FM Transmitters

This article is concerned with low powered transmitters used in some countries for interfacing personal audio devices "FM transmitter" can also refer to high powered broadcast equipment used by pirate radio and licenced broadcast stations more...

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An FM transmitter is a portable device that plugs into the headphone jack or proprietary output port of a portable audio or video device, such as an iPod (or other types of MP3 players), CD players, and satellite radio systems. The sound is then broadcast through the transmitter, and plays through an FM frequency. Purposes for an FM transmitter include playing music from a device through a car stereo, or any radio without an audio input jack.

The FM-transmitter plugs into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios. Most devices on the market typically have a short range of up to 30 feet with any average radio (up to about 75 feet with a very good radio under perfect conditions) and can broadcast on any FM frequency from 76.0 to 108.0 MHz (or 87.9 to 107.9 in the US). Some lower-cost transmitters are hard-wired to the educational band (87.7-91.9), or a certain other smaller range of frequencies.

FM transmitters are usually battery driven, but some use the cigarette lighter socket in cars, or draw their power from the device itself. They are typically used with portable audio devices such as MP3 or CD players, but are also used to broadcast other outputs (such as that from a computer sound card) throughout a home or other building.

Limitations

The relatively low power output of FM transimitters sometimes makes it unsuitable for use in some large urban areas because of the number of other radio signals. This is compounded by the fact that strong FM signals can bleed over into neighboring frequencies making the frequencies unusable with the transmitter. Removing a car's radio antenna has been found to significantly improve transmitter reception.;

Some models which connect via ports other than the headphone jack have no means of controlling the volume, which can force the sound to transmit out from the device harshly (causing overmodulation, audio distortion and possible radio interference), or too low. In theory a device could use an automatic level control or audio limiter circuit to overcome this problem although there are few (if any) devices with such a facility available on the market yet.;

European legality of FM transmitters

The European Union's Radio Spectrum body the ERO (European Radiocommunications Office) has recently introduced a recommendation document (Table/Annex 13) for Member States to include Transmitters in the FM Band for Music Devices. The underlying specification suggests that the radio transmitter will only emit a maximum of 50 nanowatts Effective radiated power. It is not known what the current "iTrip" device emits although it is known that some devices supposedly manufactured to the US "FCC Part 15" standard emit considerably more. It also has to be ratified and entered into law in each European State, meaning that consultation will normally take place with the users of the spectrum in each country, a protracted and sometimes lengthy process. Until the recommendation is put in place and the law in the country of residence changed an FM transmitter remains illegal to operate in many EU countries, in its current form due to the minuscule range of such devices to such legislation is rarely enforced against end users although retailers in some jurisdictions have been threatened with prosecution.

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