A wireless frequency modulated (FM) transmitter is an aftermarket device that allows listeners to play a portable audio source through any radio with an FM band, such as a car stereo. It has no physical connection to the radio, meaning drivers do not have extra cords trailing around the car or require an upgrade to the existing audio system. In order for a wireless FM transmitter to work, a converter takes the audio output from an external source and converts it into analog audio within the FM transmitter. FM modulation converts the signal again, this time into an FM signal. Drivers can tune the car radio to the specific frequency that corresponds to the transmitter and listen to music on the FM band as if the broadcast was coming from a regular FM station.
In the United States, all wireless FM transmitters operate within the FM frequency band from 87.7 through 107.9. Drivers need to find an empty band on the FM spectrum; otherwise, the audio clashes with live radio broadcasts. Locating an empty band can be problematic when traveling across the country and moving in and out of the range of different local FM stations.