The North American Regional Broadcasting Agreement (NARBA, ; ) refers to a series of international treaties that defined technical standards for AM band (mediumwave) radio stations. These agreements also addressed how frequency assignments were distributed among the signatories, with a special emphasis on high-powered clear channel allocations.
The initial NARBA bandplan, also known as the "Havana Treaty", was signed by the United States, Canada, Mexico, Cuba, the Dominican Republic, and Haiti on December 13, 1937, and took effect March 29, 1941. A series of modifications and adjustments followed, also under the NARBA name. NARBA's provisions were largely supplanted in 1983, with the adoption of the Regional Agreement for the Medium Frequency Broadcasting Service in Region 2 (Rio Agreement), which covered the entire Western hemisphere. However, the AM band assignments in North America largely reflect the standards first established by the NARBA agreements.
Background
Organized AM (mediumwave) radio broadcasting began in the early 1920s,FM band stations did not start to appear until the early 1940s. and the United States soon dominated the North American airwaves, with more than 500 stations by the end of 1922. Due to a change in the ionosphere after the sun sets, nighttime signals from AM band stations are reflected for distances extending for hundreds of kilometers. This is valuable in providing radio programming to sparsely settled areas using high-powered transmitters. However, it also leads to the need for international cooperation in station assignments, to avoid mutually interfering signals.
In an effort to rationalize assignments, a major reallocation went into force in the U.S. on November 11, 1928, following the standards set by the Federal Radio Commission's (FRC) General Order 40. At that time, the AM band was defined as 96 frequencies, running in 10 kilocycle-per-second (kHz) steps from 550 to 1500 kHz, which were divided into what became known as "Local", "Regional", and "Clear Channel" frequencies. The only provision the FRC made addressing international concerns was that six frequencies — 690, 730, 840, 910, 960, and 1030 — were designated for exclusive Canadian use. On May 5, 1932, through an exchange of letters, the U.S. and Canada informally endorsed and expanded the 1928 standards, including recognition of Canadian use of 540 kHz."The Havana Conference and the North American Regional Broadcasting Agreement" by Louis G. Caldwell, Variety Radio Directory, 1938–1939 edition, pages 548–553. During the 1930s, Canada also began using 1510 kHz, while in 1934, the U.S. authorized two experimental high-fidelity stations on each of 1530 and 1550 kHz."Four Get Permits On New Channels", Broadcasting, May 1, 1934, page 13."List of Broadcasting Stations: Additions to List", Radio Service Bulletin, May 15, 1934, page 7. By 1939, Cuban stations existed on frequencies as high as 1600 kHz."North American B. C. Stations by Frequencies", Radio Index, Midsummer 1939, page 75.
As other countries, especially Mexico and Cuba, developed their own radio broadcasting services, the need arose to standardize engineering practices, reduce interference, and more fairly distribute clear channel assignments. Moreover, the development of better frequency control, and especially directional antennas, made it possible for additional stations to operate on the same or close by frequencies without significantly increasing interference. A key objective for the United States was that, in exchange for receiving clear channel assignments, Mexico would eliminate the high-powered English-language "border blaster" stations that had been directing their programming toward the U.S. and causing significant interference to U.S. and Canadian stations.Border Radio by Gene Fowler and Bill Crawford, 2002, page 209. However, an initial international meeting held in Mexico City in the summer of 1933 failed, primarily due to a lack of agreement over how many clear channel frequencies would be assigned to Mexico.
1937 "Havana Treaty"
In 1937, a series of radio conferences, this time successful, was held in Havana, Cuba, and the initial NARBA agreement was signed on December 13, 1937, by representatives from the United States, Canada, Cuba, Mexico, the Dominican Republic, and Haiti."The Havana Conference and the North American Regional Broadcasting Agreement", Variety Radio Directory: '38*'39, pages 548–553. The most significant change was the formal addition of 10 broadcasting frequencies, from 1510 to 1600 kHz, with the 106 available frequencies divided into Clear Channel (59 frequencies), Regional (41) and Local (6) designations."Basic Provisions of North American Agreement", Broadcasting, December 15, 1937, page 9. The official lower limit remained at 550 kHz, as it was not possible to add stations at the bottom of the broadcast band due to the need to protect 500 kHz — a maritime international distress frequency — from interference. (Although operation on 540 kHz was not covered by the Agreement, unofficially it became an additional Canadian clear channel frequency.)"Agreement between the United States and Canada for the use of the frequency of 540 kilocycles" (Agreement between the United States of America and Canada regarding radio broadcasting: October 28 and December 10, 1938), United States Statutes at Large: 1939: Volume 53, pages 2043-2044.
Under the Agreement, most existing stations operating on 740 kHz or higher would have to change frequencies. Open frequencies were created throughout the band by "stretching out" the existing assignments, achieved by following a table which in most cases moved all the stations on a common frequency to a new, higher, dial position. This provided gaps of unassigned frequencies, most of which became clear channels allocated to Mexico and Canada. A majority of the frequency shifts were limited to between 10 and 30 kHz, which conserved the electrical height of a station's existing vertical radiator towers, an important factor for readjusting directional antenna parameters to accommodate the new frequency."How Broadcast Stations Will be Shifted", Radio Today, February 1941, page 17.
Individual stations were specified to be Class I, II, III, or IV, with the class determining the maximum power a station could use and its interference protection standards. In all of the participating countries, Class I and II stations were exclusively assigned to Clear Channel frequencies, while Class III was synonymous with a Regional frequency assignment. In the United States, Class IV stations were only assigned to Local frequencies, although in other countries they were assigned to both Local and Regional ones."Basic Provisions of the North American Pact" (section 5(b)), Broadcasting, December 15, 1937, page 72. A major change was the provision that some clear channels were allocated to be used simultaneously by two stations — those maintaining sole use of a frequency were classified as Class I-A, while stations sharing a clear channel were known as Class I-B."Protected Service Contours and Permissible Interference Signals for Broadcast Stations", Broadcasting, December 15, 1937, page 10. The Agreement assigned six Class I-A frequencies each to Mexico and Canada, and one to Cuba."Table II: Class I-A Stations", Broadcasting, December 15, 1937, page 11.
Reflecting the existence of improved radio design, the Agreement also reduced the "same market" minimum frequency separation from 50 to 40 kHz. (Mexico elected to further adopt a 30 kHz "same market" spacing, unless this was in conflict with an adjoining nation's "border zone" allocations.) This closer spacing was particularly important in the case of the two highest Local frequencies, 1420 and 1500 kHz, as stations on these frequencies were being moved to 1450 and 1490 kHz, a 40 kHz separation.
According to the Agreement's provisions, its implementation was to take place within one year after its adoption by the pact's four main signatories — the United States, Canada, Cuba and Mexico. Cuba was the first to ratify, on December 22, 1937, and was followed by the U.S. on June 15, 1938, and Canada on November 29, 1938. While waiting on Mexico, in 1939, the U.S. and Canada completed a frequency agreement based on the treaty standards. Mexico finally approved the treaty on December 29, 1939,"Mexico Ratifies Havana Treaty", Broadcasting, January 1, 1940, pages 12, 68. and work commenced on adopting its wide-ranging provisions.
March 29, 1941 implementation
An engineering conference, with representatives from the U.S., Canada, Cuba, the Dominican Republic, and Mexico, was held from January 14–30, 1941, in Washington, D.C., in order to coordinate the upcoming changes. With a few exceptions, the frequency shifts were scheduled to be implemented at 0800 Greenwich Mean Time (3 a.m. E.S.T.) on March 29, 1941, which was informally known as "moving day". (Philadelphia stations petitioned mayor Robert Lamberton to declare a "Radio Moving Day", but he refused on the grounds that "My experience has been that proclamations by the mayor mean just exactly nothing and I issue as few as I can.")"Mayor is Adamant" Broadcasting, March 24, 1941, page 48.
The frequency changes affected "about a thousand stations in 7 countries"."The Reallocation", Radio Index, May – June 1941, page 1. The following chart reviews the assignments before and after March 29, 1941, including information about individual U.S. and Canadian stations, and summarizes the most significant changes: