Some compact, high-power VHF transceivers get very hot when transmitting and can benefit from forced-air cooling. The simple circuit shown in Fig helps such radios cool themselves. It senses RF leakage from coaxial cable and turns on a cooling fan whenever the radio is transmitting.
Inductor Ll consists of 10 turns of insulated hookup wire-H2O to 24-wrapped tightly around the coax and taped into place. You need not alter the coax in any way. (Although my test circuit worked well with 10turns, your version may work better with more or less turns. You can optimize Ll by connecting a voltmeter across Rl and adjusting Ll for a peak read ing while transmitting.) Dl rectifies the signal. Cl filters the resulting de, which is applied to Ql's base to turn QI on. RI keeps QI turned off during no-drive periods by pulling Ql's base to ground.
The circuit has other uses. For instance, it could drive a TRANSMITTER ON indicator to warn you that your mobile mike is jammed between your car's seats with its push-to-talk button held down. Or, combined with a timer, it could serve as part of a transmitter-time-out warning circuit.
Author - Jay F. Hamlin, WB6HBS