Carrier current

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Carrier current is a method of low power AM radio transmission that uses the AC electrical system of a building to propagate a medium frequency AM signal to a relatively small area, such as a building or a group of buildings. In the United States, carrier current stations do not require a broadcasting license from the FCC, as long as the emissions adhere to the Part 15 Rules for unlicensed transmissions.[citation needed]

Application[edit]

This technique is used by many types of facilities that need to transmit radio to a small area. Carrier current is most often associated with college radio, high school radio, and hospital radio stations, but is also used at military bases, sports stadiums, convention halls, mental and penal institutions, trailer parks, summer camps, office buildings, and drive-in movie theaters. Many college stations that went on to obtain broadcasting licenses started out as carrier current stations because of the low cost and relative ease of starting up such a radio station.

Carrier current stations generally operate with very low power. A typical carrier current transmitter might be 5 to 30 watts, although using a building's AC wiring as an antenna is very inefficient and results in an effective radiated power of less than one watt. The usable range of the signal is usually less than 200 feet (60 meters). These signals cannot pass through utility 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.

European broadcasters[edit]

In Germany, carrier current transmission was called Drahtfunk. In Switzerland, Telefonrundspruch used telephone lines. In the Soviet Union, PLC was very common for broadcasting since the 1930s because of its low cost and accessibility, and because it made reception of uncensored over-the-air transmissions more difficult. In Norway the radiation of PLC systems from powerlines was sometimes used for radio supply. These facilities were called Linjesender.

In all cases the radio programme was fed by special transformers into the lines. To prevent uncontrolled propagation, filters for the carrier frequencies of the PLC systems were installed in substations and at line branches.

An example of the programs formerly carried by "wire broadcasting" in Switzerland:

  • 175 kHz Swiss Radio International
  • 208 kHz RSR1 "la première" (French)
  • 241 kHz "classical music"
  • 274 kHz RSI1 "rete UNO" (Italian)
  • 307 kHz DRS 1 (German)
  • 340 kHz "easy music"

Systems using telephone wires were incompatible with ISDN use which required the same bandwidth for digital data. The Swiss and German systems have been discontinued, but in Italy Filodiffusione still has several hundred thousand subscribers.

Community radio[edit]

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 three-phase electric 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.

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

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[edit]

Former stations[edit]

See also[edit]

References[edit]

External links[edit]