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Carrier current is a method of low power AM broadcasting that does not require a broadcast license in the United Statesmarker, 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. 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 from 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 alternating current. Not all listeners notice this hum, nor is it reproduced well by all receivers.

Carrier current is available for any user, and is not restricted to campus operations. There are many examples of community radio stations being operated in the United States using carrier current AM broadcasting. Signals may pass a transformer if the utility company has bypass lines installed (typically when non-conflicting carrier current-based data systems of their own are in operation). Signals may also be impressed onto the neutral leg of the 3-phase power system, a practice known as "neutral loading", in an effort both to reduce (sometimes eliminate) 60 Hz hum, and to extend effective transmission line distance. It has been successful in both ways in community and campus installations.

Extensive systems can include multiple unit installations with linear amplifiers and splitters to increase the coupling points to a large electrical grid (whether a campus, a high-rise apartment or a community). These systems would typically require coaxial cable interconnection from a transmitter to the linear amplifiers. In the 1990s, LPB, Inc., possibly the largest manufacturer of these transmission systems, designed and supplied several extensive campus-based systems that included fiber-optic links between linear amplifiers to prevent heterodyne interference.

Because of their 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.

Student-run carrier current or cable cast stations

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. Though legal, these stations are not licensed by the FCC and their call letters are entirely self-styled.

Existing stations

Former stations

See also

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