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.
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 was—literally—clear 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!
No comments:
Post a Comment