FM Transmitters

FM transmitters are used in some Christmas displays to transmit the "show audio" to viewers in their cars. This may be in addition to speakers placed around the display for visitors on foot.

The legal limit without a licence in Australia is 10µW (10 microwatts / 0.01mW / 0.00001W) of ERP (Effective Radiated Power). For more on the non-technical rules and legalities of broadcasting, see FM Broadcasting.

Other countries will likely have different rules and regulations. Please check with the government authority for communications in your area before purchasing an FM transmitter.

The frequency you choose must be within the appropriate band (87.5 Mhz to 108.1 Mhz for Australia) and not interfere with any other station. With your own transmitter off, pick a frequency where you can't hear anything but pure static. The frequency should end in an odd number.

Once you are transmitting, check the whole FM band to make sure your transmitter is not causing interference with other stations. The interference may not be only to the stations adjacent to the frequency you're using. Due to the way FM transmission works, certain frequencies can mix together and cause interference on a different (seemingly unrelated) frequency.

Check this Wikipedia page to see what FM radio stations are on what frequencies.

Most transmitters these days will be stereo. If you have a mono source, make sure you connect it to both left and right audio inputs on the transmitter.

What if you want to feed your transmitter in stereo, but simultaneously send the same audio to a mono amplifier for yard audio? Easy. You can use a couple of resistors (10K 1/4W will do) to do the summing (mixing left and right) while not affecting the stereo signal. The diagram below shows the details.

PLL stands for Phase Locked Loop. A transmitter with PLL has a very stable oscillator, set to a fixed frequency. It uses a PLL to multiply that up to the real transmission frequency. This means the frequency can be adjusted (by changing the multiplier), but the output frequency is still nice and stable over time despite voltage and temperature variations.

Usually, the antenna that comes with your transmitter will be the correct type and size. If the antenna is telescopic, make sure you extend it fully as the transmitter will be designed to feed into an antenna of a specific length. Refer to the instructions that came with the transmitter for further information.

To work out the correct length for your antenna based on your frequency you can use this calculator.

Here's a great easy way to make a dipole antenna.

Setting up an FM transmitter takes a bit of thought and consideration. By using the correct output power and frequency, you can avoid a knock on the door from the "Men in Black"!

Also remember that a better quality, lower powered transmitter will give your listeners a better experience than some rough, high powered unit. Check out what's on offer and ask other members what they've used and what they thought of it.