NoGaWaTT Power/SWR Meter
I was in need of a good power meter for QRP and QRPp levels, since having one would make it easy to tune up homebrew transmitters. It would also come in handy for monitoring power output while operating. I had heard of the NoGaWaTT power meter kit offered by the North Georgia QRP club, and so I took a look at the kit on their webpage. It seemed pretty straightforward and easy to build, and looked like just what I needed so I ordered one.
The kit consists of a PC board, two toroids, two germanium diodes, two 1 mH chokes, four .047 uF capacitors, four 100 Ohm resistors, two 25K pots, and two meters. The North Georgia club even includes four 200 Ohm 2W resistors (or some kits will instead have 6 300 Ohm resistors) to hook up as a dummy load. From my junk box I added two more 25K pots and two DPDT slider switches from old CB's, for which we'll see later.
The NoGaWaTT kit when completed will fit inside an Altoids box as many kit constructors have done, but instead I chose to use an enclosure from an A/B computer switchbox. It is considerably larger than an Altoids tin, but just large enough for the project. I was able to mount the meters on the front panel and a couple of SO-239 jacks on the rear panel. I also mounted the switches in the front. I mounted the boards in the bottom of the enclosure with double-sided foam tape.
The NoGaWaTT kit comes with two meters, one for forward power and one for reflected power. The kits even come with printed labels for the meters. All one has to do is cut the label out and attach it to the meter face behind the needle. It's very important to take care doing this. I found that it was easiest to peel the backing off the label, hold it with tweezers, and carefully place it on the face of the meter behind the needle, being sure to line up the 0 reading line with the needle. The NoGa QRP club is kind enough to supply two sets of labels (marked Forward and Reverse) for the meters in case a label is ruined during installation.
The kit only comes with one 25K pot per meter, so the stock kit will either be able to read only 6W or 2W output, whichever you calibrate it for. However, it is a simple matter to add a DPDT switch and two more 25K pots so that you can switch between reading the 2W scale or 6W scale. This allows you more accurate readings for QRPp levels when reading the 2W scale. I salvaged a couple of 25K pots from my junk box and mounted them along with the original pots on a strip of PC board, which I mounted into the enclosure next to the NoGaWaTT board. I did this because there wasn't a great deal of room on the NoGaWaTT board to solder in two pots. I would have also had to cut traces on the board in order to attach them to the DPDT switch and four separate pots. Instead, since I had plenty of room to work with, I chose to mount the pots separately and wire them together. I used computer ribbon cable to wire the switch to the boards.
As I stated before, the kit comes with resistors which make up a 50 Ohm dummy load. There's no space on the board to mount them, so I took a small piece of PC board, cut a groove down the middle, and soldered the ends of the resistors in parallel to each end. I installed this next to the NoGaWaTT board and connected it on one side to one of the slider switches. The other side of the dummy load went to ground. On the other side of the switch, I connected the antenna jack. The middle of the switch (common) was connected to the ANT connection on the NoGaWaTT board. This allows me to switch between an antenna or dummy load without having to attach an external dummy load to the meter.
The rear of the A/B switch box enclosure had holes left over from where three DB-25 jacks were installed. I cut out a piece of scrap sheet aluminum to cover the holes which is held in place by the SO-239 jacks. I painted the top half of the enclosure with black hammered texture paint. For the bottom part of the case, I used a pencil to mark where the meters would go, and then where I would need to drill holes for the slider switches. Once all the holes were drilled or cut, I painted the case with flat white spray paint. I used rub-on lettering to mark the switch positions and TX and ANT jacks, and then oversprayed the case with clear spray paint to protect the lettering. I replaced the four rubber feet on the bottom which I had removed prior to painting.
The NoGaWaTT kit is a very versatile kit. You can build it to be only a power meter using just one of the meters connected to the forward power side of the board, or you could use it as just an SWR meter showing only the reflected power. Or you could use just one of the meters and have a switch to switch between forward and reflected power. Or, you could connect both meters to the forward power, using one to show 2 Watt readings and the other 6 Watt readings. The kit can even be integrated with an antenna tuner. There are many possibilities.