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A clear-channel station is an AM band radio station in North America that has the highest protection from interference from other stations, particularly concerning night-time skywave propagation. The system exists to ensure the existence of cross-country or cross-continent radio service, and is enforced through a series of treaties and statutory laws. Usually known as class A stations since 1982, they are occasionally still referred to by their former classifications of class I-A, class I-B, or class I-N. The term "clear channel" is used most often in the context of North America and the Caribbean, where the concept originated. Since 1941, these stations have been required to maintain at least 10,000 watts of power to retain their status. ==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 kW 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 NARBA's terms, eliminating among other things the distinction between the two kinds of clear channel: the original "I-A" and "I-B" classes, and the new, U.S.-only "I-N" class, which are now all included in class A. Classes "I-A" and "I-B" still mandate a minimum efficiency of 362.10 mV/m/kW at 1 km, whereas Class "I-N" is permitted to use the lower Class B minimum efficiency of 281.63 mV/m/kW at 1 km. There exist exceptions, where a former Class B station was elevated to Class A, yet it maintained its previous antenna system, or made only minor changes thereto. 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. iHeartMedia, Inc. (formerly Clear Channel Communications until September 2014), 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Clear-channel station」の詳細全文を読む スポンサード リンク
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