THE FOX750 - 750 milliwatt transmitter

Joe Leggio WB2HOL

This three transistor 144 MHz transmitter uses a 48 MHz scanner crystal as an oscillator / tripler. A second transistor boosts the signal to 40 milliwatts, a third amplifies the power output to 750 milliwatts.

The scanner crystal I used was actually for a receive frequency 10.7 MHz higher than the two meter transmitter output. It was marked 156.21 MHz. (When you subtract the 10.7 MHz IF offset, it resulted in a transmit frequency of 145.51. Since this circuit is not the exact same circuit as the scanner, you may find that the crystal oscillates somewhere close to the marked frequency. The crystal marked as 156.21 MHz I used in this transmitter actually resulted in a signal on 145.53 MHz. )

I did get the salesperson at the local Radio Shack store a bit confused when I asked to look at his box of scanner crystals. The first thing he asked was which service I was trying to listen to... Police, Fire, Ambulance, etc.... When I said that I was looking for something for a homemade project and did not have a specific frequency in mind he understood. I did not try to tell him the crystal was to be used in a transmitter. He probably would have told me that I can't use a "receiver" crystal for a "transmitter."

As I looked at the available crystal selections, I simply took my calculator and subtracted 10.7 MHz from the marked receive frequency. I was lucky; several crystals ended up in places on the two meter band where a fox could transmit without interfering with other users. If you are not lucky, or live in an area where you must put your fox on a specific frequency, you can order a crystal through Radio Shack. Since there are still quite a few crystal controlled scanners out there, the odds are that someone in your town will have something you can use. Be aware, although most scanners use a 10.7 MHz IF, there are some with a 10.8 MHz IF. In that case, you may end up 100 kHz from where you expected.

You will need to wind several coils to build this transmitter. They are all self-supporting air-core coils made from enamel covered wire. If you follow the instructions illustrated here, you should not have any difficulty. The top of my workbench is littered with coils I have wound and tried in various circuits. I specifically avoided tapped coils or multi-winding coils to keep construction as easy as possible.

I use a simple diode detector across a 50-ohm dummy load to measure the power output while tuning up this transmitter. I did try about 5 or 6 different prototype circuits as I designed this transmitter; many were not stable. Their RF output did not smoothly change as the trimmer capacitors were adjusted but instead jumped as spurious oscillations occurred. This design is reasonably stable. It does not have a tendency to break into spurious oscillations. I used an oscilloscope to monitor the output waveform and confirm the lack of spurious oscillations. This is not really necessary as you can usually hear the instability if you monitor the output signal with your 2-meter rig.

Style of PC board construction

I built all of the transmitters using copper clad PC board as a ground plane and 1/4 inch squares of PC board as tie points. The tie points were cemented to the ground plane using "crazy-glue." This technique made constructing the transmitter go very fast. It only took a drop of glue to secure the tie points. The glue set up in seconds. It made it easy to experiment with different circuits. In a pinch, a tie point could be removed or relocated by prying upward on its side with a screwdriver. I built three transmitters using this style of construction in the past month. All have worked exactly as expected. I used a pair of shears to cut a number of 1/4 inch squares of PC board material. I then used a wide file to remove any burrs created by the shear. This allowed the square of PC board material to lay flat against the PC ground plane. (Crazy-glue is not designed to fill voids. It works best when the surfaces to be glued fit together well) Be careful as you work with "crazy-glue." It is very effective at gluing fingers together.

Tie-points are made from 1/4 inch squares of PC board material
glued with "crazy-glue" to a solid ground plane of PC board.

As I had already constructed several low power versions of this transmitter, I first glued squares of PC board material for the first two stages to the PC board ground plane. I then proceeded to add the components. I saved the output stage for last as I had not yet decided upon the interstage and output coupling curcuit design. The design I finally used was a combination of several from the ARRL handbook. I tried to use parts that could be obtained with little difficulty.

Construction is under way. The oscillator stage was constructed first and tested before proceeding.

Winding the Coils

You will need to wind several coils. I kept these as simple as possible. The wire I used was recycled from old transformers. All the coils are wound in a single layer using a drill bit as a winding form. To help you visualize how to form the coils, I have included an illustration showing how the coils are wound..

All of the coils are "close wound" with "air cores." I used a drill bit as a form and then slipped the coil off of the bit after I had cut the free ends to about 1/4 inch. I used a hobby knife to scrape the enamel paint from the ends of the wire before I removed the coil from the drill bit used as a form. This made the coils easy to solder. With any luck, you too will be able to wind your own coils for this project.

Enamel wire is close-wound on a drill bit to form the coils for this transmitter. It is easier to cut
the leads to the coil and strip the enamel from the ends prior to removing the coil from the drill
bit used as the form.

Two different gauges of enamel covered wire are used for the coils in this project. If you have wire reclaimed from an old transformer, or from an old television deflection yoke, you can determine its gauge by winding a single layer on a drill bit. You will be able to fit about 46 turns per inch if the wire is 24 gauge. You will be able to fit about 37 turns per inch if the wire is 22 gauge. If you can't find an old transformer with the correct gauge wire to recycle, the best places I have found to buy enamel covered wire are shops which specialize in rewinding electric motors. In a pinch, if you live in the USA, Radio Shack sells "magnet wire" in both 24 and 22 gauge. You will only need a few feet of each gauge wire for this project.

I used the same transistor for both the oscillator and the amplifier stage. It is important that the transistors you use in your transmitter have sufficient gain at 2 meters. The 2N5770 is one of many I could have used. Please note: a 2N3904 would probably work fine as the oscillator (at 48 MHz), it would not work well as the amplifier as it does not work well at 144 MHz.

You will need to use a heat-sink on the output transistor. I used a small press-on style. I gets warm to the touch after transmitting for a few minutes. If it gets very hot, you probably have something mis-tuned or the circuit is oscillating on it own If you follow the layout shown, you probably will not have any difficulty. If you try a different layout, be sure to keep the input and output coils of the various stages away from each other. It is too easy to create an accidental feedback path. If this occurs, the transmitter will be very unstable and almost impossible to tame. If you are experienced with VHF design, you may want to try winding the coils in toriod coil forms. Because toriods are self-shielding, you may be able to try other layouts without as much concern towards stability. Obviously, if you do this, you will have to determine the number of turns needed for the toriods. A grid dip meter would be helpful in that case.

When building the output stage, I realized that I needed to bend the base lead of the output transistor between the emitter and collector in order to install it properly.Although it did not affect performance, it did make soldering the transistor to the PC board pads a bit more difficult. If I build another version of this transmitter, I will lay out the board a bit differently so the transistor will fit better to the pads.

The tune-up of this transmitter is easy. Simply adjust each of the trimmer capacitors for maximum RF output to the antenna. (The trimmer in series with the crystal is the only exception, it is used to trim the transmitter frequency, use a frequency counter to adjust it)

Power output was between 750 milliwats and 900 milliwatts. I will probably add a three terminal voltage regulator so I can vary the output power for any specific hunt. As an experiment, I tried voltages as low as 5 volts. The power output was down to 100 milliwatts but the transmitter stayed on the air! This design works well even as the battery gets weak.


Back to RDF Projects page

Copyright 1998 - Joseph Leggio - all rights reserved.