An FM transmitter is a portable device, which converts a specific audio output into an FM radio signal. It can be plugged into a CD player, satellite radio system, headphone jack, or a portable media player. One common use of an FM transmitter is to play music from an MP3 player through the speakers of a car.

The majority of FM transmitters have a range of about 30 feet (about 9 meters). A good radio can increase that range to upwards of 75 feet (about 23 meters). Due to its low output, sometimes FM transmitters are not suitable for use in large urban areas, as the frequencies they use may become interrupted with other radio signals. This situation can be aggravated by strong FM signals that bleed into surrounding frequencies which the transmitter uses.

The FM transmitter has many helpful applications. It has, for example, made it possible to take any type of audio from a computer, including music or online debates, and transmit it to an FM radio. The FM radio, basically serves as alternative speakers. One benefit of this is that it removes the hassle of the listener from having to stay chained to a desktop PC to listen to Internet radio.

There are several types of FM transmitters on the market. Besides the obvious price differences, they also differ in power source, transmission range, and the number of FM frequencies they can use.

A digital FM transmitter helps to connect an MP3 player with a car or a home stereo system through its radio receiver. It physically connects to the MP3 player and then broadcasts a short FM transmission of the songs playing on the MP3 player. Once it is plugged into the MP3 player, the transmission frequency on the FM transmitter is set to a FM station that is not used in that area. The FM receiver is then tuned to the FM transmission station and the music on the MP3 player can be heard via the car or home stereo speakers.

For those using an FM transmitter in a car, FM transmitters typically work within a 50 foot (about 15 meter) range. A car traveling behind one with a FM transmitter can have its radio set to the same station and listen to the same transmission.

Not all transmitters are compatible with all MP3 players. Before purchasing an FM transmitter you'll want to make sure that your MP3 player is compatible with that particular transmitter. Typically, the product packaging will provide this information. You'll also want to consider the power source before purchasing an FM transmitter. While most FM transmitters are battery driven, have a connector which allows it to be plugged into a car's cigarette lighter; this option is popular for those planning to use their FM transmitters in the car.