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The Propagation Corner A Look At Mediumwave DXing—The Winter Season
by Tomas Hood, NW7US |
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The mediumwave broadcast band, also known in the United States as the AM broadcast band (or AM band) currently extends from 525 to 1700 kHz. In the United States and Canada, channels are spaced in even 10-kHz increments starting at 530 kHz. Elsewhere, channels are spaced in 9-kHz increments starting at 531 kHz. The hunt for signals from distant AM broadcasting stations is an exciting activity, especially during the late fall and winter seasons. Which distant stations you can hear depends largely upon signal propagation. Propagation at these frequencies is very different than it is for frequencies in the high frequency range (3 MHz through 30 MHz). Propagation of mediumwave signals varies depending upon the time of day, the season, and other factors. For mediumwave, the most obvious factor for good DX is the time of day. The D layer of the ionosphere almost always absorbs mediumwave radio signals during the daylight hours. As a result, nearly all mediumwave signals received during midday hours will arrive by ground wave propagation, rather than by skywaves refracted off of the ionosphere. Groundwave propagation makes reception of signals over a few hundred miles away unusual in daylight. At night, however, the ionosphere refracts these mediumwave signals, making it possible for radio stations to be heard at much greater distances, sometimes as far away as Australia, Europe, and Asia. The groundwave, as its name implies, travels along a path close to the earth’s surface. The distance a groundwave is able to travel depends upon the transmitter power, frequency, antenna pattern, and the earth’s conductivity along the path of the signal. Lower frequencies travel greater distances, all other factors remaining the same. A signal on the lowest-end of the AM broadcast band, say 540 kHz, will travels twice as far as a signal broadcast on, say 1600 kHz, if all other parameters remain the same for both stations. If the land between the transmitting antenna and the receiving antenna is rocky, a groundwave signal might only travel 150 to 300 miles. On the other hand, if the signal is moving over salt water, the groundwave signal could make it some 1,000 miles. While most groundwave signals are stable and strong, some fading and changes in reception can occur for groundwave signals. Sometimes, this fading is caused by signal cancellation due to weak skywave reception at the same point where the groundwave component is received. Groundwave propagation provides a broadcast station with reliable, stable coverage to its target audience, and radio station engineers optimize the antenna system to ensure the best delivery of that groundwave signal. During the day, because the D layer of the ionosphere so completely absorbs the mediumwave radio signals, groundwave is the only mode of propagation a mediumwave station can rely on. At night, however, because of the recombination that occurs in the D layer, and the sharp reduction in mediumwave signal absorption that results, many stations must reduce their power so that they do not interfere with other stations. Some stations must even cease transmitting during the night hours. Those stations that do not need to cease transmitting will have signals radiating up into the ionosphere and possibly refracting back to earth at far distant locations, making for AM DX.
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