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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. 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.
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.
For the U.S., a form of clear channels first appeared in 1922 when the Commerce Department moved stations which had all used three (initially two) frequencies (two for entertainment stations, one for "weather and crop reports") onto 52 frequencies. Two were used for all low-power stations and the large stations each got their own frequency. A few frequencies were used on both the East and West coasts, which were considered far enough apart to limit interference. At this time large stations were limited to 1000 watts and some licences were revoked.
On 11 November 1928, the United States implemented General Order 40, which classified each allocation in the AM band as either Local, Regional or Clear. The classification system considered stations in Canada as well. Gradually maximum power was increased to 50,000 watts (with some short lived experiments with 250–500 kilowatt "super-power" operation). This system was continued in the 1941 NARBA system although almost all stations shifted broadcast frequencies. The FCC's intent behind licensing 50,000 watt clear channel stations was to provide reliable radio service to the thousands of Americans who lived in the vast rural areas of the United States. As a result, these stations usually reached large portions of North America at night. Radio fans often call such stations "flamethrowers" or "blowtorches" because of their power.
As early as the 1930s, debate raged in Washington, D.C., and in the U.S. broadcasting industry over whether continuation of the clear-channel system was justifiable. The licensees of clear-channel stations argued that, without their special status, many rural areas would receive no radio service at all. Rural broadcasters pointed out that most of the clear-channel stations were licensed to serve large cities on the two coasts, which made little sense for a service that was meant to provide radio to the vast rural areas in the middle of the country. The clear-channel licensees requested that the power limit on the "I-A" channels in the U.S., set at 50 kW by the FCC, be lifted entirely. They pointed to WLW's successful experiments before the war, and in later years successful implementation by state broadcasters in Europe and the Middle East, as evidence that this would work and improve the service received by most Americans. Other broadcasters, particularly in the western states, argued to the contrary—that if the special status of the clear-channel stations were eliminated, they would be able to build facilities to provide local service to those rural "dark areas".
One of the most outspoken of the small-town broadcasters, Ed Craney of KGIR in Butte, Montana, went so far as to apply to move his station, then on the 1370 kHz regional channel, to a class I-A signal on 660 kHz, asking the FCC to downgrade the NBC New York flagship, WEAF, to make way for the Butte station. The FCC denied Craney's petition.
In 1941, several existing clear-channel stations applied for power increases to between 500 and 750 kW; dissemination of national defense information is cited as one reason this would be in the public interest. The FCC's engineering department presented a report on a complete reorganization of the clear-channel service in October, 1941; the report considered the possibility of "some 25 superpower stations of 500,000 watts or more, strategically located to provide maximum service" (as Broadcasting described it), and suggested that stations would have to be relocated away from the east and west coasts in such a scenario, as coastal stations waste energy over the oceans. One complication the FCC considered was the "Wheeler Resolution", passed by the United States Senate in 1938, expressing the view of the Senate that radio stations should be limited to a maximum power of 50 kW.
One station, KOB in Albuquerque, New Mexico, fought a long legal battle against the Federal Communications Commission (FCC) and New York's WABC for the right to move from a regional channel to a clear channel, 770 kHz, arguing that the New York signal was so weak in the mountain west that it served no one. KOB eventually won the argument in the late 1960s; it and several other western stations were allowed to move to eastern clear channels. (Western clear channels, like 680 in San Francisco, California, had been "duplicated" in the eastern states for many years.) These new class II-A assignments (in places like Boise, Idaho; Las Vegas and Reno, Nevada; Lexington, Nebraska; Casper, Wyoming; Kalispell, Montana; and others) began what would later be called "the breakdown of the clear channels". The class I-A station owners' proposal to increase power fifteenfold was not immediately quashed, but the new II-A stations would make it effectively impossible for stations on the duplicated channels to do so, and the owners eventually lost interest. That proposal was finally taken off the FCC's docket in the late 1970s.
On May 29, 1980, the FCC voted to limit the protection for the twenty-five clear channel stations to a 750-mile radius around the transmitter. Those stations outside the area of protection were no longer required to sign off or power down after sundown.
In 1987 the FCC changed its rules to prohibit applications for new "class-D" stations. (Class-D stations have night power between zero and 250 watts, and frequently operate on clear channels.) However, any existing station could voluntarily relinquish nighttime authority, thereby becoming a class-D, and several have done so since the rule change.
Daytimers (also known as daytime-only stations) are AM radio stations that are limited to broadcasting during the daytime only, as their signals would interfere with clear channel and other radio stations at night, when solar radiation is reduced, and medium wave radio signals can propagate much farther. Such stations are usually supposed to do one of three things: sign off, reduce power (sometimes dramatically, to only a few watts), or switch to another (typically near-by) frequency, such as Leamington, Ontario's CHIR (710 day, 730 night, now CHYR 96.7 FM), or Detroit's WNZK (690 day, 680 night). Their broadcast class is Class D. A number of these stations also broadcast on the internet and have separate streams that air when the station's air signal is off the air.
As of 2013, daytimers only exist in the United States; the last Canadian daytime station, CKOT, signed off on February 17 of that year after converting to the FM band.
List of all clear-channel stations 
The following two tables show all of the class-A stations in North America. First is the Canada, Mexico, and contiguous United States table, for the former class I-A and class I-B stations. Second is the Alaska table, for the former class I-N stations.
|Callsign||City of license|
- 690 kHz at Montreal was originally assigned under NARBA to CBF, and was later reused by CINF; after CINF closed in January 2010, CKGM applied for and was granted the frequency, and moved from 990 kHz to 690 kHz in September 2012.
- 740 kHz was used by CBC Radio One's CBL in Toronto until 2000 when the station moved to 99.1 FM. CFZM, known at the time as CHWO, acquired 740 in 2001.
- 940 kHz at Montreal was originally assigned under NARBA to CBM, and was later reused by CINW; while now silent, it remains notified to the U.S. as a class-A allotment.
- 1070 kHz was formerly used by CBA, which moved to FM in April, 2008. Canada has not withdrawn the international notification for CBA.
- 1550 kHz was originally CBE (AM), which shut down the AM station after moving to 97.5 CBEW-FM in 2011. On November 1, 2012, CBEF, traditionally on 540 kHz, also started broadcasting on the same AM transmitter and frequency that CBE had used.
- KNZR is the only U.S. class-A station licensed to operate with less than 50 kilowatts full-time; KNZR is licensed for 25 kW during the day and 10 kW at night.
- 1580 kHz was originally used by CBJ in Chicoutimi, Quebec. After that station moved to FM in 1999, CHUC applied for and was granted 1580 kHz in Cobourg, Ontario with 10 kW, but chose instead to move to FM itself (despite being notified to the U.S. as an existing station on 1580). CKDO moved from 1350 to 1580 kHz on August 13, 2006 and became that day a class A station using 10 kW. U.S. FCC record is at 
List of former clear-channel stations 
These allocations have been downgraded rather than transferred to a different station.
|Callsign||City of license||class="unsortable" Fate|
|1190||WOWO||Fort Wayne, Indiana||Downgraded to class B in 1998 by reducing night power to 9.8 kilowatts with a three tower directional antenna; Inner City Broadcasting purchased WOWO so that their other station in New York City, WLIB, could remain on the air 24 hours a day. WOWO was later purchased by Pathfinder Communications, the current owners.|
|1510||KGA||Spokane, Washington||Downgraded to class B in 2011 to make room for co-channel sister station KSFN, Piedmont, California, reducing night power to 15 kW|
- Rural Radio Magazine, Vol. 1 No. 1, Clear Channel Group (November 1938), p. 2
- "KGIR, Butte, Requests 50 kw. On WEAF Clear Channel". Broadcasting and Broadcast Advertising (Washington, D.C.: Broadcasting Publications, Inc.) 21 (9): 16. September 1, 1941.
- "WHAS Superpower". Broadcasting and Broadcast Advertising (Washington, D.C.: Broadcasting Publications, Inc.) 21 (17): 52. October 27, 1941. "A half-dozen other applications ranging from 500,000 to 750,000 watts now are pending"
- "WSB Revives Its 500-kw. Application, Seventh Stations Seeking Superpower". Broadcasting and Broadcast Advertising 21 (18): 57. November 3, 1941. "Already pending before the Commission were the applications of WLW, Cincinnati, for 650,000 watts, WOAI, San Antonio, seeking 750,000 watts, KSL, Salt Lake City, for 500,000 watts, and WSM, Nashville asking 500,000-watt operation. Similarly the application of WHO, Des Moines, for an increase to 500,000 watts is reported to be about ready for filing."
- "Superpower, Clear Channels Slated for Early FCC Probe". Broadcasting and Broadcast Advertising 21 (16): 12. October 20, 1941.
- Facts on File 1980 Yearbook, p. 519
- FCC license BL-20100527AGH
See also 
- AM Station Classes - Clear, Regional, and Local Channels (FCC Website)
- Big USA, Mexico, and Canadian AM radio stations