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It involves the broadcasting of coherent audio signals encoded into specified frequencies of the electromagnetic spectrum. These waves can then be intercepted with an appropriate antenna and tuned oscillating circuit, and decoded, amplified, and listened to via loudspeakers of your choice. The simplest receivers are all in one small package, and often run on batteries. The most complex involve many separate components, all carefully selected for their ability to produce the highest possible quality listening experience. There are also radios built to receive a large number of frequency bands.
 Radio in the U.S.
In the United States, FM, or "frequency modulation" radio, with a clumsy provision for stereo, or two channel reproduction, is usually used to transmit music; whereas AM, or "amplitude modulation" radio is nowadays the province of various genres of "talk radio".
Today's FM band was originally known as the FM2 band. The original FM band -- aka, the FM1 band -- occupied the 42-50 MHz band. This band was established by the inventor of FM, Edwin H. Armstrong, in the late 1930s. However, thanks to a bitter, long-running feud between Armstrong and David Sarnoff (president of the Radio Corporation of America), the more politically powerful Sarnoff was able to persuade the FCC to discontinue the FM1 band in exchange for the establishment of the FM2 band. (If you find an AM/FM radio manufactured between 1941 and 1947, it may have both bands.)
By FCC mandate, the FM band is divided into two sections:
- The 87.9-91.9 MHz section is reserved for non-commercial/educational (NCE) stations, with a fairly even division of public and religious stations, plus a random smattering of college and public school district stations.
- The 92.1-107.9 MHz section is mostly used by commercial stations and a few NCE stations in a few cities like Seattle.
On the commercial portion of the band, popular dance hits can usually be found promoting their beats at 105 - 107 MHz; Rock stations are usually at 94 - 97; and country stations in the fat middle of the dial from 98 - 103. These are not hard and fast regulations, except that stations near the middle of the band will experience better reception due to the antenna's lengths being a compromise that favors it.
Also by FCC mandate and an international agreement with Canada and Mexico, the AM band is divided by frequency into high-powered, medium-powered and low-powered stations. The high- and medium-power divisions are too numerous to list here. The low-power frequencies -- 1230, 1240, 1340, 1400, 1450 and 1490 KHz -- are reserved for 1000 watt stations whose daytime signals will completely fade out 30 miles (48 km) from the transmitter. In large cities, stations on these frequencies often have
oddball eclectic and/or foreign language formats.
 Cooler stuff
In addition to these large scale, usually commercial operating bands, there are parts if the spectrum reserved for amateurs of all stripes, including "ham" radio operators and those using "Citizen's Band" (CB) radio. Both of these bands permit reception and low power broadcast. Citizen's Band is the coolest because it is used by truck drivers ("big, burly..." sorta like loggers. WOOF!) and was one of the more memorable fads of the mid 1970s.
Shortwave refers to the spectrum above the AM (medium wave) and below the CB radio and television channel 2-6 bands; about half of the "ham" bands are located within the shortwave spectrum, as well as all sorts of other fun and weird stuff, including commercial and government broadcasts which can be received internationally.
For the terminally cool, one can build simple yet high powered AM radio transmitters from nothing more than several meters of common copper wire and some extremely butch capacitors. Such transmitters have the appeal not only of operating at a sufficient voltage to kill you stone dead if mishandled, but also being highly illegal due to their potential for interference with licensed broadcasters.[more detail please] It's not unknown for the terminally bored to run CB radios through massively overpowered (and very, very illegal) signal amplifiers either; in one case, a 75KW transmission using a massively modified pickup truck alternator to power it crashed the camcorder that was taking the video of the test.
The telecom industry currently wants to get their hands on as much of the radio spectrum as they can so they can gobble it up for cellular phone use and trunked multifrequency radio systems for the cops, which are completely unnecessary but a financial boon to the telecom industry who have convinced public safety agencies they need those new trunked systems in the name of homeland security. This is why broadcast TV channels 70 through 83 were discontinued in the early 1980s. They also have their eye on grabbing as much of the ham radio spectrum as they can, if not for the ARRL which is the only thing holding them back in that area.
Foreign radios are as strange as foreign languages, with bands labeled "MW", "LW", etc., which make no sense and don't pick up anything useful in America.