There still are "clear channel" stations: KFI in Los Angeles, KSL in Salt Lake City, WLW in Cincinnati, WOR in New York and a number of others across the country.
Also, with rare exceptions, stations in any given "market" are at least 20 KHz apart. That is, adjacent channels are not assigned in any given area. Almost all AM stations have a market area (intended coverage area) radius of perhaps 50 miles, often less. The "clear channel flamethrowers" running 500,000 watts are relatively few by comparison. Many AM stations today limit their audio bandwidth to 5 or 6 KHz, maximum, and even more consumer AM receivers sold today limit the audio bandwidth to 5 kHz or less! Some of the reasons for the limits at the transmitter have to do with going to digital audio. I haven't worked in the broadcast industry since digital AM started to appear so I don't know the overall issues, but I see it mentioned regularly in the broadcasting press. Of course, with receivers cutting off the audio bandpass at 5 KHz or less it is to the transmitter's advantage to limit the audio bandwidth. Doing so gives the signal more strength at the receiver. The reason receivers do it is because the listening public likes it. The average listener doesn't notice the loss of highs in the audio but they do object to the high frequency noise they hear when signals are marginal. Manufacturers found that radios, especially car radios, that aggressively roll off the audio above about 5 KHz are much preferred among the public so they all do it to stay competitive. That's been exaggerated in recent years as AM is more and more "talk radio", leaving music to FM. Some 'talk format' stations have rolled off the frequency response well below 5 KHz. Of course, suppressing high frequencies also greatly reduces the 10 kHz heterodyne in those cases where two signals are audible on adjacent AM frequency assignments, such as when driving at night between cities. A few articles I've seen in the broadcast press even note that since almost 1/2 of the American public under 50 years of age is now functionally deaf above about 6 or 7 KHz, thanks to abusing their ears with loud music as children, they are completely unaware of the change in audio response anyway. Ron AC7AC -----Original Message----- Back in the 'old days' of AM broadcasting in the US, the local stations were spread out with greater than 10 KHz spacing so they would not interfere even when received on wide bandwidth AM receivers. There were a few 'clear channel' superstations that had no competition nationwide and IIRC, they had a 20 kHz swath of spectrum. Those were mostly Westinghouse stations which ran 50,000 watts and could be heard over great distances. On normal stations, one would receive maximum fidelity with a 10 kHz IF filter (if a straight sided filter would have been available back then) because the modulation was supposed to be limited to less 5 kHz (or so rumor had it in those days). 73, Don W3FPR _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
