New full-power stations were formerly assigned sequential call signs if the permittee does not choose one of their own; these were always four letters, of which the third was the least-significant digit and the second was the most-significant digit of the sequence number. (Callsigns which were already assigned are skipped in the sequence.) Hence, many very early stations, like WMAQ Chicago (now WSCR) and WMAF Round Hill, South Dartmouth, Massachusetts (now defunct) were assigned W-A- or K-A-) call signs. The current FCC rules require a permittee to explicitly select a callsign before putting a station on the air for the first time. Prior to that time, permits for new stations are either listed simply as NEW, or referenced by the file number of the original application, in the FCC's public records.
The FCC allows FM and TV stations under common ownership with a three-letter AM or FM in the same market to use five-letter (three plus –FM or –TV suffix) call signs; for example, KGO-TV in San Francisco or WMC-FM in Memphis. In some cases, such as WIL-FM in St. Louis, the five-letter callsign may outlive the three-letter call sign on which it is based. There is also the unusual case of Baltimore's WJZ-TV, which was allowed to adopt the call sign despite the fact that there was no longer a WJZ radio; when there was, it wasn't in Baltimore; and it hadn't been owned by the same company since the 1920s. Stations which have been "conformed" in this manner may keep the five-letter call sign even after they are no longer co-owned with the "parent" station (although this was not the case prior to the mid-1980s). WWL (AM) and WWL-TV in New Orleans would be an example of eponymous stations no longer under common ownership.
Extremely early call signs used in the 1910s and into the early 1920s were arbitrary. The U.S. government began requiring stations to use three-letter call signs around 1912, but they could be chosen at random. KDKA initially broadcast as 8XK before gaining its well-known letters in 1920. The Rosicrucian Order, AMORC of San Jose, California used the call sign 6KZ.
The assignment of K and W prefixes applies only to stations in the broadcast radio and television services; it does not apply to weather radio, highway advisory radio, or time signal stations, even though these are all broadcasts in the usual sense of the word, nor does it apply to auxiliary licenses held by broadcast stations, such as studio-transmitter links and inter-city relay stations.
For example, the time signal stations WWV and WWVH are located in Colorado and Hawaii, respectively. (WWV originally began in Maryland and was later moved west. However, even ignoring that fact, U.S. government-owned stations are overseen by the NTIA and not the FCC, and are thus not subject to the FCC's rules on call signs; most do not have call signs at all.)
NOAA Weather Radio stations clustered between 162.4 and 162.55 MHz have call signs consisting of a K or W followed by two or three letters, and two digits. The K and W prefixes are both used interchangeably on both sides of the Mississippi River (e.g., KHB36 in Washington, D.C. and WXK25 in El Paso, Texas).
Highway advisory radio stations scattered throughout the AM band use call signs consisting of K and W followed by two or three letters and three digits. As with weather radio, K and W calls are both used on both sides of the Mississippi River.
Call signs in the western United States are often confused with International Civil Aviation Organization (ICAO) airport codes because both make use of four-character codes that begin with the letter K. Examples include KSFO (which simultaneously refers to San Francisco International Airport and KSFO (AM) radio), KLAX (which simultaneously refers to Los Angeles International Airport and KLAX-FM), and KDFW (which simultaneously refers to Dallas-Fort Worth International Airport and KDFW-TV).
It should be noted that the -FM or -TV suffix is not required to be assigned to TV or FM stations, except where there is another station that shares the same 3- or 4-letter base call sign. AM radio stations never have an -AM suffix.
Low-power TV and FM stations share the –LP suffix. Class A TV stations, which are LPTV stations that receive protection from interference by primary stations, use the –CA suffix. When low-power and class-A TV stations operate in ATSC digital, they instead receive the suffixes –LD and –CD, respectively.
It is fairly common for stations to choose a call sign that can be transformed into a name, such as Boston's WXKS-FM (107.9 Medford), one of many Clear Channel Communications-owned stations that call themselves "KISS." In other instances, the letters may be an initialism for a name or slogan. Some of the most famous of these include WGN (WGN and WGN-TV), owned by the Chicago Tribune, which stands for "World's Greatest Newspaper", WIS in South Carolina, which stands for "Wonderful Iodine State," and WISN, which dually stands for the station's original owner, the Wisconsin News, and the station's location in Wisconsin. Stations operated by schools and universities may adopt their school's "initials" into the call sign, such as WWVU in Morgantown, WV, the university-owned radio station of West Virginia University.
The number in the call refers to one of the 10 radio districts into which the U.S. is divided, but that only indicates where the license was issued. It is no longer necessary for a U.S. ham to change callsigns when moving to a new district. Most amateurs going to an exotic location will sign/(prefix) to show their location. Thus a station visiting American Samoa could be (regular call)/KH8. American amateurs are also permitted to operate in Canada under their own call signs with a location indicator.
Amateur stations are required to identify themselves by their call sign once every ten minutes during a transmission and at the end of the transmission.
Experimental stations use call signs out of the amateur radio sequence, with the letter following the region digit required to be an X. (All VHF stations before World War II were licensed as experimental stations.) Notable experimental stations included Major Armstrong's FM station W2XMN in Alpine, New Jersey; Powell Crosley's 500-kW superpower AM W8XO, operating nights only with WLW's programming and frequency from Mason, Ohio; and Don Lee's pioneering television station, W6XAO in Los Angeles. (Synchronous "booster" transmitters for AM stations are still considered experimental in the U.S., despite fifty years of experience in Europe, and new experimental call signs are being assigned for new licenses even now, by inserting a region digit and the letter X into the parent station's call sign.)
Special broadcast undertakings such as Internet radio, cable FM or closed circuit stations may sometimes be known by unofficial call signs such as "CSCR". These are not governed by the Canadian media regulation system, and may at times reflect call signs that would not be permissible on a conventional broadcast platform.
Four-letter call signs are the norm. Three-letter call signs are only permitted to CBC Radio stations or to commercial stations which already had a three-letter call sign before the current rules were adopted, and five-letter call signs exclusively identify CBC transmitters (which may be either rebroadcasters or SRC O&Os outside of Quebec.)
Stations of the Canadian Broadcasting Corporation or Société Radio-Canada tend to identify themselves as "CBC Radio One/Two" (English-language) or "La Première Chaîne/Espace Musique" (French-language) of a city, although they do have official three- and four- letter call signs. These generally (but not always) begin with CB.
Callsigns with four digits preceded by VF (for radio) or CH (for television) are only assigned to very-low-power local rebroadcasters; VO callsigns may only be used commercially by stations in Newfoundland and Labrador which were licensed before that province joined Canadian Confederation in 1949 (VOCM, VOAR and VOWR broadcast from St. John's long before Confederation). Only one station, VOCM-FM, has been allowed to adopt a VO callsign after 1949; it was granted the VOCM calls because of its corporate association with the AM station.
All Canadian FM stations have an –FM suffix, except for low-power rebroadcasters which have seminumeric VF callsigns. Higher-power rebroadcasters are generally licensed under the callsign of the originating station, followed by a numeric suffix and, for FM rebroadcasters of an AM station, a –FM suffix; for example, CJBC-1-FM rebroadcasts CJBC (860 Toronto), whereas CJBC-FM-1 rebroadcasts CJBC-FM (90.3 Toronto). Some rebroadcasters, however, may have their own distinct callsigns. Canadian TV stations always have the -TV suffix, with the exception of those CBC-owned stations which have a call sign in the CB-(-)T format.
For rebroadcasters which use a numeric suffix, the suffixes usually follow a 1–2–3 numeric sequence which indicates the chronological order in which rebroadcast transmitters were added. There are some cases where television rebroadcasters are suffixed with the channel number on which the transmitter broadcasts (for instance, CIII-TV's rebroadcasters are numbered with their channel assignment rather than sequentially), but this is not generally the norm.
Experimental television stations in Canada have callsigns beginning with VX9.
Canadian broadcast stations are required to identify by callsign hourly, but not at any specific time, and this rule is even more rarely enforced than the U.S. rule (see above).
Canadian amateur radio stations generally begin with VE, although some use VA. The number following these letters indicates the province, going from VA1/VE1 for Nova Scotia, VA2/VE2 (Québec), VE3/VA3 (Ontario) through VA7/VE7 for British Columbia and VE8 for the Northwest Territories, with latecomer VE9 for New Brunswick. (VE1 used to be for all three Maritime Provinces.) VE0 is for maritime mobile amateur transmissions. VY1 is used for the Yukon Territory, VY2 for Prince Edward Island, and VY0 for Nunavut. CY0 is used for Sable Island and CY9 for St. Paul Island. Special prefixes are often issued for stations operating at significant events.
The Dominion of Newfoundland prefix VO remains in active use by amateurs in the province of Newfoundland and Labrador, VO1AA atop Signal Hill in St. Johns being the most famous amateur station. Radio amateurs on the Island of Newfoundland use calls beginning with VO1, while Labrador amateurs use VO2. A popular backronym for VO stations is "Voice Of...", although this is not the VO prefix's official meaning.
Amateur radio stations in Mexico use XE1 for the central region, XE2 for the northern region, and XE3 for the southern region. XF prefixes indicate islands. Special callsigns for contests or celebrations are occasionally issued.
All of the former British West Indies colonies share the VS, ZB–ZJ, ZN–ZO, and ZQ prefixes. The list is as shown: