88-108Mhz FM Radio Transmitter


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88-108Mhz FM Radio Transmitter

This is a fairly basic radio transmitter circuit that is produced by a Project Kits company, Quasar. There are 3 stages in this version. But can be modified easily to output around 4W instead 500mW!

The main circuit diagram is here:


Parts list:


These are un-altered and taken strait from the Quasar kits manual.

Be sure to build, tune and test the circuit as the par the manual.

The range on the standard transmitter is around 1km with a single 1/2 wave antenna, and about 700m with 1/4 wave dipole.

Output Modification:

The mod I present here is to use a 2n2219 NPN transistor in place of T2 and a 2n3553 NPN transistor in place of T3. Both are VHF transistors. The last transistor will need to be put on a heatsink. I haven't found the need to make any other modifications to the circuit to make this work.

Input power is required to be 12V in it's current form.

Range should now be around 5km with a 1/2 wave antenna, but may not be stable at maximum range.

Audio Input Modification:

This will allow an audio input from an MP3 player etc.

Remove the electret microphone. R1 needs to be removed also, as this is the supply for the electret mic. Then construct the following circuit on a seperate board and connect the audio output to the mic inputs.


Antenna Calculations:

To work out an antenna wire wire length for a given frequency, use the following formula.

(V/f) / 2 = Half wave length (PI 2)


(V/f) / 4= Quater wave length (PI 4)

Where: V = Speed of Light (300 million mtrs per sec), f = Frequency in MegaHertz.

Wave Length is the physical height of the wave in meters. By diving it to an acceptable length, you are making a smaller antenna resonate at the same frequency, eg one of the multiple harmonics of the wave length.

So to put into practise:

100Mhz Half wave antenna required;

(300 / 100) / 2 = 1.5 Meters

If you want a dipole (180 degree out of phase antenna), cut this length in 2, so you have 75cm on the antenna output and 75cm on the positive rail and point them in oppose directions.


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