The Eclipse's Effects On Radio Propagation

The recordings on this page started as an experiment to observe the effects of the solar eclipse on the propagation of class C AM broadcast stations. These stations are found on 1230, 1240, 1340, 1400, 1450, and 1490 kHz. As these are relatively low powered stations intended to reach small geographic areas, there are many stations operating on each of these five frequencies. At night, each of these frequencies is a cacophony of signals unless, of course, one is fairly close to the transmitter.

The initial hypothesis was that as the zone of totality swept eastward, one station should be able to dominate each frequency, possibly followed by another station and perhaps even a third. The receiving antenna used was the homebuilt amplified loop shown below. This antenna has a figure-8 pickup pattern, which was needed in order to minimize interference from local stations on the class C frequencies (1230 and 1450 kHz) or on adjacent channels.

Mediumwave Loop Antenna

The experiment was carried out in Hillsboro, OR, where the eclipse reached its maximum around 10:18 A.M. PDT. The antenna was rotated N-S so that one of the nulls would be directed toward KRYN (1230 kHz) and KBPS (1450 kHz).

The class C frequencies often had multiple weak signals that faded in and out over each other. The first frequency checked was 1230 kHz, but nothing of interest was heard there. An unexpected result was that many signals from Seattle could be heard, even though the zone of totality swept south of Hillsboro.

Local time was 10:29 A.M. when the 1690 kHz was recorded. At this point, the antenna was rotated east-west to receive stations that might experience favorable propagation as the eclipse approached them. Scanning was started from the bottom of the band.

Aircheck Archive Page

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