Tuesday, 15 February 2011

The passive air-band receiver


The passive air-band receiver is a type of “crystal” radio which contains no local oscillator which might interferes with the on-board aircraft’s sensitive electronics. The passive aircraft receiver broadly tunes from 118 MHz to 136 MHz, and was designed to listenin

to in-flight communications between your pilot and the control tower. The passive aircraft receiver is shown in Figure . The passive aircraft receiver can be built small enough to place inside your vest pocket and it operates from an ordinary 9 volt battery. The passive aircraft receiver is basically an amplified type of “crystal radio” designed to receive AM aircraft transmissions. The “passive” design uses no oscillators or other RF circuitry capable of interfering with aircraft communications. This receiver utilizes a coil/capacitor

tuned “front-end,” which feeds an RF signal to a detector diode. The tuning capacitor may be any small variable with a range from about 5 pF to about 15 or 20 pF. The 0.15 ìH 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.) A 1N34 germanium detector diode or a Schottky diode like the 1N5711 or HP2835 should be used as the detector diode in this receiver circuit. The 10megohm resistors provide a small diode bias current for better detector efficiency.

The LM358 dual op-amp amplifier draws under 1 ma so the battery life should be quite long. Potentiometer R3 is used to adjust the gain to the second stage of the dual op-amp. The second op-amp stage drives a 100 ohm resistor in series with a 100 ìF capacitor. You must use a high impedance “crystal” headphone, since the op-amp output will not drive a speaker directly.

A speaker amplifier may be added to drive a speaker or low-z earphone, if desired, but the power consumption will increase sharply, as will the size of the receiver. The passive aircraft receiver is powered from an ordinary 9 volt transistor radio battery. The entire aircraft band receiver can be built inside a small plastic box. Simply mount an SPST slide switch on the side of the case to apply power to the circuit. If you elect, you can install a 1⁄8′′ phone jack for the “crystal”headphone. The antenna can be a 6′′ piece of #20 ga.

stiff solid copper wire or a small telescoping aluminum antenna connected to capacitor C1. Radio Shack has a good selection of whip antennas. You could elect to solder the wire antenna directly to C1 and just bring the antenna out the top of the enclosure, or you could install a small 1⁄2 ′′ mini jack for the antenna on the rear of the enclosure. 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. In order to “tune” the passive aircraft receiver you can either leave a small hole on the side of the plastic enclosure to adjust the frequency using a plastic tuning tool, or you need to find a way to attach a plastic knob onto the tuning screw; you will have to drill a larger hole in the case to bring the knob outside the case for easy tuning.

No comments:

Post a Comment