I set up a small FM transmitter and to increase the range, I put a longer antenna. But instead of increasing the range it was unstable. What happened?
Most small FM transmitters use an oscillator stage directly connected to an antenna. This means that the antenna, unless it is very well dimensioned, "carries" the circuit affecting its frequency. Thus, in these transmitters should be used a short antenna (maximum about 30 cm) so that it does not load the circuit and cause instability. You must realize that the simple approach of your hand or your movement, causes the frequency to change. So there is no use to increase the size of the antenna in these transmitters, for what is achieved is instability in range increase of place. The antenna can be increased up to ¼ of the wavelength where the circuit, in addition to the oscillator stage, also has an RF amplification stage.