Carrier current

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Carrier current is a method of low power AM broadcasting that is generally not licensed in the United States, but is allowed on the campus of any school, so long as the normal FCC Part 15 Rules are adhered to when measured at the edge of the campus. Most college radio stations started out this way, using the electrical system of a building to distribute an AM radio signal. This is one method used for college and high school radio, particularly if the signal is only intended to be picked up in a small area. Many established college radio stations originally began as carrier current stations. While the technology is still used by a number of student-run stations today, the popularity declined beginning in the 1980s, as popular music radio formats quickly migrated to the FM band. The popularity of streaming audio over the Internet has hastened this decline.

Carrier current stations generally only have an effective radiated power of a few watts. These signals cannot pass through transformers, however, and are prone to the electromagnetic interference of the alternating current. Transmitters that use carrier current are very simple, making them an effective option for students interested in radio. Transmissions can be of good quality, although there is a low frequency background hum (60 hertz in North American installations) associated with carrier current due to the cycle of the alternating current. Not all people notice this hum, depending on how sensitive their hearing is to that frequency range.

Because of the simple design and the fact that the transmitter doesn't need an external antenna, carrier current listening devices have found some use in the world of espionage.

[edit] Student-run carrier current or cable cast stations

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As with most other student-run stations, these stations often operate on sporadic schedules. Most of these stations are also supplemented by other broadcasting methods, such as LPFM, closed circuit, and streaming audio. Many carrier current stations have been, and continue to be, replaced by these technologies as well. These stations are NOT licensed by the FCC and their call letters are entirely self-styled.