Application
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 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.
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