Closed-circuit radio

A closed-circuit radio is a radio that emits over a very small range, typically a college campus. This system can be achieved from two ways:

• By tying into the PA System^[1]
• By using existing power lines, telephones lines, pipes, etc. to transmit a carrier current^[2]

References

1. Radio Industry: closed circuit radio station, school radio station, college dormitories
2. The early days of armed forces radio