28.5.
2021 Aircraft Receiver
Passive Aircraft Receiver
The Passive Aircraft Receiver is basically an amplified "crystal radio" designed to
receive nearby AM aircraft transmissions. The "passive" design uses no oscillators or other RF
circuitry capable of interfering with aircraft communications so it should be fine inside the
cabin of the aircraft. Nevertheless, check the regulations before using this receiver on a
commercial airliner. New security regulations probably prohibit this device on commercial
flights. Do not expect to hear two-way aircraft transmissions with this receiver! It is a short-
range receiver only.
The detector diode is a 1N5711, HP2835 or similar Schottky detector diode. The 10 megohm
resistors provide a small diode bias current for better detector efficiency. The tuning
capacitor may be any small variable with a range from about 5 pF to about 15 or 20 pF. The
0.15 uH inductor may be a molded choke or a few turns wound with a small diameter.
Experiment with the coil to get the desired tuning range. The aircraft frequencies are directly
above the FM band so a proper inductor will tune FM stations with the capacitor set near
maximum capacity. (The FM stations will sound distorted since they are being slope
detected.) Other capacitor and inductor combinations may be selected to tune other
bands if desired. (Try the CB band at 27 MHz.) The LM358 dual op-amp draws under 1 ma so
the battery life is quite long. A speaker amplifier may be added to drive a speaker or low-z
earphone. The antenna can be a couple of inches if the receiver is near the transmitter or a
couple of feet for maximum range. The selectivity is reduce as the antenna length is
increased so best performance is achieved with the shortest acceptable antenna. Try
increasing the 1.8 pF capacitor value when using very short antennas and decreasing it for
long antennas. The receiver could be built into a small plastic box with a short antenna
inside.
See the Amazing All-Band Receiver for additional circuit ideas.
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