Carrier current

Carrier current transmission (originally called wired wireless) employs guided low-power radio signals, which are transmitted along electrical conductors. The transmissions are picked up by receivers that are either connected to, or a short distance from, the conductors. Carrier current transmission is used to send audio and telemetry to selected locations, and also for low-power broadcasting that covers a small geographical area, such as a college campus. The most common form of carrier current uses longwave or mediumwave AM radio signals that are sent through existing electrical wiring, although other conductors can be used, such as telephone lines.

Carrier current generally uses low power transmissions. In cases where the signals are being carried over electrical wires, special preparations must be made for distant transmissions, as the signals cannot pass through standard utility transformers. Signals can pass through transformers if the utility company has installed bypass lines, which typically has already been done when carrier current-based data systems are in operation. Signals can also be impressed onto the neutral leg of the three-phase electric power system, a practice known as "neutral loading", in order to reduce or eliminate mains hum (60 hertz in North American installations), and to extend effective transmission line distance.

For a broadcasting installation, a typical carrier current transmitter has an output in the range 5 to 30 watts. However, electrical wiring is a very inefficient antenna, and this results in a transmitted effective radiated power of less than one watt, and the distance that signals can be picked up is usually less than 60 meters (200 feet) from the wires. Transmission sound quality can be good, although it sometimes includes the low frequency mains hum interference produced by the alternating current. However, not all listeners notice this hum, nor is it reproduced well by all receivers.

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.

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