Medium Wave - Medium Wave in The Americas

Medium Wave in The Americas

See also: North American Radio Broadcasting Agreement

Initially broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters (833 kHz), with stations required to switch to 485 meters (619 kHz) when broadcasting weather forecasts, crop price reports and other government reports. This arrangement had numerous practical difficulties. Early transmitters were technically crude and virtually impossible to set accurately on their intended frequency and if (as frequently happened) two (or more) stations in the same part of the country broadcast simultaneously the resultant interference meant that usually neither could be heard clearly. The Commerce Department rarely intervened in such cases but left it up to stations to enter into voluntary timesharing agreements amongst themselves. The addition of a third "entertainment" wavelength, 400 meters, did little to solve this overcrowding.

In 1923, the Commerce Department realized that as more and more stations were applying for commercial licenses, it was not practical to have every station broadcast on the same three wavelengths. On May 15, 1923, Commerce Secretary Herbert Hoover announced a new bandplan which set aside 81 frequencies, in 10 kHz steps, from 550 kHz to 1350 kHz (extended to 1500, then 1600 and ultimately 1700 kHz in later years). Each station would be assigned one frequency (albeit usually shared with stations in other parts of the country and/or abroad), no longer having to broadcast weather and government reports on a different frequency than entertainment. Class A and B stations were segregated into sub-bands.

Nowadays in most of the Americas, mediumwave broadcast stations are separated by 10 kHz and have two sidebands of up to ± 5 kHz in theory, although in practice stations transmit audio of up to 10 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ± 4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US and Canada the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2 megawatts daytime.

Most United States AM radio stations are required by the Federal Communications Commission (FCC) to shut down, reduce power or employ a directional antenna array at night in order to avoid interference with each other due to night-time only long-distance skywave propagation ("skip"). Those stations which shut down completely at night are often known as "daytimers". Similar regulations are in force for Canadian stations, administered by Industry Canada.