A closed-circuit radio is similar to traditional radio communication, though its signal is sent via pre-laid speaker wires or utility lines. It has a very limited range, typically within a building or a college campus.
An existing PA System may be used to send the audio signal. This method is typically applied in schools, such as by permitting students to use a central microphone to deliver daily announcements. Existing power lines, telephone lines, pipes, etc. can transmit a carrier current. Radio receivers at close range may then tune to the signal emitted from the utility line.The early days of armed forces radio Closed-circuit communications signal distribution systemRadio behind bars – The Polish caseClosed-circuit Radio Relays GameCollege Closed-circuit Radio Station Gets New, More Powerful TransmitterClosed-circuit Radio Network in Homes Seen
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