NARBA

The North American Regional Broadcasting Agreement, commonly abbreviated as 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, current AM band assignments in North America largely reflect the standards first established by the NARBA agreements.

Organized AM (mediumwave) radio broadcasting began in the early 1920s, 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 radio signals from AM band stations are reflected for distances extending for hundreds of kilometers. This is valuable in proving 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. During the 1930s, Canada also began using 1510 kHz, while the U.S. authorized experimental high-fidelity stations operating on 1530 and 1550 kHz. By 1939, Cuban stations existed on frequencies as high as 1600 kHz.

...
Wikipedia