AM VHF PASSIVE AIR-BAND RECEIVER The passive or non-emissive Airband Receiver is basically an amplified "crystal radio" designed to receive nearby AM aircraft transmissions (121 - 133 MHz). The "passive" design uses no oscillators or other RF circuitry capable of interfering with aircraft communications; so it should receive on board VHF "comms" from within the cabin of the aircraft. Useful for "listening-in" to the pilot if you are on a commercial flight. Because of low sensitivity, it is a short-range receiver only. You cannot 'pick-up' two-way aircraft transmissions with this receiver! The input tuned cct 'L' is a 2 turn loop, 30mm dia (measured at 0.15uH on my L-C Meter ) which intercepts RF directly as opposed to an LC cct fed with external aerial Tuning capacitor is a 30pF Philips Beehive trimmer, with a short length of plastic tube glued - as a tuning shaft. Capacitance (meas' on LC meter) runs from 28 to 7pF; which by formula gives a freq' range of 77 ~ 155MHz Detector uses a biased 1N5711 (or similar) schottky diode (lowest forward-biassed voltage drop). The two 10M resistors bias the detector diode and the op-amp input near mid-rail for better detector efficiency The LM358 dual op-amp draws less than 1 ma so the battery drain is minimal. Insertion of earphones plug completes supply circuit and therby acts as an on/off switch. The componentry (inc battery) fits neatly inside a 30mm PVC tube 130mm long I acknowledge inspiration from the following webpages: Passive Aircraft Receiver Non-Emissive Airband Receiver Non-Emissive Air-Band Receiver (Implementation & Use)