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Question(s) about daytime and nightime coverage areas of AM radio stations

I

Iowan

1. First of all, why are there like two coverage areas for AM radio stations (one for day and one for night)?

2. What time does this daily/nightly back-and-fourth switch take place? Or does it vary between stations? For example, what times do KJOC ("True Oldies 1170") switch between coverages?
 
Iowan said:
1. First of all, why are there like two coverage areas for AM radio stations (one for day and one for night)?
Click to expand...

This isn't the case for all AM stations. There are quite-a-few that operate at the same power level and with the same directional pattern day and night. AM can be received across great distances after dark because of the changes in the atmosphere after the sun goes down. As a result, the potential for AM's to interfere with each other goes up dramatically. That's why you see the changes in power levels and directional patterns. The US and Canada, by the way, are about the only countries that use directional antennas. Most other countries just have AM's reduce power after dark. It's to protect other stations on the same or nearby frequencies, sometimes hundreds of miles away. FM, on the other hand, does not reflect against the upper layers of the atmosphere. So, you can predict the interference much more reliably.

2. What time does this daily/nightly back-and-fourth switch take place? Or does it vary between stations? For example, what times do KJOC ("True Oldies 1170") switch between coverages?
Click to expand...

Each station has different "sunset" times as specified on their FCC licenses. It depends on the stations they have to protect as well as other factors. For example, some stations on the West Coast may have to power down in the middle of afternoon drive, at East Coast sunset times, while other stations on the East Coast can sometimes operate at a much higher power well into the evening, until West Coast sunset. I don't know KJOC's specific sunset time, but I suspect it has to power down to protect KFAQ 1170 in Tulsa, which means it would be near your local sunset time as Tulsa and the Quad Cities are in the same time zone.
 
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