KC8AON's QRP PROJECTS

MINIATURE SWR/PWR METER CIRCUIT

AN SWR CIRCUIT THAT WILL FIT INSIDE MOST QRP RIGS

BUILD THIS LITTLE CIRCUIT INTO YOUR NEXT TUNER PROJECT !

After building my Z-match tuner, I wanted to design a circuit board for a Stockton type swr/pwr meter that I could reproduce easily and see how small I could make it. The result is in the above picture. It is a board that measures a scant 1" x 1.5" and is made from common single sided copper clad board. I drew the lines on the board with a pencil, and then used a Dremel tool and an abrasive wheel to cut the traces in the copper that isolates the individual pads that the components solder to. The circuit is a variant of the Stockton type of bridge used in many commercial swr/pwr meters and kits these days. The design calls for a short piece of coax going through each toroidal sensor with the shield grounded at one end only to eliminate harmonic currents, but I am going to experiment with just an ordinary piece of insulated wire due to the fact that my circuit will be used at QRP levels only. Will let you all know how this works out once I get it all tested.

Figure 1 above shows the PC board dimensions and pad locations. You could etch this board, but it is so simple, it is real easy to just sketch the lines on the board using a pencil and ruler and then use the Dremel tool to cut the traces.

Figure 2 above shows the board with the parts location marked. As you can see, the design is straight forward and parts count is minimal. The circuit and board is so small that it will fit inside many QRP rigs. The only thing wrong with that is the fact that it may be hard to find a panel meter small enought to fit the rig. The circuit will fit easily inside an Altoids tin, and there are many old CB radios out there that you can rob the meter out of that is small enough to fit the Altoids tin also. You can also fit this circuit inside that QRP antenna tuner you have been planning on building ! It's the same circuit that I built in my Z-match tuner, except I used a Radio Shack general purpose perf board for that one.

And finally, here is the schematic for the swr/pwr meter circuit. T1 & T2 in most Stockton type bridges call for an FT50-61 core, but I used the smaller FT37-61 cores to save space, and have had no problems with them when using QRP power levels. T1 & T2 each have 12 turns of #24 enamel wire, and the secondary is usually a short section of coax with the shield grounded at one end only to form what is known as a Faraday shield that helps eliminate harmonic currents that may cause the meter readings to be inaccurate. But like I said, since I am building this one to be used at QRP levels only, I decided to experiment with just an ordinary piece of insulated wire in place of the coax just to see how it works. If it works ok, it will make the reproduction of this circuit much easier in the future because the coax seems to be the hardest part of the circuit to deal with. At least is is for me anyway ! The 50 ohm resistors in the circuit are made by connecting two 100 ohm 1/4 watt carbon resistors in parallel, so you will need 4 100 ohm resistors to make 2 50 ohm resistors unless you can find some 1/4 watt 50 ohm resistors. I think you can get 51 ohm resistors, and if you get the 5% tolerance resistors, that should be close enough and since you will only need 2 of them, it saves even more space on the circuit board. The schematic only shows one meter with a switch to select either forward or reflected power. You can eliminate the switch and use 2 identical meters if you have them and monitor both forward and reflected power at the same time. Calibrating the meters is done by adjusting the variable resistors on the board. I usually just set mine to read full scale for the power level that I am using on the forward side, and set it to just read zero on the reflected side with full power applied and a 50 ohm dummy load connected to the output and use them for a relative reading instrument. I'm really not worried about accurate measurement of forward power, and on the reflected side, if I tune until the meter reads zero, then I know that I'm real near 50 ohms and that's all I'm looking for. Of course, you could calibrate yours anyway you want !

Here's a picture of one of my very first power meter projects that I built way back in 1996. It uses the same type of circuit shown above, but is on a much larger scale and will handle about 500 watts max. you will notice that there is a "QRP" switch on the front panel. With this switch in the QRP position, this meter will read full scale with about 6 watts applied. It's just too big to pack around for portable use though ! It's the meter that I used with my first QRP rig though, my old Heathkit HW-8 ! I got lucky with this one, a friend knew that I liked to build things and gave me the 2 meters for this one - they are a matched pair from an old CB test set !

73 !