Description
Certain mediumwave frequencies were set aside under the North American Radio Broadcasting Agreement (NARBA) for nighttime use by only one or two specific AM stations, covering a wide area via skywave propagation; these frequencies were known as the clear channels, and the stations on them are thus clear-channel stations. Where only one station was assigned to a clear channel, the treaty provides that it must operate with a nominal power of 50 kilowatts or more; stations on the other clear channels, with two or more stations, must use between 10 and 50 kW, and most often use a directional antenna so as not to interfere with each other. In addition to the frequencies, the treaty also specified the specific locations where stations on this second kind of channel (known as class I-B) could be built.
Some of the original NARBA signatories, including the United States, Canada and Mexico, have implemented bilateral agreements that supersede its terms, eliminating among other things the distinction between the two kinds of clear channel: the original "I-A", "I-B", and "I-N" station classes are now all included in class A.
Clear-channel stations, unlike all other AM stations in North America, have a secondary service area—that is, they are entitled to protection from interference to their nighttime skywave signals. Other stations are entitled, at most, to protection from nighttime interference in their primary service area — that which is covered by their groundwave signal.
Many stations beyond those listed in the treaty have been assigned to operate on a clear channel (and some had been long before NARBA came into effect in 1941). In most cases, those stations operate during the daytime only, so as not to interfere with the primary stations on those channels. Since the early 1980s, many such stations have been permitted to operate at night with such low power as to be deemed not to interfere; these stations are still considered "daytimers" and are not entitled to any protection from interference to their nighttime signals. Another group of stations, formerly known as class II stations, were licensed to operate on the former "I-B" clear channels with significant power at night, provided that they use directional antenna systems to minimize radiation towards the primary stations.
Clear Channel Communications, a San Antonio, Texas-based company which owns over 900 U.S. radio stations, was originally formed to purchase one clear-channel station, WOAI. The company now owns more than a dozen such stations.
Read more about this topic: Clear-channel Station
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