The North American Radio Broadcasting Agreement, usually referred to as NARBA, is a treaty that took effect in March 1941 and set out an international bandplan and interference rules for mediumwave AM broadcasting in North America. NARBA accommodated much of the U.S. bandplan of 1928, with accommodation to Canada and Mexico.
Although mostly replaced by other agreements in the 1980s, the basic bandplan of NARBA has remained to the present day. Among its major features were the extension of the broadcast band from its former limits of 550 kHz to 1500 kHz to its 1941 limits of 540 kHz to 1600 kHz to its present limits of 540 kHz to 1700 kHz and the shift of most existing AM stations' frequencies to make room for additional clear-channel station allocations for Canada and Mexico.
The agreement eventually governed AM band use in the United States, Canada, Mexico, Cuba, the Dominican Republic, and Haiti. In accordance with the treaty, clear channel frequencies were set aside across, roughly, the lower half of the radio dial (with a few regional channels thrown in), and regional channels across, roughly, the upper half of the radio dial (with a few clear channels thrown in).
The replacement 1230, 1240, 1340, 1400, 1450, and 1490 kHz local channels (formerly 1200, 1210, 1310, 1370. 1420 and 1500 kHz) were reserved for local channel stations (these are regional channels if located outside the North American continent, in which case regional channel stations could be allocated to those channels).
The agreement also officially reduced the "same market" minimum channel spacing from 50 kHz to 40 kHz, although Mexico elected to enforce a 30 kHz "same market" channel spacing, unless such reduced spacing was in conflict with an abutting nation's "border zone" allocations, in which case 40 kHz was enforced.
It required that most existing AM stations change frequencies according to a well-defined "table", which attempted to conserve the electrical height of the extant vertical radiator(s) and thereby controlling possible interference, while resulting in a wholesale yet predictable shuffling of radio station dial positions.
There were about 100 stations which were not changed according to the "table" and in these cases every attempt was made to move an existing clear channel station to a possibly distant clear channel (and not to a regional channel) and to move an existing regional channel station to a possibly distant regional channel (and not to a clear channel); local channel stations were not moved outside of the "table" as the "table" accommodated every eventuality, including even the cases of stations on the two highest local channels, 1420 and 1500 kHz, an 80 kHz spacing, as the new "same market" spacing of 40 kHz accommodated this case (these moved stations would be allocated to 1450 and 1490 kHz, a 40 kHz spacing).
At the same time, this spacing protected the international medium-wave distress frequency, 500 kHz, which must be protected by all stations everywhere.
Read more about North American Radio Broadcasting Agreement: Agreement, Frequency Changes, Current Status
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