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, 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.
- 1 Technology
- 2 Initial development
- 3 Long-distance communication
- 4 Home entertainment services
- 5 Low-power broadcasting stations
- 6 See also
- 7 References
- 8 External links
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 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 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.
Home entertainment services
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.
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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.
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:
- 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"
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." Therefore, to operate legally, U.S. carrier current station broadcast emissions must 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.
- Bulls Radio 1620 - University of South Florida, also heard on licensed WMNF-HD2
- Radio Laurier Macdonald 560 AM at Laurier Macdonald High School in St. Leonard, Montreal, Quebec, Canada
- Radio SNHU 1620 at Southern New Hampshire University
- UMSLRadio "The U" 1620 AM. University of Missouri–St. Louis, University City, Missouri (The Carrier Current Station information is broadcast at the top-of-the-Hour.)
- Wolfpack Radio 1700 at the University of Nevada
- KAMP/1570 at the University of Arizona
- KANM/1580 at Texas A&M University
- KASR/1330 Arizona State University
- KCIZ 1650 at Mora High School, Mora, Minnesota
- KDUP/1580 at University of Portland, (Portland, Oregon) 
- KJACK 1680 - Northern Arizona University
- KLBC/1610 at Long Beach City College
- KMSC/1500 at Minnesota State University Moorhead
- KRFH at Humboldt State University
- KRIO 1660 at Rio Linda High School
- K-ROCKS RadioOne AM Stereo 1670 and AM Stereo 710 in Casper Wyoming 
- KSSU 1580 AM at California State University, Sacramento
- KSUB at Seattle University in conjunction with 8 mW low-power broadcasting KXSU-LP 102.1 FM and Internet radio
- KUR 1670/88.3 at Kutztown University
- KUTE/1620 University of Utah
- WALT/1610 at Davidson College
- WCXQ 1690 AM in Isabela, Puerto Rico
- WERW 1670 AM at Syracuse University
- WEXP at La Salle University
- WGCC 650 AM at Genesee Community College
- Studio U 1710 AM at the University of Oklahoma
- WMAX 540 at Mount Washington College in Manchester, New Hampshire
- WPPJ/670 Point Park College
- WPMD/1700 at Cerritos College
- WQMC/1290 at Queens College, City University of New York in Kew Gardens Hills, Queens
- WSIN/1590 at Southern Connecticut State University
- WSLU 1620/100.1 at Saint Leo University, St. Leo, Florida
- WTBU 640 AM/89.3 FM at Boston University
- Brown Student Radio at Brown University in Providence, Rhode Island, on A.M. 600.
- CHRW-FM at the University of Western Ontario started as a carrier current station at 610 kHz
- K.C. AM at Colby College, now WMHB
- KARL-AM at Carleton College, now KRLX
- KAL at University of California, Berkeley - now KALX
- KCC at Chabot College, Hayward, California - now KCRH 89.9
- KCCS at University of Missouri at Columbia - Residence Hall station
- KCWS-AM at Central Washington State College - now KCWU-FM
- KDVS, originally KCD at University of California, Davis
- KFRH at Washington University in St. Louis - now KWUR/90.3
- KHSC at Humboldt State College - now KHSU 90.5
- KMPS-AM at University of Alaska Fairbanks - now KSUA/91.5
- KNAB at Chapman University, Orange, California - ceased carrier current in 1991, now internet-only station ChapmanRadio.com
- KNMA 660 AM at New Mexico State University circa 1963
- KOWL at Rice University - now Rice Radio
- KRLK 97.5 at Rio Linda High School, California - now KRIO 1660
- KSLU at Saint Louis University in Saint Louis, Missouri - originally KBIL, now online
- KSU at Stanford University – now KZSU
- KSWC at Southwestern College in Winfield, Kansas - now at 100.3 FM
- KTTC (originally MD2) at Texas Tech College - now KTXT-FM 88.1 at Texas Tech University
- KUOK at the University of Kansas - now KJHK
- KVUC on 620 AM at Union College (Nebraska) in Lincoln, Nebraska from 1952 to 1968. Later licensed at 91.3 FM with call letters KUCV.
- WAMU(AM) American University, Washington, DC
- WBMB at Baruch College, CUNY; started as a carrier current station at 590AM
- WBSC on 640 AM at Bloomsburg State College (now Bloomsburg University) in Bloomsburg, Pennsylvania, but has merged with 91.1 FM WBUQ after that station signed on in 1985. For several years, both stations operated independent of one another prior to the merger.
- WCAR 550 AM at the University of North Carolina at Chapel Hill - now WXYC-FM 89.3
- WCCT on 830 KC, later on 540 KC at Clarkson College of Technology and SUNY Potsdam, in Potsdam, NY from 1961 to sometime in the 1980s, and evolving into WTSC-FM, an FCC licensed non-commercial station.
- WCHP 650 AM at Central Michigan University at Mount Pleasant Michigan
- WCUR as WSCS 640 AM, and WCUR 680 AM at West Chester University in West Chester, Pennsylvania, but has since migrated to 91.7 FM, as well as online at wcur.org
- WDBS 560 AM at Duke University - now WXDU 88.7
- WDCR (AM)
- WDGN AM 600 at Downers Grove North High School, became WDGC-FM, Downers Grove, Illinois
- WERU 710 AM/104.7 FM at Embry–Riddle Aeronautical University - now WIKD-LP 102.5
- WESB on 640 AM at University of Dayton Ohio in the 1970s
- WFAL 1610 AM at Bowling Green State University
- WFIB University of Cincinnati, Cincinnati, Ohio, on 800 AM
- WFRS 560 AM at Ferris State College, Big Rapids, Michigan (Now Ferris State University) From the 1950's through the 1980's
- WFVS-LP 530 AM and recently LPFM at 100.5 at Fort Valley State University in Fort Valley, Georgia (fiber-optic linked carrier current system)
- WGBC 640 AM at State University of New York at Geneseo
- WHAT 530 AM at Johns Hopkins University, later WHSR and now WJHU
- WHEN 640 AM at the University of Delaware
- WHEN 570 AM "The Rock of Macomb" at Western Illinois University
- WHRM 580 and 620 AM at Hiram College
- WINO 840 AM (and later WRFX) at Central Michigan University at Mt. Pleasant, Michigan
- WJJC at John Jay College of Criminal Justice, 1970s
- WJHU at Johns Hopkins University, Baltimore, Maryland.
- WJJX 640AM at the University of Michigan from 1952 to mid-1990s, predecessor to (and previously using the call letters) WCBN now on FM 88.3
- WJPZ at Syracuse University, now 89.1 FM
- WKC at Knox College, Galesburg, Illinois - now WVKC-FM
- WKCO at Kenyon College, Gambier, OH - now WKCO-FM
- WKDU-FM 91.7, the student-run radio station of Drexel University formally WMAX (1958)
- WKDT 89.3 FM, the cadet radio station, United States Military Academy, West Point, New York
- WLCR-AM 640 (originally WCDW-AM 830) operated by Summer campers at Camp Shaw-Mi-Del-Eca in Lewisburg, West Virginia.
- WLKR AM Lake Superior State University, Sault Ste. Marie, Michigan - Now WLSO 90.1 FM "The Sounds of Lake State"
- WMSN Michigan State University, East Lansing, Michigan flagship station for the now defunct Michigan State Network, with affiliates housed in dormitories and originating their own local programming including WBRS (Brody Hall), WKME (Shaw Hall), WEAK (Wonders Hall), WMCD (McDonnell Hall), and WFEE (Fee Hall)
- WMTU-FM at Michigan Technological University; started as a carrier-current AM station
- WMUC (AM) at the University of Maryland, College Park; started in 1948; ended in 1999 on 650 AM. WMUC-FM continues serving College Park.
- WNTC on 640 KC at Clarkson College of Technology and SUNY Potsdam, in Potsdam, NY from 1948 to 2001
- WNYU on 800 AM in New York University's dorms at its Lower Manhattan campus
- WOCR-650, a "pirate" carrier current station in Ocean City, Maryland, in 1973
- WOLF at North Carolina State University in Raleigh, North Carolina - now WKNC
- WPSM at Penn State's McKeesport campus (now Penn State Greater Allegheny) - now an internet station WMKP
- WQAD/WFQR/WIN/WIUS at Indiana University - now WIUX-LP
- WRAF on 590 AM at Binghamton University - now WHRW
- WRCC-AM at 640 AM at Rockland Community College
- WRCR-AM Rockford College
- WRFX at 840 AM at Central Michigan University at Mt. Pleasant, Michigan
- WRGW 540 AM at George Washington University in Washington, D.C. Now online only.
- WRCT on 900 AM at Carnegie Mellon University - now WRCT-FM
- WRIU Studio B, now only online, at the University of Rhode Island and licensed to Kingston, Rhode Island, on A.M. 580.
- WRPS-730 AM at SUNY Potsdam (NY), now WAIH 90.3
- WRLC-AM on 1110 and 1150 AM at Rutgers University, now WVPH-FM
- WRUC on 640 AM at Union College in Schenectady, New York, now 89.7 FM.
- WRUR-AM 1090 at the University of Rochester in Rochester, New York - now WRUR-FM; see also campus radio
- WSAC 710 AM at St. Anselm College, Goffstown, New Hampshire
- WSGR (South Green Radio) at Ohio University in Athens, Ohio
- WSMC 650 AM at St. Mary's College, St. Mary's, Maryland. Put on the air 1971.
- WSND 640 at the University of Notre Dame
- WSOE on 1200 AM at the Milwaukee School of Engineering in Milwaukee, Wisconsin - now WMSE on 91.7 FM
- WTAS 610 AM at Hope College in Holland, Michigan
- WTGR 530 AM (1969) at Memphis State University in Memphis, Tennessee, now the University of Memphis - now WUMR on 91.7 FM
- WTMC 530 AM (1973) at the Boston Housing Authority's Bromley-Heath Housing Project, Jamaica Plain, MA
- WUFI-540AM at Florida International University, now WRGP 88.1 & 95.3 FM
- WUVA 640 AM at the University of Virginia in Charlottesville from 1947 until 1979, when it was licensed to 92.7 MHz FM
- WUVT 640 AM at Virginia Tech
- WVAT (Voice of Alfred Tech) at SUNY, Alfred, New York - started broadcasting January 1965, now WETD-FM
- WVAU on 610 AM at American University, Washington, D.C. (station is still present, but they no longer broadcast carrier current)
- WVBU on 640 AM at Bucknell University later licensed to 90.5 MHz FM, carrier current turned off several years later
- WVOF on 620 AM at Fairfield University, Connecticut
- WVYC 640 AM at York College of Pennsylvania, sister station to FCC-licensed WVYC FM 99.7, also on the Internet
- WWRC on 640 AM at Rider College Rider University, Lawrenceville, NJ, 1962-84. Ended carrier current, became WRRC 107.7 in 1984
- WXOU 88.3 FM at Oakland University in Rochester Hills, Michigan (licensed to Auburn Hills, Michigan)
- WXPN and WQHS-730 at the University of Pennsylvania
- WYBC/640 at Yale University
- "Heinrich Hertz", The Electrician, July 20, 1894, page 333. Hertz's paper was titled "On the Propagation of Electric Waves along Wires".
- "Multiplex Telephony and Telegraphy by Means of Electric Waves Guided by Wires" by George O. Squier, Proceedings of the American Institute of American Engineers, May, 1911, pages 857-862. Squier assigned ownership of his U.S. patents to "the American People". He later unsuccessfully tried to claim that this had not exempted commercial concerns from paying royalties on his patents.
- "Telephony over Power Lines (Early History)" by Mischa Schwartz, "Presented IEEE History Conference, Newark, New Jersey, August 2007 and annotated since". (ethw.org)
- "Interplant Telephonic Communications Established Over High-Tension Lines", Electrical World, July 17, 1920, page 141.
- "Giving the Public a Light-Socket Broadcasting Service" by William Harris, Jr., Radio Broadcast, October 1923, pages 465-470.
- "Dr. George Abraham, Ph.D" (collegebroadcasters.us)
- "David W. Borst" (collegebroadcasters.us)
- The Gas Pipe Networks: A History of College Radio 1936-1946 by Louis M. Bloch, Jr., 1980, pages 11-13.
- Bloch (1980) pages 102-103.
- "Radio Pipe Broadcasters" by Erik Barnouw, The Saturday Evening Post, May 24, 1941, pages 36, 79-80.
- Bloch (1980) page 45.
- "Low Power Radio" (fcc.gov)
- "Welcome to BullsRadio.org". Bullsradio.org.
- RadioSNHU. "RadioSNHU". radio.snhu.edu.
- "The U". umslradio.com.
- "Kamp Student Radio". kamp.arizona.edu.
- "KANM 99.9 FM Cable - 1600 AM". 5 December 1998. Archived from the original on 5 December 1998.
- "Archived copy". Archived from the original on 2008-09-29. Retrieved 2008-09-29.CS1 maint: Archived copy as title (link)
- "KDUP – Bluff Radio". kdup.up.edu.
- http://www.kjack.org/ http://www.kjackradio.com/
- "KLBC 1610AM - "Truly Underground Radio"". www.klbc.org.
- "K-ROCKS Radio One - We Don't Do Cute at KROCKS!". krocksradioone.com.
- https://kxsuseattle.wordpress.com/ https://web.archive.org/web/20160323002125/http://www.ksubseattle.org/ http://www.seattlespectator.com/2015/10/21/seattle-u-student-radio-entering-a-new-era/ https://www.seattleu.edu/alumni/su-voice-alumni-blog/from-ksub-to-kxsu---big-changes-for-seattle-university-student-radio.html http://www.seattleu.edu/ksub/default.aspx?id=42100
- https://web.archive.org/web/20111121074604/http://www.kute.utah.edu/ http://kuteradio.org/
- "WPPJ - Point Park University". www.pointpark.edu.
- https://web.archive.org/web/20170922094625/http://wpmd.org/ https://web.archive.org/web/20161211110723/http://www.wpmdonthenet.com/about/
- "WSIN - SCSU Independent Radio". radio.southernct.edu.
- "Archived copy". Archived from the original on 2014-05-03. Retrieved 2014-05-03.CS1 maint: Archived copy as title (link)
- email@example.com, Jerry Peek,. "KRWG Radio, Las Cruces, NM -- History and FM's 10th Anniversary". www.jpeek.com.
- "Category:WCBN - Ann Arbor - ArborWiki:". 2011-01-07. Retrieved 2011-09-27.
- "CBN History Part I: Residence Hall Studios:". Retrieved 2011-09-27.
- "WMUC - The University of Maryland, College Park". 9 October 2002. Archived from the original on 9 October 2002.
- "Join a Student Organization - Penn State Student Affairs". www.clubs.psu.edu.
- WRCR alumni site
- "88.1 - WRGP Radiate FM - 95.3". www.wrgp.org.
- YaleRadio (17 April 2011). "This Is College Radio (1956) - WYBC Yale Radio & the Ivy (Radio) Network" – via YouTube.