VHF FM Aircraft Receiver

VHF FM Aircraft Receiver is a superregenative receiver developed for listening to FM transmitters but also tunes the aircraft band and the top portion of the FM broadcast band. Receives both AM and FM (107mHz to 135 MHz). You can use this receiver with the any FM transmitter. The receiver is amazingly simple using only one transistor for the receiver section and one IC for the audio section. This circuit is a self-quenching regenerative RF receiver also known as a superregenerative receiver. A superregenerative receiver performs two basic functions. First it feeds back a portion of the received signal from it’s output in phase to its input; and second a super audible quenching oscillator drives the amplifier through the point of oscillation and maximum sensitivity and then quenches the oscillation repeatedly. This keeps the feedback from driving the circuit into self-oscillation and allows the signal to be regenerated over and over again. In this version of the circuit, both functions are performed by the circuitry associated with Q1. The rest of the circuit, shown to the right of L3 in the schematic, comprise the audio amplification circuit and are centered on the LM386 Audio Amp IC. In this configuration the LM386 is set at a gain of 200 and feeds it’s output to a standard 1/8-inch diameter stereo phone jack. The audio can then be heard by plugging any standard stereo headset into the jack.

The controls consist of C1, C2, R3 and R6. C2 provides the frequency selection. C1 provides fine-tuning, R3 adjusts sensitivity and R6 is the volume control.

Operation:
Connect a 9-volt radio battery to the battery connector and a stereo headset to the speaker jack. You should hear a hissing noise if your receiver is working. Tune the receiver by adjusting C2 with a non-ferrous tool. C1 can be used to fine-tune the circuit or to shift the tuning range of C2 (increase the capacitance of C1 if you want to receive FM broadcast below 107 MHz). With C1 In its highest capacitance position C2 will tune the receiver through the lower portion of the aircraft band and the top portion of the FM broadcast band. With C1 in its lowest capacitance position C2 will tune the receiver through the aircraft band and a little higher.

C2 is at its highest capacitance when the small dab of solder on the top of C2 is lined up with the middle pin. A good way to set the receiver to receive a signal from a 108 MHz transmitter like the FM108 tracking transmitter is to set C2 slightly to the right of its highest capacitance and then tune C1 until you hear the signal from the 108MHz transmitter.

Listening to the aircraft band

You may have to tune around a bit and listen for a while before hearing any aircraft transmissions. Pilot communications are generally short and to the point. Transmission is limited to a few seconds. Since VHF communications are “line-of-sight” you will be able to hear aircraft at 30,000 feet a100 miles away or more but may not be able to hear the control tower that is only 10 or 20 miles away if your view is obstructed by buildings or hilly terrain..

One thing to watch out for is strong local signals. The received signal may be garbled if the signal is too strong. This is usually the case when tuning FM broadcast band stations. To remedy this problem turn the sensitivity to its lowest point and put the antenna in the down position.

Radio Tracking the FM108 transmitter
Hold the radio with the antenna fully extended against the front of your body shielding the antenna with your body. Slowly turn while listening to the “beeping” signal from the FM108 transmitter. The beeping will be the loudest when you are facing the transmitter. If the “beeping” is loud when facing all directions then lower the antenna to the point where you can barely hear the signal when facing in one direction. Continue to reduce the length of the antenna as you approach the transmitter.

Why doesn't the VHF1 receiver use a directional beam or yagi antenna??
A directional beam antenna for this frequency would be to large for field work. Such an antenna would be over four feet wide and would present a considerable problem moving through brush and trees. The relative short telescoping antenna when shielded by the operator’s body works very effectively for direction finding and is relative easy to work through heavy brush and trees. This technique is sometimes referred to as "Body Fade" and it produces a cardioid sensitivity pattern (see picture). The peak null position is exactly 180 degrees opposite the transmitter.

As mentioned above, the operator should take a reading by holding the receiver against their stomach (the edge of the receiver opposite the antenna should be touching the stomach area). The antenna should extend vertically and be about 6 inches in front of the face. Turn slowly listening for the strongest signal (loudest beeping). Move in the direction of the strongest signal (opposite the direction of the weakest signal). Periodically stop and take another reading, adjust the course and continue to work towards the transmitter. Since the “null” point is much narrower than the maximum signal point it may be easier to use the “null” point to establish the most accurate direction to the transmitter. With a little practice a person can become quite efficient in locating the transmitter although the path taken will be somewhat zigzagged.

Components:
L1 .12 uH coil
L2 .15uH
L3 .68 uH coil
L4 .82 uH coil
Air wound; 1.5 turns
on 3/8 form
#26 insulated wire
R1 680 ohm
R2 33K
R3 50K pot
R4 10K
R5 6.8K
R6 5K pot
C1 2 - 5 pF trimmer
R7 10 ohm
C2 2.5-12 pF trimmer
C3 22pF
Q1 NTE 108 Transistor
C4 6.8pF
IC1 LM386
C5,C11 .002 uF
C6 .001 uF
C7 1uF
C8 1000 uF
C10 .047 uF
C12 10 uF
Antenna
C13, C15 0.1 uF
C14 220 uF

VHF FM Aircraft Receiver

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