Mobile FM transmitter

This telephone FM transmitter connects in series with your telephone line and transmits your phone conversation over the FM band when you pick up the handset. The transmitted signal can be tuned to any FM receiver. The circuit includes a "Broadcasting" LED indicator and a switch to turn the transmitter off. A unique feature of the circuit is that it is battery-free, as it is powered directly from the telephone line.  

 The transmitter uses only a short, wire antenna about 4 inches/10 centimeters long to transmit signals, and some of the radio frequency signals are also radiated through the telephone line itself.