A very nice simple and useful circuit diagram of an FM transmitter is sown in this schematic . This fm transmitter circuit is very simple and it has a acceptable transmission . The signal transited from this fm transmitter circuit can be received at almost 300 meters in open air The circuit require a 3volts operating voltage and can be tuned anywhere in the FM band.
You can use this rf transmitter circuit to transmit signal from your house to garden or from room to room . To listen the signal you can use any radio (portable or not ) that can work on FM band .
The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in diameter.
After the coil in soldered into place spread the coils apart about 0.5 to 1mm so that they are not touching. If you don’t have a trim cap you can use a fixed value capacitor and you can vary the TX frequency by adjusting the spacing of the coils or placing a small piece of ferrite inside the coil , but the better way to change the transmission frequency is to use a variable capacitor .
Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively.
The calibration of this rf transmitter circuit is very simple and you need just to place a radio at some distance from the transmitter and set it somewhere about 89-90MHZ( chose the transmission frequency ) and after that vary the transmitter oscillator frequency , by modifying the value of the capacitor . The transmission frequency is set to the desired frequency just when you can hear the transmitted signal.