Call signs in the United States

20th Century

The earliest identification, used in the 1910s and into the early 1920s, was arbitrary. The U.S. government began requiring stations to use three-letter call signs around 1912, but they could be chosen at random. This system was replaced by the basic form of the current system in the early 1920s. Examples of pre-1920 stations include 8XK in Pittsburgh, Pennsylvania, which became KDKA in November 1920, and Charles Herrold's series of identifiers from 1909 in San Jose, California: first "This is the Herrold Station" or "San Jose calling", then the call signs FN, SJN, 6XF, and 6XE, then, with the advent of modern call signs, KQW in December 1921, and eventually KCBS from 1949 onward. (KDKA remains the call sign for the Pittsburgh station mentioned here, even though it is hundreds of miles east of the Mississippi River.)

For most of the 20th century, new full-power stations were also assigned their four-letter call signs sequentially by the Federal Communications Commission (FCC) if the permit holder did not choose a call sign prior to full licensing. For four-letter call signs beginning with W, sequential call signs were incremented with the fourth letter being least significant and changing each time, the second letter being the next most significant, and the third letter being the most significant and changing only after every combination of second and fourth letter was exhausted. Thus, the first sequential four-letter call signs in the W sequence were of the "W_A_" form, such as (the now defunct) WMAF in South Dartmouth, Massachusetts followed soon after by WMAQ in Chicago (now WSCR). The "K" sequence was also sequential, but with a more complicated history concerning the second letter; the third and fourth letters, however, were incremented in the most obvious way, with the fourth letter least significant, then the third letter, then the second letter. For both K and W, any call signs which were already assigned were simply skipped over in the sequential system.

21st Century

The FCC has since begun requiring permit holders to choose the call sign prior to licensing; stations not yet given a call sign show up in FCC electronic records with the word "NEW" or the file number of the original station application instead. Stations may change their call signs whenever they wish to, and often they do so in connection with a change of radio format. Call signs become available again after 30 days of non-use, although stations (frequently under common ownership) can swap call signs at the same time.

Records for officially deleted stations still remain in the electronic FCC database for some time, but with the last call sign prefixed with a "D", which is not allocated for valid station call signs in the United States; for example, a station whose last call sign was KXXX before deletion will appear in the database as DKXXX so as not to conflict with any station that may be assigned KXXX in the future. Call signs beginning with "D" are not in use in the United States and are unlikely to ever be assigned in the United States.

Short call signs

In the 1920s, many stations were assigned three-letter call signs. These have been grandfathered under the current system, even though many of these stations have changed owners. Such stations include WWJ (AM) and WJR (AM) in Detroit, KOA in Denver, WGN in Chicago, and WRR (FM) in Dallas, which was originally assigned WRR-FM in 1948 as a sister station to WRR (AM) from 1921. (WRR is an unusual case in that the call sign was moved from the original AM station to a commonly owned FM station, formerly WRR-FM, before the AM station was sold.) For decades, the Federal Communications Commission carried out a policy of "drop it and lose it forever" with respect to the three-letter call signs, but it allowed the radio station KKHJ in Los Angeles to reclaim its historic three-letter call, KHJ.

The FCC allows FM radio and television 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 call sign may outlive the three-letter call sign on which it is based. There was also the unusual case of Baltimore's WJZ-TV, which was allowed to adopt this call sign despite the fact that no form of the WJZ call sign had been in use for over four years prior, and when WJZ did exist, it had been in a different region and owned by a different company since the 1920s. The call signs WJZ and WJZ-FM were later reused for Baltimore sister stations of that new WJZ-TV.

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.

K and W

New broadcast stations are assigned call signs beginning with "K" if their city of license is located west of the Mississippi River, and beginning with "W" if they are licensed east of the river. No broadcast stations are assigned call signs beginning with "N" or "AA"–"AL". Again, some early stations have been grandfathered, so there are four broadcasters with a "K" prefix east of the Mississippi, and a few dozen with a "W" on the west side. (There are more grandfathered "W" stations because the dividing line used to be two states farther west.) Some examples of stations with a now-unusual first letter are KDKA in Pittsburgh, Pennsylvania, KYW in Philadelphia, WBAP in Fort Worth, Texas, and WACO-FM in Waco, Texas, which also has the distinction of being one of only three radio stations whose call sign is the same as its community of license. Stations located near the Mississippi River or that straddle it such as Minneapolis-St. Paul, as well as some in northern Minnesota and southern Louisiana, may have either letter, depending on the precise location of their community of license and on historical contingencies. Metropolitan areas that straddle different states on both sides of the river, such as St. Louis, Memphis, and the Quad Cities area encompassing Iowa and Illinois, have stations with both call letter prefixes, because of the stations' communities of license are located on either side of the river.

The FCC allows derived call signs in the same market as a commonly owned AM or FM without respect of the boundary, so stations may establish common branding across bands and services. One famous example was the case of the former KWK in St. Louis, which after several petitions was permitted to change the call sign of its sister FM station in Granite City, Illinois, then WWWK (FM), to KWK-FM. Later, the AM station would change its call sign and the FM station became KWK (FM), thereby becoming an exemplar of both categories of grandfathered stations.

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 respectively located in Colorado and Hawaii (WWV originally began in Maryland and was later moved west; however, even ignoring that fact, U.S. government-owned stations are overseen by the National Telecommunications and Information Administration (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 letters, and two digits. The "K" and "W" prefixes are both used interchangeably on both sides of the Mississippi River (for example, 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 radio station KSFO), KLAX (which simultaneously refers to Los Angeles International Airport and KLAX-FM), and KDFW (which simultaneously refers to Dallas-Fort Worth International Airport and television station KDFW).

Suffixes

FM radio and television call signs may be followed by a dash and the two-letter class of station: "-FM", "-LP", "-TV", or "-CA". For digital television, since June 2009, stations have the latitude to choose a "-TV" or "-DT" suffix for their calls, but as of 2014, very few stations have chosen to use "-DT" call signs. Occasionally, an FM or television station may have one or more boosters, which retransmit the main station's signal to overcome terrain obstacles. In this case, the main portion of the call sign remains the same (unlike translators), and the boosters are given sequential numeric suffixes like "–FM1".

It should be noted that the "-FM" or "-TV" suffix is not required to be assigned to television or FM radio stations, except where there is another station that shares the same three- or four-letter base call sign without an accompanying suffix. AM radio stations never have an "-AM" or any other suffix. If a station has no suffix, the FCC uses parentheses to identify the station unambiguously in documentation (such as in rulemaking proceedings), in the same manner that Wikipedia handles disambiguation of article names (except that there is no space between the two). This ensures that, for example, WIKI(AM) is not mistaken for WIKI-FM and WIKI-TV just because it was identified as only WIKI. This occurs regardless of whether there is actually another station using the call sign.

Low-power television and FM radio stations share the "–LP" suffix. Class A television stations, which are LPTV stations that receive protection from RF interference by primary stations, use the "–CA" suffix. Low-power television stations operating an ATSC digital signal, they instead receive the suffix "-LD", although digital television stations which have their RF channel numbers and sequential letters use only "–D", as in W08EG-D. The "-CD" suffix is assigned for digital class-A stations, but a small number of such stations have "-D", "-LD", or "-CA" call signs.

The first suffix to be introduced was "-FM", on November 1, 1943, when the call signs of all 45 then-licensed FM stations were changed. Call signs under the old system consisted of W or K followed by the last two digits of the frequency (from 31, for 43.1 MHz, to 99, for 49.9 MHz) and one or two letters indicating the city served. No provision had been made for conflicting city codes (for example, whether an "M" could stand for Milwaukee or Minneapolis, or both), severely limiting the application of this system in the anticipated post-war boom of FM station construction. Public confusion was cited as another reason to eliminate the old system. At this time, FM stations had to be licensed to the same organization and serve the same city as the station with the unsuffixed call sign in order to be eligible for an "-FM" suffix. Non-commercial educational radio stations such as WNYE and WBEZ, in the 41–43 MHz band, already used regular four-letter call signs. There was no "-TV" suffix as yet, so the few commercial television stations as then existed had to use distinct call signs from the radio stations they were associated with (for example, WRGB rather than WGY-TV).

Translators

Translator stations on FM radio and television are assigned sequential call signs. These stations use an appropriate initial letter followed by a two- or three-digit channel number, and then a two-letter sequential suffix. For example, a translator on VHF channel 4 might have the call sign K04AX. Digital translator stations are assigned call signs in the same manner, except that the letter "-D" may be appended (e.g., K04AX or K04AX-D). The FM band also has channel numbers starting with 87.9 MHz as channel 200, although they are almost unknown to regular listeners who usually tune into a station based on its frequency. W201AA was the first FM translator at 88.1 MHz in the east, for example. Such call signs are never reused by another station, though it is unclear if this could occur in the future due to exhaustion of the 676 (26²) two-letter combinations. As of 2014, channels 7, 9, and 13 in the Western United States (where the mountainous terrain makes translators a necessity) are up to K07Zx, K09Zx, and K13Zx, respectively.

The FCC makes no differentiation between translating and originating LPTV stations, thus either type of station may have an alphanumeric or a regular "-LP" or "-LD" call sign.

In 2009, the FCC began allowing digital television stations to apply for "digital replacement" translator stations, which are not given a separate call sign, instead taking on the one of the primary station. This is only in the case of areas that would lose coverage due to the digital television transition.

Station identification

Many stations prefer not to use call signs at all, since a moniker or slogan is more easily remembered by listeners (and those filling in diaries for the Nielsen Audio radio ratings). However, in the United States, the Federal Communications Commission does require periodic station identification using the formal call sign, as close to the beginning of each hour as possible, at a "natural break in programming". However, this rule is now rarely enforced. Stations are also required to identify their community of license. Only the frequency, name of the licensee, channel number, and/or network affiliation may come between the two.

HD Radio stations must identify clearly that they are using a digital transmission mode, and must identify each program stream individually, but need not do so in any particular form; many licensees have chosen to identify as "WXXX HD2" and so on but this is not part of their call sign. Translator stations only need to have their call signs announced three times a day (at particular times) through the main station, or though some automated means (by voice or Morse code) hourly by the translator.

There are some unusual cases, such as the low-frequency WWVB time station. Because of the station's very narrow-bandwidth signal, that station only broadcasts a one bit per second signal that cannot usually be understood by human beings, so the station is identified by shifting the broadcast carrier wave's phase by 45 degrees twice an hour [see Phase-shift keying (PSK)].

A common method of station identification by radio is along the lines of the call sign, the city of license (and/or market/area it serves), the frequency, and the ownership (for example: "KQKS Lakewood/Denver: A Lincoln Financial Group Station. This is KS1075").

Because television is a visual medium, television stations instead usually display a small graphic or text (a D.O.G. or "bug") at the bottom of the screen listing their call sign, community of license, and so forth. Many such stations display their station identification non-intrusively in small type during short promotions, either for an upcoming show or their next local newscast (even incorporating these identifications at the start of newscasts), that air just before the top of each hour.

Call signs as names or initials

It is fairly common for stations to choose a call sign that can be transformed into a name, such as Boston, Massachusetts'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 (AM) and WGN-TV), owned by the Tribune Company, parent of the Chicago Tribune, which stands for the latter's longtime slogan "World's Greatest Newspaper"; WIS in Columbia, South Carolina, which stands for "Wonderful Iodine State"; and WISN in Milwaukee, which stands for both the station's original owner, the Wisconsin News, and the station's location in Wisconsin.

The owned-and-operated stations of the Big Three television networks (ABC, CBS and NBC) in New York City and Los Angeles get their call signs from their network, with New York City stations adding the "W" and Los Angeles (or, in the case of KCBS, San Francisco) stations adding the "K". Stations operated by schools and universities may adopt their school's initials into the call sign, such as WWVU in Morgantown, West Virginia, the university-owned radio station of West Virginia University. West of the Mississippi, this can be seen with KKSU in Manhattan, Kansas, the former AM station and current television station of Kansas State University.

Call signs as numbers

It is also common for television stations to choose call letters that either directly or indirectly reference the station or channel number upon which they broadcast. Some examples of this are: in Winston–Salem, North Carolina, the call sign WXII was chosen to represent the Roman numeral for 12 (same thing as KXII in Sherman, Texas), the channel on which the station broadcasts; the former WIIC (now WPXI) in Pittsburgh was chosen to accent the fact that the station broadcast on VHF channel 11; Philadelphia's WPVI was chosen to remind viewers that the station broadcast on VHF channel 6. Roman numerals can precede another letter suggesting a city, as with WIVB for channel 4 in Buffalo, New York. WTWO in Terre Haute, Indiana references its position on channel 2 (as does KTWO out of Casper, Wyoming). Phonetic spellings such as KFOR-TV (representing its channel 4 allocation) in Oklahoma City, and near-misses such as WSYX in Columbus, Ohio (representing its channel 6 allocation) also fit the pattern.

This is not always reliable; until 1973, WSIX-TV (now WKRN) in Nashville, Tennessee had broadcast on Channel 2. (WSIX-TV, however, received its call sign through sister radio station WSIX, and not in reference to a channel number.)

Experimental and non-broadcast stations

United States amateur radio call signs are issued with one or two letters, followed by a single digit, and then one to three more letters. Generally the shorter the call (up to a 1x2 or 2x1 format) the higher the grade of license, but an amateur who upgrades is not required to change his or her call sign. In any case some of the available blocks have been used up. The 1x1 call signs, such as K6O, are for short-term special event stations. Outlying areas have special calls. For example, those issued in Hawaii can (like other American call signs) start with "A", "K", "N", or "W", but then will have "H6" or "H7" before the one to three additional letters. Other Pacific possessions use other "H" numbers. For example, a station on Guam could be "KH0–". Stations in Alaska have "L" as their second prefix letter, and stations in the Caribbean region (such as the Virgin Islands) use "P" for their second letter.

The number in the call sign refers to one of the ten radio districts into which the United States is divided, but that only indicates where the license was issued. It is no longer necessary for an American amateur radio operator to change his/her call sign 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 communication and at the end of the communication.

Experimental stations use call signs out of the amateur radio sequence, with the letter following the region digit required to be an "X". (All television and most FM stations that were operational 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, California. (Synchronous "booster" transmitters for AM stations are still considered experimental in the U.S., 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.)

Leisure craft boats with VHF radios may not be assigned call signs, in which case the name of the vessel is used instead. Ships wishing to have a radio license anyway are under FCC class SA: "Ship recreational or voluntarily equipped." Those calls follow the land mobile format of the initial letter K or W followed by one or two letters, followed by three or four numbers (such as KX0983 or WXX0029).

U.S. territories

Puerto Rico, Navassa Island, and the U.S. Virgin Islands all use the American standard call signs of "W" (being east of the Mississippi River). Guam and the Northern Mariana Islands use "K". American Samoa uses "K" as well, but WVUV was grandfathered in. All of these areas are under the jurisdiction of the U.S. Federal Communications Commission (FCC).