Very low-power personal radio

SUMMARY:The Ramsay FM-10 is a build-it-yourself stereo transmitter with surprisingly good sound quality. Even if you have never built electronic project kits before, putting the kit together is easy. The only required knowledge before starting is how to use a soldering iron. Once all parts are soldered in place you are ready to prepare to broadcast.

COMPONENTS:There are 2 audio inputs (left and right), a power switch and a jack for an external antenna. The Ramsey kit operated off a 9-V battery, making it very portable, or you can substitute a 12-VDC power supply for slightly increased output. The carrier frequency is totally adjustable across the FM band (88-108 MHz).

The heart of the FM-10 is the BA-1404 stereo FM transmitter chip (or integrated circuit). The actual output of the FM-10 kit is about 1/100th of a watt. However, with the proper antenna you can get an effective range of up to 1/2 km. It is possible to add additional amplifier stages to boost the power of the FM-10.

BROADCASTING:The first thing to do is to find a frequency on the FM band that is not being used by another station. The frequency of the FM-10 is controlled by an adjustable induction coil and one of the capacitor positions. The kit includes three capacitors that can be installed in this position, one for the bottom third of the FM band, one for the middle third, and one for the top end. Choose which third of the FM band you want to operate in (the least busy)and then fine-tune the frequency by adjusting the induction coil.

Once you’ve found an unoccupied frequency to broadcast on, you then need to adjust the sub-carrier tuning, which can be done by turning the sub-carrier trimming capacitor until the stereo light goes on on your receiver. A trimming resistor controls the stereo balance which is optimized simply by listening to your receiver for the best sound quality and stereo separation. You should attach an audio input to the FM-10 at this point to test the tonal quality.

The kit lacks an audio input level control, so attach something with an adjustable line level output (eg. a cassette or CD player with a headphone jack). The simplest type of antenna to use is a flexible T-shaped FM dipole, commonly found in stereo or electronic shops. Ideally the length of your antenna sections should follow this formula: length (in feet)=234/frequency (in MHz). It should be noted that the FM-10 is not a very stable transmitter; changes in temperature cause the frequency to drift somewhat.