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.
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Technology
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.
Initial development
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 phenonmenon 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.
Long distance communication
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 U.S. and Europe. The majority of power line communication installations use transmissions in the longwave band, to avoid interference to and from standard AM stations.
United States
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.
Europe
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 powerlines 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.
In these systems programs were fed by special transformers into the lines. To prevent uncontrolled propagation, filters for the service's carrier frequencies were installed in substations and at line branches. Systems using telephone wires were incompatible with ISDN which required the same bandwidth to transmit digital data. Although the Swiss and German systems have been discontinued, the Italian it:Filodiffusione still has several hundred thousand subscribers.
Programs formerly carried by "wire broadcasting" in Switzerland included:
Low power broadcasting stations
Carrier current technology is also used for broadcasting radio programs that can be received over a small area by standard AM radios. This is most often associated with college radio and high school radio, but also has applications for hospital radio stations and at military bases, sports stadiums, convention halls, mental and penal institutions, trailer parks, summer camps, office buildings, and drive-in movie theaters. Transmitters that use carrier current are very simple, making them an effective option for students interested in radio.
Carrier current broadcasting began in 1936, when students at Brown University in Providence, Rhode Island developed a carrier current station initially called "The Brown Network". This station was founded by George Abraham and David W. Borst, who had originally installed an intercom system between their dormitory rooms. The intercom links were first expanded to additional locations, and then the system was replaced by distributed low-powered radio transmitters, which fed their signals into various buildings' electrical wires, allowing nearby radio receivers to receive the transmissions.
The carrier current station idea soon spread to other college campuses, especially in the Northeastern United States. The Intercollegiate Broadcasting System (IBS) was formed in February 1940, to coordinate activities between twelve college carrier current stations and to solicit advertisers interested in sponsoring programs geared toward their student audiences. The innovation received a major publicity boost by a complimentary article that appeared in the May 24, 1941 issue of The Saturday Evening Post, and eventually hundreds of college stations were established. Responding to the growing phenomenon, a 1941 release issued by the U.S. Federal Communications Commission (FCC) stated that because of the stations' very limited ranges, it had "not promulgated any rule governing their operation", and U.S. carrier current stations currently do not require a broadcasting license as long as their emissions adhere to the FCC's Part 15 Rules for unlicensed transmissions.
Educational institution carrier current and cablecast stations
Many college stations that went on to obtain FM broadcasting licenses started out as carrier current stations because of the low cost and relative ease of starting one up. Although college-based carrier current stations have existed for over 80 years, their numbers are steadily declining, becoming supplemented, or replaced, by other transmission methods, including low-power FM (LPFM), closed circuit over cable TV channels, and Internet streaming audio. As with most other student-run facilities, these stations often operate on sporadic schedules.
In the United States, unlike Educational FM stations, carrier current stations can carry a full range of advertising. Due to their low power these stations do not require an FCC license, and are not assigned official call letters. However, in keeping with standard radio industry practice, they commonly adopt their own call letter-like identifiers.