Carrier Current Broadcasting
Carrier current radio transmission operates by injecting an low power AM radio frequency broadcast signal into the AC power lines of the building where reception of the broadcast signal is desired. This method of transmission not only allows for reception by AM receivers powered by the building's AC power system but also allows reception by nearby portable AM radio receivers since the electrical system acts as an intentional radiator.
Carrier Current History:
Carrier current transmission enjoyed growing popularity during the 1960's and 1970's when LPB Incorporated had manufactured equipment allowing colleges and universities to have their own campus-limited radio station without requiring an FCC license. LPB Inc had ceased operation years ago leaving Radio Systems Incorporated of Logan Township, New Jersey as the only commercial manufacturer of carrier current AM broadcasting equipment.
Carrier current broadcasting has been used to provide radio service for campus buildings as well as on-site dormitories. These same systems have also been utilized for high school radio, community and hospital radio broadcasting as well as drive-in movie theaters.
Carrier Current Limitations:
Due to the nature of power distribution most radio frequency signals will not pass through the power transformers that most power utility companies use. This is because the RF frequencies of the AM band are much higher than the 60 Hertz frequency used for power distribution in North America.
Some power utilities use equipment that allows them to control the load of the power grid by controlling residential customers' air conditioning equipment via remote control. In this scenario their control equipment accepts commands from signals sent over the power lines. It may be possible in this case for a carrier current AM signal to travel further than what is considered typical.
Part 15 Regulations:
In addition to defining the requirements for leaky coax AM installations, Part 15.221 of the Part 15 regulations defines acceptable operation for carrier current broadcast installations. Carrier current systems may operate in the band 525-1705 kHz provided the field strength levels of the radiated emissions do not exceed 15 uV/m measured at a distance of 47,715/(frequency in kHz) meters from the electric power line.
Typical Part 15 AM compliant transmitters designed for use with an intentional radiator (i.e., antenna) would be incapable of providing sufficient RF levels needed for acceptable carrier current transmission. Commercially manufactured transmitters specifically made for carrier current systems range in power from 5 to 30 watts.
System Implementation:
Appropriate coupling methods must be employed to ensure the transmitter as well as personnel aren't exposed to the lethal voltage of the building's electrical service. The purpose of the transmitter coupler is the provide impedance matching between the transmitter and the electrical system.
The AM broadcast signal does not easily pass through the transformers of the local power utility, allowing the signal to remain confined. Greater coverage for other buildings on the same property is possible by using supplying each building by injecting the RF into each building's electrical service panel. This would be accomplished by splitting the main transmitter's signal and sending it via standard (i.e., non-radiating) coaxial cable to couplers located in each building. Longer runs usually require amplification to supply adequate signal to the outlying building's coupler.
An alternative to distributing a common RF signal would be to install independent AM transmitters and couplers in the buildings where reception of the locally originated AM broadcast signal is desired. With this type of installation the audio signal would have to be distributed to each transmitter's audio input connection. In this implementation it is important to control signal levels to prevent interference if operation on a common broadcast frequency is desired.
Signal coverage may be extended to some extent by implementing "neutral loading" where the RF signal in injected into the neutral wire of the electrical service. Care must be made to ensure any radiated signal continues to comply with Part 15.221.
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