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Millions of people regularly listen to terrestrial radio. AM (amplitude modulation) is an older standard than FM (frequency modulation) and is easier to implement. AM signals can travel between 100 and 300 miles while FM signals are limited by the curvature of the Earth, giving them a maximum distance of about 50 or 60 miles. While AM signals fade with distance, FM is consistent within the receiving area, making AM ideal for news broadcasting and FM ideal for music.
All FM radio stations end in an odd number (88.3, 96.1, etc.), though this is purely conventional and regulated by the FCC. AM frequencies are measured in kilohertz, while FM frequencies are measured in megahertz. AM stations range from 520 kHz to 1710 kHZ, with stations spaced 10 kHz apart, while FM stations range from 88 mHZ to 108 mHz. 1 megahertz is equal to 1,000 kilohertz, so technically AM stations range from FM 0.520 to FM 1.7 while FM stations range from AM 88000 to 108000. (Remember, all radio waves are part of the same electromagnetic spectrum — AM and FM are just different ranges of that spectrum.) In theory, tuning to AM 99100 is the same as tuning to FM 99.1.
You've probably noticed you can pick up nearby channels on an unused frequency. For example, you can probably pick up the radio station FM 100.1 on FM 100.3 or AM 600 on AM 610. But do these numbers mean anything? Are some frequencies "better" than others? To transmit a signal, one simply broadcasts audio at a specific frequency. Lower frequencies require less power and travel farther, but are lower quality. Higher frequencies require more power and don't travel as far, but are higher quality. That is, the frequencies that will be transmitted farthest are at the lower end of the spectrum — the 520 kHZ and FM 88.1 frequencies will travel the farthest from the broadcast station and have the lowest quality. In theory, stations at lower frequencies transmit further and have the worst quality. For more information, see this article online.