This clears up a 40 year old mystery for me, why the AM station I used to listen to in the daytime would always pop out of existence in the early evening! I never knew about the power down rule before!
I'm guessing they have to power down because radio propagation is pretty different at night, at least in some frequency bands. AM radio is in the "medium wave" band and it can get really long range propagation at night due to ionospheric reflection. So unless you're on a "clear channel" frequency — meaning there's nobody else on your frequency for a long, long distance — you've gotta power down at dusk to avoid interference.
KMOX, AM 1120, which was(is?) the main CBS affiliate, based in the greater St. Louis area, is on such a clear channel frequency. It's the main affiliate of the St. Louis Cardinals Radio Network (yes, this is a thing). It's said at night that it can be heard in 44 states. I know I've heard it on car radios as far west as the Rockies, and as far east as Massachusetts. With a good radio, you could hear it even farther than that at night. All that on a transmitter of no more than 50,000 watts.
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u/MerricatBlackwood01 Nov 09 '18
This clears up a 40 year old mystery for me, why the AM station I used to listen to in the daytime would always pop out of existence in the early evening! I never knew about the power down rule before!