Tuning the Oscillating Circuit

Commonly known as the 'Tank Circuit'

I will refer to the 'tank circuit' in the 30mW FM Transmitter, since this is where most novices start out making a VHF unit. Although this tank circuit is the same exact one in the other two transmitters...the 200mW and the 7 Watt.

Understanding The Components in the Tank Circuit

The tank circuit is made up of a variable capacitor (5-30pF) and a 'tapped' Air-Core Coil. With just these two devices making up the tank circuit, one can alter (change) the transmitting frequency of the 30mW transmitter.

There are three different ways to alter the frequency

1. You can alter the frequency by changing the value of the variable (trimmer) capacitor.

2. You can alter the frequency by stretching or sqeezing the 'tapped' air-core coil.

3. You can also alter the frequency by manipulated both.

The 'Resonant Frequency'

The resonant frequency is the 'main' frequency of the transmitted signal. There is only one 'main' signal. Other frequencies that radiate off the 'main' signal are called 'Harmonic' frequencies. These harmonic frequencies are always associated with any transmitted signal.

Tuning In The 30mW FM Transmitter

With a non-metallic tool needed for turning the knob on the variable capacitor and the 'tapped' air-core coil pre-stretched at 7/16 of an inch, you are now ready to have the unit transmit at your desired FM frequency...the 'tank circuit' has been set to transmit a signal around the 88 Mhz. With a FM receiving radio about fifty feet from the transmitter and with the volume control set to about 1/3 to 1/2, and the radio tuned to around 88 Mhz (make sure you have good 'clean' static...if an FM station happens to occupy that frequency, slightly tune the radio just to where the stations fall off and you have good 'clean' static), go back to the transmitter. Now turn the transmitter on.

With you non-metallic tool, start turning the variable capacitor slowly while you whisper real close in the electret microphone. Soone or later you will catch your whispering voice on the FM radio. I do believe there are two different places on the variable capacitor where your voice will be heard....decide which of the two places sounds the best. If they both sound the same...use either one.

Once you have spotted your position on the variable capacitor, you may 'fine' tune the voice by slightly sqeezing or stretching the coil itself. The variable capacitor cannot be fine-tuned as well as adjusting the coil. The variable capacitor is there to 'catch' the whispering voice in the FM radio...but after that, it would be wise to start stretching or sqeezing the 'tapped' air-core coil for getting the voice sharp and clear.

Once all this is done and are happy with the results...you may then proceed to find out much output power is being dissapted by the transmitter. Refer to the main webpage for a link on how to make and apply a 'power meter' to the 30mW FM transmitter.

...and let the project continue!

...your friend, Patrick