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An FM transmitter is a portable electronic device that is designed to convert a specific and fixed audio output into an FM radio signal that can be received on radios in the immediate vicinity. Most devices work by finding available bandwidth and then broadcasting the music or other sound across that specific wavelength. This can be a practical and inexpensive way to play music that is stored on a portable music player or is streaming online to more than one listener. One of the most popular ways to use this technology is in the car; harnessing the car’s radio and sound system is often more effective than simply playing the music from a phone or other device. Other cars following nearby can also pick up the signal, which is often desirable for things like group road trips or caravans. The devices can also be used in homes or offices, though they work best in areas where there isn’t a lot of competition for available bandwidth. In downtown or densely populated places, it can be hard to find a free signal and, even then, that signal can cut in and out or overlap with other nearby broadcasts.
FM bandwidth is nearly ubiquitous in most parts of the world, and it works on a system of frequency: different broadcasters can reserve or use certain specific frequencies, usually designated numerically, and can then use that space to play music, host conversations, or provide any other sort of audio programming. Commercial broadcasters usually use strong antennae to project their transmissions out over a great distance, which is how people can listen to broadcasts from many miles away. The idea behind a personal FM transmitter is similar, just on a smaller scale. The main goal is usually to broadcast certain pre-recorded selections to the immediate vicinity, and in most cases there is no option to add commentary or other conversations. The device simply relays the already-recorded sounds and nothing more.
Most of these devices are quite small, and they’re usually designed to be as easy to use as possible. There are a couple of different styles, but in general they all have a port for receiving audio output, a cable for connecting to an outside player or sound source, and a transmission tuner. They usually have an antenna, too, but this is almost always internal and usually goes unnoticed.
Most transmitters are fairly similar in terms of operation. The first thing they’ll usually do is scan the airwaves for an available frequency. Then, they will broadcast the sound inputted into it out over that frequency, such that anyone in the immediate vicinity could tune their radios to the identified channel and hear whatever the transmitter is sending out. People often plug these into portable music players or smartphones in order to share audio content more widely.
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 these 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.
Signal range limitations also mean that these sorts of devices aren’t really suited to actually broadcasting to more than a single home or car, though sometimes cars traveling together and maintaining close proximity can participate together. They are usually thought of as a way to get the benefits of surround sound or synched sound systems in houses, offices, or cars — just without the expense of actually setting up those more complicated systems. Harnessing available frequencies is usually cost effective and doesn’t require much technical know-how.
Not all transmitters are compatible with all music players. Before purchasing an FM transmitter, consumers would be wise to do a bit of research to be sure all components will work together. Power sourcing is also an important consideration. While most transmitters are battery driven, some also have a separate connector that allows them to be plugged into a car's cigarette lighter. This option is popular for those planning to use their transmitters for longer road trips.
I thought these were largely obsolete with the advent of auxiliary jacks that are built into most modern car audio systems and allow any device with a headphone jack to be connected directly.
I had a few of those FM transmitters and learned in a hurry that the inexpensive ones were awful. Sure, they might work OK for a time, but invariably interference from stronger radio signals would stomp all over their signals.