The WM-2 QRP Wattmeter from Oak Hills Research

The WM-2 from Oak Hills Research is a cool QRP wattmeter kit, which can measure forward and reflected power in three ranges: 100mW, 1W and 10W. With an accuracy of 5% of the full scale it should be possible to measure powers down to 5mW using this instrument. The WM-2 has a big, easy to read scale and can operate either from a built-in 9V battery or an external DC source.

Building Experience

When properly designed, a wattmeter is a relatively easy instrument to build. There are relatively few parts and the only challenge is the construction of the directional coupler. The WM-2 is no exception and it can be easily assembled during a weekend. The PCB layout is well designed and there is good space to mount all the parts. The most diffcult part, the construction of the directional coupler, is also quite easy if one follows the very detailed instructions of the assembly manual. There is no doubt that the manual has been written with beginners in mind.

There are a number of wires that have to be mounted between the PCB and the peripherals like switches and plugs. The design here is really excellent, because coloured wires are included in the kit and the manual gives exact lengths for each piece of wire needed for the complete assembly. All the wires are prepared before use. Actually, the wires are included in the form of a cable with eight single wires and one is supposed to pull them out one by one. I had a little trouble pulling out the wires out from the outer jacket, nevertheless, I managed to separate the wires by carefully cutting the whole outer jacket. You can see the reason the the picture below

Mounting the PCB and peripherals into the cabinet is easy, although I decided to cut the unused pins of the range switch before trying to mount it. The cabinet, which by the way has all holes pre-punched, is made of thick aluminium and it feels very robust. In fact, the whole construction feels robust if the srews are properly tightened.

Alignment

The alignment procedure is straight forward and can be carried out using either a battery or an external DC source. The only instrument needed for alignment is a DC voltmeter capable of measureing between 100 mV and 3 V.

I have a few remarks concerning the alignment. Some of the potentiometers are a little difficult to access and some of them are a little difficult to adjust to the required voltage. R6, for example, is first adjusted to 2.56 V, later on it has to be adjusted down to 251 mV. The manual says nothing about the required precision, thus one has to assume that it is necessary to hit the 251 mV exactly, which corresponds to a precision of 1/3000 (the full range of the trimmer is more than 3 V). It is possible to do it, however, it would have been much easier if the R6 would have been a multi-turn potentiometer. The next problem is cutting the jumper JP1. This has to be done when the alignment is finished and one must be extremely careful not to touch R6, which more or less shadows the jumper as you can see on the picture below.

Operation

Operation is as easy and simple as with any other analog wattmeter. One selects the desired range with one switch and then selects either forward or reflected power with the other switch. One of the great strengths of this instrument is definitely the large scale, which makes reading and interpolation very easy.

OHR writes in the manual that the WM-2 has an accuracy of 5% of the full scale, which would correspond to 5 mW on the 100 mW scale. I have no way to check this so I just have to trust them. Nevertheless I have tried to do some rough measurements using my FT-817ND and my DL-1 dummy load from Elecraft, which can be used to measure the absorbed power using a voltmeter. The table belows shows the power setting of the FT-817, the number of bars on the ft-817 SWR meter, the measured forward power on the WM-2, the measured voltage and power on the DL-1 dummy load and, finally, the current drawn by the FT-817. The last column has not much meaning in the present context but is shown for information.

FT-817		OHR WM-2		DL-1	FT-817
Power [W]	Bars	10W scale	1W scale		Amps
0.5	1	0.5	0.45	0.45	0.96
1.0	3	0.8	1.0	0.9	1.15
2.5	5	2.0	–	2.3	1.49
5.0	8	4.1	–	4.6	1.92

Keeping in mind that the DL-1 only gives a rough idea of the power, I am satisfied with the acquired accuracy. I could go on and compare to many other wattmeters but at the end it will only be statistics. I trust that OHR had much better intruments at their disposal when they designed the WM-2. In any case, I am sure it gives a much better readings than the built-in meter of the FT-817 

Conclusion

The WM-2 QRP wattmeter from Oak Hills Research is a simple, well designed and easy to build kit. The kit is 100% complete, including a robust cabinet with pre-punched holes and printed stickers for both the front and the back panel. Both the electronic and mechanical parts seem to be of high quality resulting in an instrument with professional look and good for many years.

Alignment is easy and straight forward, but can be made more convenient by using multi-turn potentiometers, at least for R6. Furthermore, placing the jumper JP1 closer to the edge of the PCB, instead of hiding it behind R6 would help a lot, in particular at later times when one has to realign the wattmeter (even better, why not use a jumper from PC hardware?). The instrument is also very easy to operate and the large scale is a real pleasure to look at. OHR made a very good choice by using this instead of a standard size scale.

The WM-2 has three ranges: 100 mW, 1 W and 10 W. Somehow I feel that a 5 W range would have been nice, partly to expand the scale between 1 and 5 watts, but also because 5 W is the official output poower limit for QRP. The assembly manual is good and very detailed but there is no technical description of how the wattmeter works. I think beginners would appreciate a short technical description about how SWR meters work in general.

Despite the few minor issues that I have mentioned I can definitely recomend this kit for any QRP operator. In fact, I think it is indispensable for anybody who likes to use powers below 10 W, in particular people with mojo running 1 W or less. Have fun!

References

1. WM-2 pages at Oak Hills Research.
2. OHR WM-2 reviews on eHam.net.
3. Frequenctly Asked Questions about the WM-2.
4. Local copy of the WM-2 Documents.