There are many techniques to make an FM transmitter stable so it will not change its frequency as the battery runs down. The rf circuit shown below is a mini stable FM transmitter which is using a zener diode to provide accurate voltage to the oscillator while the battery runs down, due to which it will
not drift its frequency. The circuit is simple using few components but provide good range. The coil L is equal to 8 to 10 turns of #22 enameled wire wound on 6.30 millimeters diameter form. The input voltage can be 9 or 12 volts DC. Keep all the components as near as possible and use 12 inch wire for antenna or use 30 inch wire antenna for greater range. If you require a long range with this circuit use diy fm antenna.