Before there were radio call letters, there were other identifiers that contributed to the development of call letters. Two of these were the signal flag codes used to identify ships at sea and the calls used by land telegraph stations. They are discussed below.
Pre-radio call letters
Signal flag codes
An international set of flags was used at sea to identify individual ships. Each ship would set up a group of four flags that identified it; each of the flags was designated by a letter, and no letter could be repeated because ships carried only one of each flag for signaling. In order to ensure that these were unique, the Bureau of Navigation, established by law in 1884, was charged with the assignment of signal letters to U. S. ships. These were published in books issued at intervals, called "Merchant Vessels of the United States."
Land telegraph calls
Every station and operator along a telegraph line was assigned a short "call" or "signal." These were generally one or two letters, which were generally self-designated; as each station only communicated with other stations operated by the same telegraph company, there were no requirements for any coordination to avoid conflicts between different stations.
Ship call letter assignments prior to 1912
As commercial license call letters were issued under international conventions that were first developed for ship calls, the history of these assignments is presented here first. All early radio work was done in Morse code, and spelling out an operator's full name or location would have been cumbersome, so the system of assigning call letters was adapted from land telegraphy, as described above. Radio transmitter calls were assigned following the landline telegraph practice. These were usually one-to-three-character identifiers which were frequently based on geographic location or personal or ship names. Thus, stations "calling" each other were able to link up with a minimum of sorting out identities. During this era there were few standards, and as call signs were self-assigned, this often resulted in problems when, for example, two or more ships chose the same call. It became clear that unique identifiers, organized by national origin, were needed in order to determine exactly which vessel was in danger when a distress call was being received.
In 1906, a convention was established in Berlin, Germany, which included a regulation specifying that coastal and ship stations were to have call letters "distinguishable from one another and each must be formed of a group of three letters." Evidence from callsign lists published at the time shows that many stations were slow to make the switch to three-letter calls. Initially, as well, there seems to have been very little in the way of procedure for insuring unique calls. Most stations in Germany had adopted 3-letter calls as early as 1905; Marconi-operated stations used station calls beginning with an M, mostly having converted by 1908. In the US, the Revenue Cutter Service (now the U. S. Coast Guard) adopted calls consisting of RC and a third letter, and on November 20, 1909, the U. S. Navy switched to three-letter calls starting with N from a wide range of two-letter calls. There were still however, as late as 1912, many ships using one- and two-letter calls, and a dearth of international coordination. (See this table, entitled Wireless Telegraph Stations of the World.)
The 1912 move to national prefixes
While the US had not initially signed onto the Berlin Convention, it formally ratified it on April 3, 1912. Eugene Tyler Chamberlain, head of the Bureau of Navigation, was concerned because call letters which had been adopted by the various ships did not conform to the convention, and often duplicated (he issued a report in 1911 which pointed out that "In some instances three letters of the alphabet are used, in others a letter and a number, and in others a number alone. Where two letters are used, in 6 cases the same two letters have been assigned to three vessels, and in 25 cases the same letters have been assigned to two vessels.") The Bureau determined that the 1884 act (mentioned earlier) which gave it the right to assign signal-flag codes to ships could be used as a basis to assign radio call signs as well. At some time between January 1, 1912 and June 30, 1912 new three-letter call signs were adopted. It was apparently expected that the United States would be given the right to assign call letters beginning with the letters K and W, so call letters beginning with K were used for Atlantic- and Gulf-based ships and those beginning with W were assigned to Pacific-based ships.
In 1912, shortly after this assignment of new call signs was made, the London International Radiotelegraphic Convention was signed. This allocated call signs to various countries (see this document, issued by the Bureau over Chamberlain's name), and while the US got all the letters N and W, it was given only the block of K signs beginning KD to the end. (In fact, it was not until 1929 that the KA-KC letters were assigned to the United States.) So signs beginning KA through KC had to be reassigned.
Also in 1912, a new Radio Act was passed, giving the Bureau of Navigation the right to license ship and land radio stations, and thus the right to control the call letters of land stations (which to that point had not been regulated). It initially intended to follow the same plan as for ships for call letters (K prefixes in the east, W in the west), according to the above-mentioned publication of the Bureau, but the actual assignments were the reverse. In addition, N calls were reserved for government stations, and those beginning WU, WV, WX, WY, and WZ for the Army, so only KD to KZ, WA to WT, and WW calls were available.
In many cases, shore stations which had two-letter calls simply had a K or W prefixed, while those with three-letter calls had the first letter changed to K or W as appropriate. The initial list is given in this document.
Amateur and experimental calls
Whether because of the terms of the treaty, or because of the way it was read by the authorities in the United States, it was felt that Amateur and Special Land stations did not qualify for "international" calls, and the International Bureau of the Telegraph Union at Berne was not notified of their existence, nor were these stations felt to require the national K/W prefix. As a result, an entirely different system was used to assign amateur and experimental station calls. These stations were assigned a one-digit and two-letter call. The first digit was used to indicate a region of the country (See the sections labeled "Amateur Stations" and "Special Classes of Stations" in this document). For these, it was contemplated that there might not be enough calls available using one digit and two letters, so authorization was given to add a third letter. (Note that the document did not, however, contemplate assigning four-letter calls to ships and shore stations.)
In this Wiki, only those stations in these categories are shown which ultimately developed into standard broadcasting stations. (See this index.)
According to the June 30, 1928 Radio Service Bulletin, the requirement for amateur and experimental stations to carry national letter prefixes became effective on January 1, 1929. The United States Government decided to add W (and K, in the territories) prefixes to amateur calls effective October 1, 1928, preceding the start of the international requirement, so that this could be acknowledged in publications prior to 1929.
The early years under the agreements
Initially after the 1906 agreements on call letter assignments, three-letter calls were issued to ships and shore stations that communicated with them. Because of World War I, with German submarines sinking US vessels, it became more necessary for ships to have radio stations to call for help. As a result, the supply of three-letter calls became quickly exhausted, and the U. S. Government began issuing four-letter calls. Also, because of the Panama Canal connecting the two oceans, ships no longer could easily be divided into East Coast and West Coast fleets. The four letter calls were all issued with K prefixes, and it was decided that the first letter after the K should be a vowel, so KE, KI, KO, and KU calls were issued to ships, generally in alphabetical sequence. Land stations, less numerous, continued to be given three-letter calls, and the number of available three-letter calls was augmented by two groups of calls formerly assigned to ships: because of superstitions, calls of ships that had sunk were not re-issued; in addition, ships which were sold to another country had their call retired.
The 1920 policy change
Along about June 1920, the last of the KU four-letter calls available for ships was used up. Since KA through KC were still assigned to Germany, the first available four-letter calls of the form K+consonant+two more letters, alpabetically, were KD calls. About this time, the Bureau of Navigation decided to cease making a distinction between land and sea station call letters, and began to assign KD calls to all new stations. On October 27, 1920, Westinghouse established a new station in East Pittsburgh, Pennsylvania. The last call assigned was KDJZ, (to a ship, Montgomery City), so the next call in alphabetical sequence was KDKA. Soon afterward, in April, 1921, policy was changed again, and three-letter calls were resumed (W in the east and K in the west), so that Westinghouse's next station, for example, was assigned the call letters WJZ. During that period from June 1920 to April 1921, two other stations, KDPM, Cleveland, Ohio, and KDPT, San Diego, California (originally non-broadcasting service stations but later transferred to the broadcasting service) were licensed, but they did not survive. It is only because of this policy change that was effective for less than a year that KDKA got its unusual call; it would have received a three-letter W call if its license had been granted a few months earlier or later.
As three-letter calls ran out, four-letter calls were used; however, three-letter calls retired from ships were still issued as they became available. The population distribution of the United States meant that the eastern part of the country, receiving W calls, ran out of three-letter combinations first. The US Government established a policy that three letters in a row could not be used in a call letter assignment, so the first four-letter broadcast station assignment (other than KDKA, KDPM, and KDPT) was WAAB, assigned to a station whose license application was submitted by the New Orleans Times-Picayune on April 4, 1922. In 1922, predominantly four-letter calls were issued. A peculiar sequence was used, in which all the call letters with A as third letter were issued before B in the third letter was assigned, rather than normal alphabetical order: after WAAZ would come WBAA, after WBAZ would be WCAA, and WABA would not be issued until WZAZ had been reached, in April 1923. For some reason, in mid-1928 a jump was made from W-B- calls to WHDH, skipping the rest of the W-B- sequence, all the W-C- group, and the beginning of the W-D- group; from that point, the W-E- and W-F- calls were issued. In the West, however, a normal alphabetical sequence was followed; since all the KE calls had been issued, after the KD calls would follow KF and then KG. (KH calls were reserved in 1927 for a new service category, so KG calls were followed by KI.) The first four-letter call issued in the West under this plan was KDYL in Salt Lake City, Utah, which was also the longest-surviving of the stations of this era. (The station changed its call letters to KCPX on December 21, 1959, by which time no other KD-- call in the West remained.) From 1923 on, three-letter calls were issued by request if they were available from retired ships. Also, four-letter calls could be requested out of their alphabetical sequence if they were not already in use. The last new three-letter call issued was either KOH, issued to Jay Peters on September 13, 1928 or WIS in Columbia, South Carolina on January 23, 1930, depending on the source. From that point on, three-letter calls were only issued to stations that had previously used that same call (see WHN.)
FM and TV calls
Early FM and TV calls
Originally FM and television stations were considered experimental, and stations in those services had experimental calls, the first letter being W or K, then a digit denoting the geographic area, then an X and two more letters.
For FM stations, a new call-letter plan was adopted in 1941 that was intended to be systematic; first a K or W, then two digits representing the frequency, then one or two letters representing the location. Frequencies at the time were in the range of 43 to 49 MHz, and the two-digit frequency code was obtained by dropping the "4" and the decimal point; thus W81SP was on 48.1 MHz in Springfield, Massachusetts. (See this list of K calls and this list of W calls.) Experimental-type call letters were only gradually phased out; while W71NY was operating by July 1, 1941, W2XMN continued into 1949 (even after the frequency-based call letters had been eliminated and the stations reassigned to the new higher-frequency band) and KE2XCC continued well into the 1950s.
The 1941 systematic (frequency and city-based) call letter scheme lasted only until November 1, 1943, and at that time the call letters were normalized, except (as stated in the previous paragraph) for some experimental-type calls.
FM and TV calls, after normalization
Originally, the FCC required FM and TV stations to have unique call letters, so the FM station owned by WHN was WHNF and the TV station owned by WCBS was WCBW. After a few years, the FCC allowed the stations to use five- or six-letter calls consisting of the three- or four-letter call of the affiliated AM station followed by an -FM or -TV suffix. Originally all stations with -FM or -TV suffixes were affiliates of AM stations, but when WJZ in New York City became WABC in 1953, Westinghouse, which had originally started WJZ, requested the right to restore those call letters, and eventually, after over five years, was granted the right to use WJZ-TV for its Baltimore station on channel 13. This became the first case of a station with a -TV suffix not affiliated with an AM station. (However, in 2008, the former Baltimore station WJFK became WJZ(AM).) A year later the first station with an -FM suffix not affiliated with an AM station was created by the University of Texas when they were granted permission for a new educational FM station in Austin, Texas. As the University had operated KUT in 1925-1932, they requested, and were permitted to use, the call letters KUT-FM.
In 1987 a change in the rules was made, and stations with -FM and -TV suffixes no longer have to be under the same ownership or in the same market as AM stations with the corresponding call. (Within each service, they still need to be unique.) This has led to stations with -FM and -TV suffixes unrelated to AM stations with the same call. (See, for example, WBIG and WBIG-FM.)
FM translator calls
Recently, a new category of FM stations, rebroadcasting local AM stations in areas where reception is poor, known as FM translator stations, has been established. For these, a new type of call sign was developed, resembling the frequency-based call signs of the 1940s. The call consists of a K or W, a three-digit numeral which starts with 201 for 88.1 MHz and goes up to 300 for 107.9 MHz, and two more letters; however these seem to be assigned chronologically; as of May 2016, only AA through FJ seem to be in use. It should be noted that the frequency itself does not appear but is coded, so if the frequency is 88.1 + x, the numeric appearing in the call sign is 201 + 5x.
Individual call letter chronologies can be reached from the following links: