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AM radio was established with clear channels so that major communication centers (ie NYC, LA, Seattle, Chicago) could transmit completely across the nation. Those AM frequencies included 660,710, 770, 880, 890, 1020, 1030, 1060, 1130, and 1210 Khz. Stations on those frequencies were expected to be received throughout that half of the nation or farther.
One might expect this created a radio version of a nationwide commerical network. However it was done first and foremost in the late 1920s for a national information system.
During the Cold War, a Conelrad system was created so that commercial stations would go off air. Then national defense transmissions existed on 640 and 1240 Khz. And again, no commercial purpose.
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