Carrier current

Carrier current transmission, originally called wired wireless, employs guided low-power radio-frequency signals, which are transmitted along electrical conductors. The transmissions are picked up by receivers that are either connected to the conductors, or a short distance from them. 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 medium wave AM radio signals that are sent through existing electrical wiring, although other conductors can be used, such as telephone lines. Carrier current transmission, originally called wired wireless, employs guided low-power radio-frequency signals, which are transmitted along electrical conductors. The transmissions are picked up by receivers that are either connected to the conductors, or a short distance from them. 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 medium wave 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 bridge transformers if the utility company has installed high-pass filters, 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 over which 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. The ability for electrical conductors to act as waveguides for radio signals was noted in the earliest days of radio experimentation, and Heinrich Hertz published the first review of the phenomenon in 1889. By 1911, Major General George Owen Squier was conducting some of the earliest studies designed to put carrier current transmissions, which he called 'wired wireless', to practical use. To be effective, the radio transmitter must be capable of generating pure continuous-wave AM transmissions. Thus, the technology needed to set up carrier current transmissions would not be readily available until the late 1910s, with the development of vacuum tube transmitters and amplifiers. The first commercial applications of carrier current technology included the setting up of long-distance telegraph, telemetry, and telephone communication by electrical companies over their high-voltage distribution lines. This approach had a major advantage over standard telegraph and telephone lines, because radio signals can readily jump over any small gaps in cases when there is a line break. In May 1918, the Imperial Japanese Electro-Technical Laboratory of Tokyo successfully tested 'wave telephony' over the Kinogawa Hydro-Electric Company's 144-kilometer (90 mile) long power line. In the summer of 1920, a successful test transmission over 19.2 kilometers (12 miles) of high-tension wires was reported from New Jersey, and by 1929 one thousand installations had been made in the United States and Europe. The majority of power line communication installations use transmissions in the longwave band, to avoid interference to and from standard AM stations. In 1923, the Wired Radio Service Company, a subsidiary of the local electric company, set up a subscription news and entertainment service at Staten Island, New York that used carrier current transmissions over the electrical power lines. To receive the transmissions, subscribers had to lease a receiver costing between two and five dollars a month. However, despite the power company's optimism that the system would eventually be installed nationally, the effort proved unable to compete with the free offerings provided by standard radio stations. General Squier continued to unsuccessfully promote the technology for home entertainment, until 1934, when he helped found the Muzak company, which focused on the business market. Carrier current home entertainment services would prove to be more popular in Europe. Previously, there had been a few successful telephone newspaper services, which sent entertainment to subscribers over standard telephone lines. However, carrier current transmissions had the ability to provide programs over telephone lines without affecting the regular telephone service, and could also send multiple programs simultaneously. In Germany, the carrier current service was called Drahtfunk, and in Switzerland Telefonrundspruch. In the Soviet Union, this approach was very common beginning in the 1930s because of its low cost and accessibility, and because it made reception of uncensored over-the-air transmissions more difficult. In Norway radiation from power lines was used, provided by the Linjesender facility. In Britain such systems were used for a time in areas where reception from conventional BBC radio transmitters was poor.