Oak Hills Research OHR WM-2 Watt meter

I'm in the process of building an OHR WM-2 QRP Watt Meter from Marshall Emm's Oak Hills Research (part of Morse Express).

The quality of OHR kits is fantastic, as was demonstrated on their OHR RFL-100 dummy load (see my build here.

I was so impressed that I ordered a few more kits to get into making some more useful and fulfilling projects for ham radio: watt meter, digital counter/frequency display and 20m CW transceiver.

Building up kit-making experience, the dummy load was a very gentle intro to following instructions and soldering. No real challenges there.

Moving onto the WM-2 Watt Meter, the supplied kit is again of very high quality. The aluminum case is robust, light, well-finished and very accurately cut/stamped. Rather than using paint, the facia of the watt meter is a thick plastic sheet with high strength adhesive. No chance of this peeling off with time, as long as you cleaned the aluminum case surface well enough.

What follows is a visual diary of the build. The instructions from OHR are first-rate, very clear, well organised and complete with inventory list of every single part (right down to #4 lock washers,... all my components were in the box without fail) and circuit diagram.

The first component - a resistor

High quality PCB with screen printed component shapes and labels, and solder mask

Diodes go in. The 3 critical germanium diodes are matched for forward voltage bias by OHR. My 3 agreed to within a 2mV error.

ICs are used for the bridge and op amp for the meter needle. They are soldered directly to the PCB without IC holders.

Trimmer pots are added to calibrate and align the meter need full deflection at specific voltages. Three of them are standard kit items, used to adjust the meter need full deflection points. The fourth is used to adjust a voltage drop to an accurate value, down to the millivolt. I have read from other builders that the standard trimmer pot (1 turn) is a bit time consuming to achieve the correct millivolt readings. So, I installed a 25 turn trimmer pot in its place, hoping to control the resistance, and hence required voltage drop, with ease to the required millivolt accuracy. This is seen in the photo above as the blue rectangular item.

My very first hand-wound toroids. I remain uncertain at the time of writing if the coils extend far enough around the toroid ferrites. In this photo they rest at approx 2/3 the circumference. 

Finished toroids and coax cable signal sampling. Together, they form transformers that tap the RF power flowing through the meter. There are two transformers, once for  forward and the other for reverse power. Marshall Emm's technique for tinning the enameled wire works well: apply solder iron tip to heat the end of the wire and apply a glob of solder. Keep the iron in contact and the enameled coating with bubble up through the solder glob. Run the iron down the wire, feeding solder as needed to maintain coverage and bubbling. The wire ends naturally seem to straighten!

First transformer mounted. The toroidal coil coverage is now closer to 270 degrees (3/4s) circumference, since the ends of the wire need to be drawn parallel together to be inserted into the PCB holes. 

The instructions advise to try to make the toroids as nearly identical as possible to tap power in the same way. This removes variation from the comparison of forward vs reverse power measurements.
I may look at increasing the toroidal coil spacing to get the coils to cover more of the cores (say 300 degrees around).

Getting ready to cut and prep the wiring loom

Tinning

The PCB and switches are done. Now for final assemble of the case and hardware.

The workspace. The watt meter is coming along well.

One thought on powering the watt meter. There is no stabilised voltage regulator in the circuit. It is designed to work from an internal 9v PP3 battery or external DC supply from 9v-13v. This wider DC range is only used to power the ICs and meter needle. These op amps have voltage regulators built in, so there should be no need to add an LM7809 for example.


To test the finished meter, I plan to use my Elecraft KX3. This is an SDR digital radio (without the computer) and offers me the best 'reference' on power outputs and SWR via its built-in ATU.

I plan to control the KX3 power output to assist in calibrating the meter once the set up without the rig is done. This will offer me some validation of the WM-2 setup.