 20 dB or more. If you are acquainted with Mr. Murphy, you know which to expect! The difference in path lengths can be estimated from the path profile, and then translated into wavelengths to give the phase relationship. Then we have to account for the reflection itself, and this is where things get interesting. The amplitude and phase of the reflected wave depend on a number of variables, including conductivity and permittivity of the reflecting surface, frequency, angle of incidence, and polarization.

It is difficult to summarize the effects of all of the variables which affect ground reflections, but a typical case is shown in Fig. 8 [2]. This particular figure is for typical ground conditions at 100 MHz, but the same behavior is seen over a wide range of ground constants and frequencies. Notice that there is a large difference in reflection amplitudes between horizontal and vertical polarization (denoted on the curves with "h" and "v", respectively), and that vertical polarization in general gives rise to a much smaller reflected wave. However, the difference is large only for angles of incidence greater than a few degrees (note that, unlike in optics, in radio transmission the angle of incidence is normally measured with respect to a tangent to the reflecting surface rather than a normal to it); in practice, these angles will only occur on very short paths, or paths with extraordinarily high antennas. For typical paths, the angle of incidence tends to be of the order of one degree or less - for example, for a 10 km path over smooth earth with 10 m antenna heights, the angle of incidence of the ground reflection would only be about 0.11 degrees. In such a case, both polarizations will give reflection amplitudes near unity (i.e., no reflection loss). Perhaps more surprisingly, there will also be a phase reversal in both cases. Horizontally-polarized waves always undergo a phase reversal upon reflection, but for vertically-polarized waves, the phase change is a function of the angle of incidence and the ground characteristics.



Figure 8 Typical Ground Reflection Parameters

The upshot of all this is that for most paths in which the ground reflection is significant (and no other reflections are present), there will be very little difference in performance between horizontal and vertical polarization. For very short paths, horizontal polarization will generally give rise to a stronger reflection. If it turns out that this causes cancellation rather than enhancement, switching to vertical polarization may provide a solution. In other words, for shorter paths, it is usually worthwhile to try both polarizations to see which works better (of course, other factors such as mounting constraints and rejection of other sources of multipath and interference also enter into the choice of polarization).

As stated above, for either polarization, as the path gets longer we approach the case where the ground reflection produces a phase reversal and very little attenuation. At the same time, the direct and reflected paths are becoming more nearly equal. The path loss ripples up and down as we increase the distance, until we reach the point where the path lengths differ by just one-half wavelength. Combined with the 180° phase shift caused by the ground reflection, this brings the direct and reflected signals into phase, resulting in an enhancement over the free space path loss (theoretically 6 dB, but this will seldom be realized in practice). Thereafter, it's all downhill as the distance is further increased, since phase difference between the two paths approaches in the limit the 180° phase shift of the ground reflection. It can be shown that, in this region, the received power follows an inverse fourth-power law as a function of distance instead of the usual square law (i.e., 12 dB more attenuation when you double the distance, instead of 6 dB). The distance at which the path loss starts to increase at the fourth-power rate is reached when the ellipsoid corresponding to the first Fresnel zone just touches the ground. A reasonably good estimate of this distance can be calculated from the equation

(11)

where h1 and h2 are the antenna heights above the ground reflection point. For example, for antenna heights of 10 m, at 915 MHz ( = 33 cm) we will be into the fourth-law loss region for links longer than about 1.2 km.

So, for longer-range paths, ground reflections are always bad news. Serious problems with ground reflections are most commonly encountered with radio links across bodies of water. Spread spectrum techniques and diversity antenna arrangements usually can't overcome the problems - the solution lies in siting the antennas (e.g., away from the shore of the body of water) such that the reflected path is cut off by natural obstacles, while the direct path is unimpaired. In other cases, it may be possible to adjust the antenna locations so as to move the reflection point to a rough area of land which scatters the signal rather than creating a strong specular reflection.

Other Sources of Reflections

Much of what has been said about ground reflections applies to reflections from other objects as well. The "ground reflection" on a particular path may be from a building rooftop rather than the ground itself, but the effect is much the same. On long links, reflections from objects near the line of the direct path will almost always cause increased path loss - in essence, you have a permanent "flat fade" over a very wide bandwidth. Reflections from objects which are well off to the side of the direct path are a different story, however. This is a frequent occurrence in urban areas, where the sides of buildings can cause strong reflections. In such cases, the angle of incidence may be much larger than zero, unlike the ground reflection case. This means that horizontal and vertical polarization may behave quite differently - as we saw in Fig. 8, vertically polarized signals tend to produce lower-amplitude reflections than horizontally polarized signals when the angle of incidence exceeds a few degrees. When the reflecting surface is vertical, like the side of a building, a signal which is transmitted with horizontal polarization effectively has vertical polarization as far as the reflection is concerned. Therefore, horizontal polarization will generally result in weaker reflections and less multipath than vertical polarization in these cases.

Effects of Rain, Snow and Fog

The loss of LOS paths may sometimes be affected by weather conditions (other than the refraction effects which have already been mentioned). Rain and fog (clouds) become a significant source of attenuation only when we get well into the microwave region. Attenuation from fog only becomes noticeable (i.e., attenuation of the order of 1 dB or more) above about 30 GHz. Snow is in this category as well. Rain attenuation becomes significant at around 10 GHz, where a heavy rainfall may cause additional path loss of the order of 1 dB/km.

Path Loss on Non-Line of Sight Paths

We have spent quite a bit of time looking at LOS paths, and described the mechanisms which often cause them to have path loss which differs from the "free space" assumption. We've seen that the path loss isn't always easy to predict. When we have a path which is not LOS, it becomes even more difficult to predict how well signals will propagate over it. Unfortunately, non-LOS situations are sometimes unavoidable, particularly in urban areas. The following sections deal with some of the major factors which must be considered.

Diffraction Losses

In some special cases, such as diffraction over a single obstacle which can be modeled as a knife edge, the loss of a non-LOS path can be predicted fairly readily. In fact, this is the same situation that we saw in Figures 1 and 2, with the diffraction parameter > 0. This parameter, from equation (8), is


To get d, measure the straight-line distance between the endpoints of the link. Then measure the length of the actual path, which includes the two endpoints and the tip of the knife edge, and take the difference between the two. The geometry is shown in Fig. 7(a), the "positive h" case. A good approximation to the knife-edge diffraction loss in dB can then be calculated from

(12)

Example 3. We want to run a 915 MHz link between two points which are a straight-line distance of 25 km apart. However, 5 km from one end of the link, there is a ridge which is 100 meters higher than the two endpoints. Assuming that the ridge can be modeled as a knife edge, and that the paths from the endpoints to the top of ridge are LOS with adequate Fresnel zone clearance, what is the expected path loss? From simple geometry, we find that length of the path over the ridge is 25,001.25 meters, so that d = 1.25 m. Since = 0.33 m, the parameter , from (8), is 3.89. Substituting this into (12), we find that the expected diffraction loss is 24.9 dB. The free space path loss for a 25 km path at 915 MHz is, from equation (6a), 119.6 dB, so the total predicted path loss for this path is 144.5 dB. This is too lossy a path for many WLAN devices. For example, suppose we are using WaveLAN cards with 13 dBi gain antennas, which (disregarding feedline losses) brings them up to the maximum allowable EIRP of +36 dBm. This will produce, at the antenna terminals at the other end of the link, a received power of (36 - 144.5 + 13) = -95.5 dBm. This falls well short of the -78 dBm requirement of the WaveLAN cards. On the other hand, a lower-speed system may be quite usable over this path. For instance, the FreeWave 115 Kbps modems require only about -108 dBm for reliable operation, which is a comfortable margin below our predicted signal levels.

To see the effect of operating frequency on diffraction losses, we can repeat the calculation, this time using 144 MHz, and find the predicted diffraction loss to be 17.5 dB, or 7.4 dB less than at 915 MHz. At 2.4 GHz, the predicted loss is 29.0 dB, an increase of 4.1 dB over the 915 MHz case (these differences are for the diffraction losses only, not the only total path loss).



Figure 9 Diffraction by a Rounded Obstacle

Unfortunately, the paths which digital experimenters are faced with are seldom this simple. They will frequently involve diffraction over multiple rooftops or other obstacles, many of which don't resemble knife edges. The path losses will generally be substantially greater in these cases than predicted by the single knife edge model. The paths will also often pass through objects such as trees and wood-frame buildings which are semi-transparent at radio frequencies. Many models have been developed to try and predict path losses in these more complex cases. The most successful are those which deal with restricted scenarios rather than trying to cover all of the possibilities. One common scenario is diffraction over a single obstacle which is too rounded to be considered a knife edge. There are different ways of treating this problem; the one described here is from Ref. [3]. The top of the object is modeled as a cylinder of radius r, as shown in Fig. 9. To calculate the loss, you need to plot the profile of the actual object, and then draw straight lines from the link endpoints such that they just graze the highest part of the object as seen from their individual perspectives. Then the parameters Ds, d1, d2 and are estimated, and an estimate of the radius r can then be calculated from

(13)

Note that the angle is measured in radians. The procedure then is to calculate the knife edge diffraction loss for this path as outlined above, and then add to it an excess loss factor Lex, calculated from

(14)

There is also a correction factor for roughness: if the object is, for example, a hill which is tree-covered rather than smooth at the top, the excess diffraction loss is said to be about 65% of that predicted in (14). In general, smoother objects produce greater diffraction losses.

Example 4. We revisit the scenario in Example 3, but let's suppose that we've now decided that the ridge blocking our path doesn't cut it as a knife edge (ouch!). From a plot of the profile, we estimate that Ds = 10 meters. As before, d1 = 20 km, d2 = 5 km and the height of the ridge is 100 meters. Dusting off our high school trigonometry, we can work out that = 1.43, or 0.025 radians. Now, plugging these numbers into (13), we get r = 188 meters. Then, with = 0.33 m, we can calculate the excess loss from (14):


So, summed with the knife edge loss calculated previously, we have an estimated total diffraction loss of 37.3 dB (assuming the ridge is "smooth" rather than "rough"). This is a lot, but you can easily imagine scenarios where the losses are much greater: just look at the direct dependence on the angle in (14) and picture from Fig. 9 what happens when the obstacle is closer to one of the link endpoints. Amateurs doing weak signal work are accustomed to dealing with large path losses in non-LOS propagation, but such losses are usually intolerable in high-speed digital links.

Attenuation from Trees and Forests

Trees can be a significant source of path loss, and there are a number of variables involved, such as the specific type of tree, whether it is wet or dry, and in the case of deciduous trees, whether the leaves are present or not. Isolated trees are not usually a major problem, but a dense forest is another story. The attenuation depends on the distance the signal must penetrate through the forest, and it increases with frequency. According to a CCIR report [10], the attenuation is of the order of 0.05 dB/m at 200 MHz, 0.1 dB/m at 500 MHz, 0.2 dB/m at 1 GHz, 0.3 dB/m at 2 GHz and 0.4 dB/m at 3 GHz. At lower frequencies, the attenuation is somewhat lower for horizontal polarization than for vertical, but the difference disappears above about 1 GHz. This adds up to a lot of excess path loss if your signal must penetrate several hundred meters of forest! Fortunately, there is also significant propagation by diffraction over the treetops, especially if you can get your antennas up near treetop level or keep them a good distance from the edge of the forest, so all is not lost if you live near a forest.

General Non-LOS Propagation Models

There are many more general models and empirical techniques for predicting non-LOS path losses, but the details are beyond the scope of this paper. Most of them are aimed at prediction of the paths between elevated base stations and mobile or portable stations near ground level, and they typically have restrictions on the frequency range and distances for which they are valid; thus they may be of limited usefulness in the planning of amateur high-speed digital links. Nevertheless, they are well worth studying to gain further insight into the nature of non-LOS propagation. The details are available in many texts - Ref. [3] has a particularly good treatment. One crude, but useful, approximation will be mentioned here: the loss on many non-LOS paths in urban areas can be modeled quite well by a fourth-power distance law. In other words, we substitute d4 for d2 in equation (5). In equation (6), we can substitute 40log(d) for the 20log(d) term, which would correspond to the assumption of square-law distance loss for distances up to 1 km (or 1 mile, for the non-metric version of the equation), and fourth-law loss thereafter. This is probably an overly optimistic assumption for heavily built-up areas, but is at least a useful starting point.

The propagation losses on non-LOS paths can be discouragingly high, particularly in urban areas. Antenna height becomes a critical factor, and getting your antennas up above rooftop heights will often spell the difference between success and failure. Due to the great variability of propagation in cluttered urban environments, accurate path loss predictions can be difficult. If a preliminary analysis of the path indicates that you are at least in the ballpark (say within 10 or 15 dB) of having a usable link, then it will generally be worthwhile to give it a try and hope to be pleasantly surprised (but be prepared to be disappointed!).

Software Tools for Propagation Prediction

Although there is no substitute for experience and acquiring a "feel" for radio propagation, computer programs can make the job of predicting radio link performance a lot easier. They are particularly handy for exploring "what if" scenarios with different paths, antenna heights, etc. Unfortunately, they also tend to cost money! If you're lucky, you may have access to one of the sophisticated prediction programs which includes the most complex propagation models, terrain databases, etc. If not, you can still find some free software utilities that will make it easier to do some of the calculations discussed above, such as knife edge diffraction losses. One very useful freeware program which was developed specifically for short-range VHF/UHF applications is RFProp, by Colin Seymour, G4NNA. Check Colin's Web page at http://www.users.dircon.co.uk/~netking/freesw.htm for more information and downloading instructions. This is a Windows (3.1, 95 or NT) program which can calculate path loss in free space and simple diffraction scenarios. In addition to calculating knife edge diffraction loss, it provides some correction factors for estimating the loss caused by more rounded objects, such as hills. It also allows changing the distance loss exponent from square-law to fourth-law (or anything else, for that matter) to simulate long paths with ground reflections or obstructed urban paths. There is also some provision for estimating the loss caused when the signals must penetrate buildings. The program has a graphical user interface in which the major path parameters can be entered and the result (in terms of receiver SNR margin) seen immediately. There is also a tabular output which lists the detailed results along with all of the assumed parameters.

Special Considerations for Digital Systems

We have previously looked at the effect of multipath on path loss. When reflections occur from objects which are very close to the direct path, then paths have very similar lengths and nearly the same time delay. Depending on the relative phase shifts of the paths, the signals traversing them at a given frequency can add constructively to provide a gain with respect to a single path, or destructively to provide a loss. On longer paths in particular, the effect is usually a loss. Since the path lengths are nearly equal, the loss occurs over a wide frequency range, producing a "flat" fade.

In many cases, however, reflections from objects well away from the direct path can give rise to significant multipath. The most common reflectors are buildings and other manmade structures, but many natural features can also be good reflectors. In such cases, the propagation delays of the paths from one end of the link to the other can differ considerably. The extent of this time spreading of the signal is commonly measured by a parameter known as the delay spread of the path. One consequence of having a larger delay spread is that the reinforcement and cancellation effects will now vary more rapidly with frequency. For example, suppose we have two paths with equal attenuation and which differ in length by 300 meters, corresponding to a delay difference of 1 µsec. In the frequency domain, this link will have deep nulls at intervals of 1 MHz, with maxima in between. With a narrowband system, you may be lucky and be operating at a frequency near a maximum, or you may be unlucky and be near a null, in which case you lose most of your signal (techniques such as space diversity reception may help, though). The path loss in this case is highly frequency-dependent. On the other hand, a wideband signal which is, say, several MHz wide, would be subject to only partial cancellation or selective fading. Depending on the nature of the signal and how information is encoded into it, it may be quite tolerant of having part of its energy notched out by the multipath channel. Tolerance of multipath-induced signal cancellation is one of the major benefits of spread spectrum (SS) transmission techniques.

Longer multipath delay spreads have another consequence where digital signals are concerned, however: overlap of received data symbols with adjacent symbols, known as intersymbol interference or ISI. Suppose we try to transmit a 1 Mbps data stream over the two-path multipath channel mentioned above. Assuming a modulation scheme with 1 sec symbol length is used, then the signals arriving over the two paths will be offset by exactly one symbol period. Each received symbol arriving over the shorter path will be overlaid by a copy of the previous symbol from the longer path, making it impossible to decode with standard demodulation techniques. This problem can be solved by using an adaptive equalizer in the receiver, but this level of sophistication is not commonly found in amateur or WLAN modems (but it will certainly become more common as speeds continue to increase). Another way to attack this problem is to increase the symbol length while maintaining a high bit rate by using a multicarrier modulation scheme such as OFDM (Orthogonal Frequency Division Multiplex), but again, such techniques are seldom found in the wireless modem equipment available to hobbyists. For unequalized multipath channels, the delay spread must be much less than the symbol length, or the link performance will suffer greatly. The effect of multipath-induced ISI is to establish an irreducible error rate - beyond a certain point, increasing transmitter power will cause no improvement in BER, since the BER vs Eb/N0 curve has gone flat. A common rule of thumb prescribes that the multipath delay spread should be no more than about 10% of the symbol length. This will generally keep the irreducible error rate down to the order of 10-3 or less. Thus, in our two-path example above, a system running at 100K symbols/s or less may work satisfactorily. The actual raw BER requirements for a particular system will of course depend on the error-control coding technique used.

Although it is commonly believed that SS modulation schemes solve the multipath ISI problem, this is not really the case. As stated above, SS can convert a flat-faded channel into one which has selective fading, which is a good thing. In the case of Frequency Hopping (FHSS), it means that signal cancellation due to multipath will occur only a fraction of the time (i.e., only on some of the channels we hop to), and we can recover the data by means of Forward Error Correction (or by error detection and retransmission). In the case of Direct Sequence (DSSS), only a fraction of the transmitted spectrum is notched out by the multipath cancellation. This causes some degradation of the BER, but again error control coding can be used to compensate for this. In both cases, SS modulation has given us a form of frequency diversity. For DSSS, the large continuous spread bandwidth allows us to resolve many of the multipath components (those separated by delays of approximately the reciprocal of the spread bandwidth, or more). These appear as separate peaks in the DSSS receiver correlator output. A diversity receiver using the RAKE principle can take advantage of some of the multipath signal power by combining it constructively before making the bit decisions. More commonly, however, only the largest correlation peak is used, and all of the other multipath energy represents wideband interference. Regardless of whether a diversity receiver structure is used, however, ISI (and hence BER degradation) will still occur when the multipath delay spread approaches the same order of magnitude as the information symbol length. An excellent discussion of these concepts can be found in chapter 9 of Ref. [11].

As an illustration, consider again the WaveLAN product, which is a DSSS system using DQPSK modulation, a spread bandwidth of 11 MHz, and a symbol length of 1 µsec. Tests of WaveLAN using a channel simulator [12] have shown that its performance degrades when the delay spread exceeds 84 nsec (0.084 µsec), which is only about 10% of the symbol length.

Delay spreads of several microseconds are not uncommon, especially in urban areas. Mountainous areas can produce much longer delay spreads, sometimes tens of microseconds. This spells big trouble for doing high-speed data transmission in these areas. The best way to mitigate multipath in these situations is to use highly directional antennas, preferably at both ends of the link. The higher the data rate, the more critical it becomes to use high-gain antennas. This is one advantage to going higher in frequency. The delay spread for a given link will usually not exhibit much frequency dependence - for example, there will be similar amounts of multipath whether you operate at 450 MHz or 2.4 GHz, if you use the same antenna gain and type. However, you can get more directivity at the higher frequencies, which often will result in significantly reduced multipath delay spread and hence lower BER. It may seem strange that high-speed WLAN products are often supplied with omnidirectional antennas which do nothing to combat multipath, but this is because the antennas are intended for indoor use. The attenuation provided by the building structure will usually cause a drastic reduction in the amplitude of reflections from outside the building, as well as from distant areas inside the building. Delay spreads therefore tend to be much smaller inside buildings - typically of the order of 0.1 µsec or less. However, as WLAN products with data rates of 10 Mbps and beyond are now appearing, even delay spreads of this magnitude are problematic and must be dealt with by such measures as equalizers, high-level modulation schemes and sectorized antennas.

Conclusions

Radio propagation is a vast topic, and we've only scratched the surface here. We haven't considered, for example, the interesting area of data transmission involving mobile stations - maybe next year! Hopefully, this paper has provided some insight into the problems and solutions associated with setting up digital links in the VHF to microwave spectrum. To sum up, here are a few guidelines and principles:

  • Always strive for LOS conditions. Even with LOS, you must pay attention to details regarding variability of refractivity, Fresnel zone clearance and avoiding reflections from the ground and other surfaces. Non-LOS paths will often lead to disappointment unless they are very short, especially with the high-speed unlicenced WLAN devices. Their low ERP limits and high receive signal power requirements (due to large noise bandwidths, high noise figures and sometimes, significant modem implementation losses) leave little margin for higher-than-LOS path losses. Hams are not encumbered by the low ERP limits, but it can be very expensive to overcome excessive path losses with higher transmitter powers.

  • Use as much antenna gain as is practical. It is always worthwhile to try both polarizations, but horizontal polarization will often be superior to vertical. It will generally provide less multipath in urban areas, and may provide lower path loss in some non-LOS situations (e.g., attenuation from trees at VHF and lower UHF). Also, interfering signals from pagers and the like tend to be vertically polarized, so using the opposite polarization can often provide some protection from them.

  • There are advantages to going higher in frequency, into the microwave bands, due to the higher antenna gains which can be achieved. The tighter focusing of energy which can be achieved may result in lower overall path loss on LOS paths (providing that you can keep the feedline losses under control), and less multipath. Higher frequencies also have smaller Fresnel zones, and thus require less clearance over obstacles to avoid diffraction losses. And, of course, the higher bands have more bandwidth available for high-speed data, and less probability of interference. However, the advantage may be lost in non-LOS situations, since diffraction losses, and attenuation from natural objects such as trees, increase with frequency.

Radio propagation is seldom 100% predictable, and one should never hesitate to experiment. It's very useful, though, to be equipped with enough knowledge to know what techniques to try, and when there is little probability of success. This paper was intended to help fill some gaps in that knowledge. Good luck with your radio links!

Acknowledgements
The author gratefully acknowledges the work of his daughter Kelly in producing the figures for this paper. WaveLAN is a registered trademark of Lucent Technologies, Inc.
References
[1] ARRL UHF/Microwave Experimenter's Manual (American Radio Relay League, 1990).

[2] Hall, M.P.M., Barclay, L.W. and Hewitt, M.T. (Eds.), Propagation of Radiowaves (Institution of Electrical Engineers, 1996).

[3] Parsons, J.D., The Mobile Radio Propagation Channel (Wiley & Sons, 1992).

[4] Doble, J., Introduction to Radio Propagation for Fixed and Mobile Communications (Artech House, 1996).

[5] Bertoni, H.L., Honcharenko, W., Maciel, L.R. and Xia, H.H., "UHF Propagation Prediction for Wireless Personal Communications", Proceedings of the IEEE, Vol. 82, No. 9, September 1994, pp. 1333-1359.

[6] Andersen, J.B., Rappaport, T.S. and Yoshida, S., "Propagation Measurements and Models for Wireless Communications Channels", IEEE Communications Magazine, January 1995, pp. 42-49.

[7] Freeman, R.L., Radio System Design for Telecommunications (Wiley & Sons, 1987).

[8] Lee, W.C.Y., Mobile Communications Design Fundamentals, Second Edition (Wiley & Sons, 1993).

[9] CCIR (now ITU-R) Report 567-4, "Propagation data and prediction methods for the terrestrial land mobile service using the frequency range 30 MHz to 3 GHz" (International Telecommunication Union, Geneva, 1990).

[10] CCIR Report 1145, "Propagation over irregular terrain with and without vegetation" (International Telecommunication Union, Geneva, 1990).

[11] Pahlavan, K., and Levesque, A.H., Wireless Information Networks (Wiley & Sons, 1995).

[12] Hollemans, W., and Verschoor, A., "Performance Study of WaveLAN and Altair Radio-LANs", Proceedings of the 5th IEEE Symposium on Personal, Indoor and Mobile Radio Communications, September 1994.

Appendix
Cable Type144 MHz 220 MHz450 MHz 915 MHz1.2 GHz 2.4 GHz5.8 GHz
RG-586.2

(20.3)

7.4

(24.3)

10.6

(34.8)

16.5

(54.1)

21.1

(69.2)

32.2

(105.6)

51.6

(169.2)

RG-8X4.7

(15.4)

6.0

(19.7)

8.6

(28.2)

12.8

(42.0)

15.9

(52.8)

23.1

(75.8)

40.9

(134.2)

LMR-2403.0

(9.8)

3.7

(12.1)

5.3

(17.4)

7.6

(24.9)

9.2

(30.2)

12.9

(42.3)

20.4

(66.9)

RG-213/2142.8

(9.2)

3.5

(11.5)

5.2

(17.1)

8.0

(26.2)

10.1

(33.1)

15.2

(49.9)

28.6

(93.8)

99131.6

(5.2)

1.9

(6.2)

2.8

(9.2)

4.2

(13.8)

5.2

(17.1)

7.7

(25.3)

13.8

(45.3)

LMR-4001.5

(4.9)

1.8

(5.9)

2.7

(8.9)

3.9

(12.8)

4.8

(15.7)

6.8

(22.3)

10.8

(35.4)

3/8" LDF 1.3

(4.3)

1.6

(5.2)

2.3

(7.5)

3.4

(11.2)

4.2

(13.8)

5.9

(19.4)

8.1

(26.6)

LMR-6000.96

(3.1)

1.2

(3.9)

1.7

(5.6)

2.5

(8.2)

3.1

(10.2)

4.4

(14.4)

7.3

(23.9)

1/2" LDF 0.85

(2.8)

1.1

(3.6)

1.5

(4.9)

2.2

(7.2)

2.7

(8.9)

3.9

(12.8)

6.6

(21.6)

7/8" LDF 0.46

(1.5)

0.56

(2.1)

0.83

(2.7)

1.2

(3.9)

1.5

(4.9)

2.3

(7.5)

3.8

(12.5)

1 1/4" LDF 0.34

(1.1)

0.42

(1.4)

0.62

(2.0)

0.91

(3.0)

1.1

(3.6)

1.7

(5.6)

2.8

(9.2)

1 5/8" LDF 0.28

(0.92)

0.35

(1.1)

0.52

(1.7)

0.77

(2.5)

0.96

(3.1)

1.4

(4.6)

2.5

(8.2)

Table 1 - Attenuation of Various Transmission Lines in Amateur and ISM Bands in dB/ 100 ft (dB/ 100 m)

Notes

Attenuation data based on figures from the "Communications Coax Selection Guide" from Times Microwave Systems (http://www.timesmicrowave.com/products/commercial/selectguide/atten/) and other sources.

The LMR series is manufactured by Times Microwave. 9913 is manufactured by Belden Corp. RG-series cables are manufactured by Belden and many others. The LDF series are foam dielectric, solid corrugated outer conductor cables, best known by the brand name HELIAX (®Andrew Corp.).