VHF2 Tracking Receiver

(designed for use with the FM108 series transmitters)

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The plans that follow will lead you through the assembly of a reciever designed to be used with FM 108 MHz tracking transmitters.   This receiver when used with the “Beeping tracking Transmitter (108 MHz)“ makes a good transmitter locating system suitable for model rockets, balloons and assorted other objects.

 The kit contains a Printed Circuit Board (PCB), all electronic components and an antenna.  The builder will need to provide a 9 volt radio battery and a headset.  The receiver is designed to be used with the common stereo headsets used on most portable radios, CD players and MVD players.  It should have a 1/8 inch diameter plug.

 Construction is simple and reliable if reasonably good soldering practices are followed.

 Circuit Description:

This circuit is a self-quenching regenerative RF receiver also known as a  super regenerative receiver (fig 1).  A super regenerative receiver performs two basic functions.  First it feeds back a portion of the received signal from its 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 its 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 C2, and R6. C2 provides the frequency selection.  R6 is the volume control.


 Construction is made simple and reliable by utilizing PCB construction and several fixed inductors.  Only one coil needs to be hand wound.

 Please refer to Fig 2 "Component Placement:".

 NOTE:  Take you time and use good soldering technique in order to prevent solder bridges and “cold”solder connections.  Use a 25 to 40 watt soldering iron with a pencil sharp point; trim all leads after soldering and check each solder joint with a magnifying glass for bridges and solid connection. 

 1.      Please refer to the "Component Placement" illustration when installing parts.  All components are installed on the blank side of the board (not the foil side).

2.      When installing components bend the leads to match the hole spacing.  Push the leads through the holes at the proper location being careful of polarity on the IC and transistor.

3.      Solder the leads on the foil side of the board being careful not to produce solder bridges between the foil tracings.  Trim all excess leads immediately after each component installation.

4.      Assemble the circuit by starting on the bottom left side of the PCB and work your way to the right being careful to place the components in the correct positions. Common component markings are list in parentheses after each part.  Abbreviations s are:
                  BK= black
                  BR= brown
                  RD= red
                  OR= orange
                  YL= yellow
                  GN= green
                  BU= blue
                  GY= gray
                  SL= silver
                  GD= gold

pF = pico Farad
uF = micro Farad
uH = micro Henry
PCB = Printed Circuit Board

5.      Install L1  (SL/GN/BR/RD/SL) 0.12 uHcoil

6.      Clean ¼ inch of the lacquer from both ends of L2 (1 ½ turn air wound coil) using a knife blade, file, or emery paper.  Clean to the bottom of the coil winding.

7.      Next install coil L2. If it is not already wound or if it has become malformed you can form it by winding 1 1/2 turns of #22 wire on a 5/16 inch form.  You can wind the coil on the shaft of a 5/16 inch drill bit and then remove. The coil does not have to be exact but needs to be close. Be sure to clean the ends of the coil wire before soldering to the PCB.  The spacing between the loops will be approximately 1/16 inch.

8.      Install C2 ( not marked  ) 3 -20 pF adjustable capacitor.

9.      Install C3 (22) 22 pF capacitor

10.  Install Q1 (NTE108).  Make sure that the flat side is facing towards the “top” of the PCB.

11.  Install C4 (6.8) 6.8 pF capacitor

12.  Install C5 (202) .002 uF capacitor

13.  Install C7 (105 or .1) 1 uF capacitor  (“+” towards middle of PCB and “–“ towards the edge)

14.  Install L3 (SL/GN/BU/GY/SL) .68 uH coil

15.  Install R1 (BU/GY/BR/GD)  680 Ohm resistor

16.  Install C6 (102) .001 uF capacitor

17.  Install R3 (OR/OR/OR/GD ) 33K Ohm resistor

18.  Check your work.  All holes on the bottom left quarter of the PCB should have components in them

19.  Install L4 (SL/GN/GY/RD/SL) .82 uH coil

20.  Install R2 (OR/OR/OR/GD) 33k Ohm resistor

21.  Install R4 (BR/BK/OR/GD) 10K Ohm resistor

22.  Install C11 (202) .002 uF capacitor

23.  Install R5 (BU/GY/RD/GD) 6.8K Ohm resistor

24.  Install C12 (10uF) 10uF capacitor. “+” side toward top (battery side of PCB)

25.  Install IC1 (386).  Install with PIN #1 to the bottom left.  See illustration for pin location.  Pin “1” will have a dot on the case over its location.

26.  Install C10 (473) .047 uF capacitor.  May need to straighten leads to fit hole spacing.

27.  Install C13 (104) .1 uF capacitor

28.  Install R6 (potientometer). you will need to use moderate force to push the leads through the PCB holes.

29.  Install C14 (220uF) 220 uF capacitor.  “+” side towards edge of PCB, “-“towards center of PCB.

30.  Install C15 (104) .1 uF capacitor

31.  Install C16 (104) .1 uF capacitor

32.  Install R7 (BR/BK/BK/GD) 10 Ohm resistor

33.  Install C8 (1000 uF) “-“ toward edge of PCB, “+” towards center of PCB.

34.  Install J1 the phono jack.  You will need to use moderate force to push the leads through the PCB holes.

35.  Install SW the switch. You will need to use moderate force to push the leads through the PCB holes.

36.  Install H1 the battery holder to the foil side of the PCB using two of the small screws.  Using a little force screw them through the battery holder and into the PCB. Note that this part is mounted on the foil side of the PCB

37.  Install the battery snap connector.  The red wire goes into the hole next to the “+” sign and the black wire goes into the hole next to the “-” sign.

38.  Install T1 the antenna.  Push and turn the antenna screw through the PCB from the foil side and screw into the base of the antenna.  Make this a tight connection.

39.  Install the battery and connect it to the battery snap.

40.  Check the operation of the radio.  Plug a headset into the headset jack and turn the switch to “on” (lever toward the edge of PCB).  You should hear a rushing sound if the circuit is working.   Using a small screw driver turn the screw on C2.  You should be able to tune FM radio stations above 107 MHz and may hear aircraft communications or any nearby NOAA weather radio stations.  If this doesn’t work recheck all part placements and solder connections.

41.  Cut out the drill guide 

42.  Center the drill guide on the top of the case lid.  Secure with some tape.

43.  Place a wood block on the underside of the case lid and then lightly center punch the centers of each of the holes as marked on the “hole guide” taped to the top of the lid.  You can use a 4” piece of 2x4 or 1x4 board for the block and the sharp end of a nail as the center punch.  Remove the “hole guide” before drilling the holes.

44.  With the wood block still supporting the back of the lid drill holes in the lid with the bit sizes indicated on the “hole guide”.  If you do not have the exact drill size you can drill the holes a little smaller and use a round file or tapered grinding stone to enlarge to the desired size. A hobby motor tool works well for doing this. Note that the metal is very thin and will tear easily if you do not drill the holes very very slowly and very carefully.

45.  Push the insulation sleeve (small tube ¾” long) over the adjustment shaft of C2 carefully pushing it all the way to the surface of C2.  You will need to use moderate force to push the sleeve over the capacitor screw.

46.  Fasten the assembled PCB to the lid as shown in the diagram.  Note that you may need to push the jack, antenna, potientometer and switch a little to get them lined up with holes.

47.  Note that the large knob pushes on to the shaft of R6 and the small knob has an allen screw that needs to be tighten after slipping over the insulation sleeve extending from C2.  Note the allen screw needs to be tight against the sleeve or the knob will not be able to turn the sleeve and the capacitor tuning screw.

48.  Slide the larger plactic sleeve over the antenna and through the lid.  The antenna should not touch the lid.

49.  This completes the assembly of the radio

A complete parts listing is included (table 1) with Mouser Electronics (www.mouser.com) part numbers.  Mouser is not the only source for the parts; in fact the parts are readily available from most electronic parts supply companies.


 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 (small knob).   

 Turn on your FM108 transmitter and place several feet away from the receiver. Adjust C2 (small knob) until you hear a strong signal.  Note that there may be more than one spot that you will receive the signal so tune to the spot with the loudest signal.

You should also be able to hear a couple of FM broadcast band stations and possibly some aircraft communications as well as the FM 108 transmitters.

 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.

 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. 

 Note that the strongest signal will be received from the FM108 transmitter when the VHF1 antenna is in the same plane as the transmitter’s antenna.  Since the transmitter’s antenna position is not always known the user should initially try orienting the VHF1 antenna is both a vertical and a horizontal plane to determine which way works best


.Listening to the aircraft band

The VHF2 receiver is not primarily designed as an aircraft band receiver but it does cover the entire aircraft band frequency.

 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


Component Side View

Table 1




Mouser Part Number













.12 uH coil




680 ohm




Air wound; 1.5 turns
 on 3/8 form
#22 insulated wire






.68 uH coil







.82 uH coil












291- 6.8K






10K pot



Not used




10 ohm



3-20 pF trimmer











NTE 108 Transistor











.002 uF







.001 uF














1000 uF




stereo jack



.047 uF




battery clip




Not used



battery snap



10 uF








0.1 uF




Screw for antenna


Small screw


220 uF





5006-sw (from MPJA)




















Spacer #4 1/2







Nut 4-40





















Insulation sleeve

Small tube





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Copyright ©2011 Jerry Baumeister
Revised - 9-23-02, 4-24-2011