The president & CEO of PBS station WNET in New York, Bill Baker, has a weblog, and in it he has a very interesting post on clear channel radio stations -- those "blowtorch" AM radio stations which he describes as:
... A clear channel station -- not to be confused with the group station owner Clear Channel Communications -- means that the station is almost always 50,000 watts and during hours of darkness has interference-free coverage for about 750 miles around its city of license. Other stations on the same frequency sign off at sunset or go directional to protect the huge service areas of the clear channel stations. The clear channel stations also have wide daytime coverage, sometimes extending 100 miles or more in many directions. ...
Turns out, though, that Clear Channel Communications owns 16 of the 58 U.S. clear channel stations and takes its name from clear channel station WOAI in San Antonio. The next largest owner with 13 is CBS, he says. That and much more info in this scholarly post.
This post was a real nostalgic treat because I got my interest in radio from DXing on the AM band from about age 12-16 and collected reception verifications from them. Most of these stations were in my collection because they were the easiest to receive. I know that Dr. Baker has a ham radio license, as do I (though I'm not very active), so maybe he got interested in broadcasting the same way. If I recall correctly, he came to WNET from Westinghouse, which founded clear channel pioneer station KDKA, 1020 kHz, in Pittsburgh (now owned by CBS Radio) in 1920.
P.S.: WNET also has another blog called Daily Media Briefing that's worth checking out. It uses a one post/many short items format that contains lots of interesting tidbits. Both it and Bill Baker's blog have RSS feeds.