The following is a digest version of materials I have written in Popular Communications and other publications. If you have a chance, sign up for a subscription to Popular Communications and follow my monthly column, where I explore propagation and look at current, past, and future conditions on the radio waves.
(copyright, 2003, Tomas Hood)
A Look at Medium Wave DXing - The Winter Season
The medium wave (MW) 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. Propagation at these frequencies is very different than for frequencies in the high frequency range (3 MHz through 30 MHz). Propagation of medium wave signals varies depending upon the time of day, the season, and other factors.
For medium wave, the most obvious factor for good DX is the time of day. The D Layer of the ionosphere almost always absorbs medium wave radio signals during the daylight hours. As a result, nearly all medium wave 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 past a few hundred miles away unusual during the day. At night, however, the ionosphere tends to refract these medium wave signals, making it possible for radio stations to be heard at much greater distances. When conditions are just right, you might hear stations 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 1000 miles away. For this reason, groundwave is the mode of propagation during the day.
At night, however, because of the recombination that occurs in the D layer and the sharp reduction in MW signal absorption that results in the D layer, 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.
The ionosphere is therefore directly responsible for MW DX signals. After sunset these MW signals tend to be refracted off of the E and F layers back to the earth, much like a flashlight beam might be reflected off of a mirror. The distance of the skywave skip is anywhere from ten to 500 miles, or so. MX DX signals may travel farther, if the ground is highly conductive, providing a reflection of the signal back up into the ionosphere. Multiple hop skywave signals can enable a broadcast signal at night to span thousands of miles. It is typical to hear European and Asian stations over the salt water of the oceans.
Reception of medium wave signals tends to be better in winter than in summer, due to lower levels of atmospheric noise and longer hours of darkness. During times of severe geomagnetic storms, when the planetary K index is above 4, auroral ionization can absorb the skywave medium wave signals, causing any higher-latitude broadcast signals to disappear which would allow weaker mid- and low-latitude stations to be heard. At the same time, it has been observed that mid- and low-latitude skywave signals may be enhanced during these times because ionospheric tilting and other phenomena. DXing of stations from south of the equator is often possible during highly active geomagnetic storms.
One of the most exciting aspects of medium wave DXing is known as the "sunrise and sunset DXing window." The most fruitful times to reap distant MW signals are from just before sunset to a few hours after sunset and again just before sunrise to a few hours afterward. The sunset skip period is particularly useful to DXers in the eastern part of North America, because stations in time zones farther west become audible after local daytime stations have stopped transmitting. Western DXers, on the other hand, have an advantage in being able to pick up many eastern stations as they begin their broadcast days in the morning. A good overview on working the sunrise and sunset DX window on MW frequencies is at < http://www.nrcdxas.org/articles/sunriseset.html
Because of the seasonal decrease in geomagnetic activity during December and January, and because of the longer hours of darkness in the Northern Hemisphere, you will find a rich selection of medium wave AM signals from as far away as Europe, South America, Asia, and even the South Pacific. Let me know your experiences.
(copyright, 2003, Tomas Hood. All rights reserved.)