Carrier current

Carrier current is a method of low-power broadcasting that uses the electrical system of a building to distribute an AM radio signal. This is one method used for college radio, particularly if the signal is only intended to be picked up in a small area. They generally only have an effective radiated power of a few watts. Transmitters that use carrier current are very simple, making them an effective option for students interested in radio. Transmissions can be of good quality, although there is reportedly a 60 hertz background hum (in North American installations) associated with carrier current.

Because of the simple design and the fact that the transmitter doesn't need an external antenna, carrier current listening devices have found some use in the world of espionage.