Tuesday, September 24, 2013

Why does FM radio go from 87.9 to 107.9?

In the United States, the frequency modulation ("FM") radio band stretches from 87.7 MHz (megahertz) to 108 MHz. The center frequencies are probably 87.9 - 107.9 on your car radio.

Image courtesy paulswansen.

Stations from 88-92 MHz (the "reserved band") are legally reserved for noncommercial radio. This probably explains why NPR is usually on the lower number stations.

Why is FM radio here? The Law (tm)!

The Federal Communications Commission ("FCC") allocated FM radio stations in this part of the radio frequency spectrum after World War II.

The frequency spectrum is like a neighborhood, with frequencies carved out by the International Telecommunications Union ("ITU") for different parts of the world. The ITU, a specialized agency of the United Nations headquartered in Geneva, Switzerland, was originally founded as the International Telegraph Union under the 1865 International Telegraph Convention.

The United States, in turn, carves out its spectrum for various uses, including television, satellite, GPS, and radio in the Frequency Allocation Table, a regulation published at 47 C.F.R. § 2.106. The FCC allocates civilian commercial uses, while the National Telecommunications and Information Administration ("NTIA") allocates military uses.

So who are FM radio's "neighbors," if you could extend your car radio below 87.9 and above 107.9? Just underneath the FM band are what used to be analog television channels 5 and 6. And above: satellite communications.

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