Here is a FM transmitter design which can accept audio signal input from either microphone or audio input jack. The electret microphone can accept audio input from user talking into the microphone. With the audio jack, one can input audio signal from mobile phone or PC into the FM transmitter. The audio input selection is done using a switch indicated as SW1 in the circuit diagram below. Also this transistor has a tuning capacitor to tune to different FM frequency. This is done using the variable capacitor VC1.
The following is the schematic diagram.
This FM transistor can be easily build at home with DIY PCB which is illustrated in the tutorial How to make FM Transmitter.
Consider building LC oscillator for100MHz FM transmitter. If inductor is 0.25uH then capacitor is 10pF. This can be calculated using online LC circuit calculator. The base to collector capacitance is 2 to 5pF. A trimmer capacitor of 30pF can be used. The inductor with 0.25uH inductance is a #22 gauge wire coil with 7 turns having diameter of 1/4inch or 6.35mm diameter. To build an inductor you can use the air coil online inductance calculator.
The
FM antenna is build such that it is 1/10th of the wavelength of the
signal. For 100MHz, the wavelength is about 3 meters so 1/10th of it is
30cm.
