Propagation
During the daytime, medium and high powered mediumwave AM radio stations have a normal reception range of about 20 to 250 miles (32 to 400+ km), depending on the transmitter power, location, and the quality of the receiving equipment, including the amount of man-made and natural electromagnetic noise present. Long-distance reception is normally impeded by the D layer of the ionosphere, which during the daylight hours absorbs signals in the mediumwave range.
As the sun sets, the D layer weakens, allowing Medium Wave radio waves from such stations to bounce off the F layer of the ionosphere, producing reliable, long distance reception of (especially) high-powered stations up to about 1,200 miles (2,000 km) away on a nightly basis. Aside from the more or less regular reception of certain high powered transmitters, variable conditions allow reception of different stations at different times - for example, on one night a medium-powered broadcaster from Cleveland, Ohio may be audible in Duluth, Minnesota, but not on the following night. Much of the hobby consists in trying to receive and log as many of these stations as possible, identifying target stations and frequencies to listen to and log.
Near or on the coastlines, trans-oceanic reception is quite common and a favored target of DXers in those areas. Very distant inter-continental DX from stations several thousands of miles away is possible even far inland, but may require exceptionally good conditions and a good receiver and antenna on the listening side.
DX stations evaporate from the dial as the sun rises. However, sunrise and sunset ("SRS" and "SSS") periods can provide interesting loggings.
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