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In the United States, FM radio stations end in odd numbers, but AM stations do not. We are grieved to report there is absolutely no reason for this. A long time ago somebody at the FCC made the call, and that's the way it's been ever since. The FCC is responsible for divvying up the radio spectrum for commercial use. It has assigned spectrums for cell phones, baby monitors, radio-controlled cars, air traffic controllers, and even the Mir space station. At this point, much of the process of assigning bandwidths simply depends on what's still available. As HowStuffWorks explains, FM radio operates between 88 and 108 megahertz, or millions of cycles per second. Individual stations
are allotted 200 kilohertz (thousands of cycles) "slices" between odd number fractions: 90.1, 90.3, 90.5, etc. AM radio, however, operates between 535 kilohertz and 1,700 kilohertz (note: that's kilohertz, not megahertz), and the stations have even-numbered call signs: 610, 620, etc. FM operates at a higher frequency than AM simply because it came later: AM radio started in the '20s, but FM didn't become popular until much later.
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