OHR WM-2 Power Meter
The WM-2 QRP wattmeter is an extremely useful station accessory. Manufactured by Oak Hills Research, a division of Milestone Technologies, this kit is very easy to assemble and works great for measuring the output of QRP transmitters.
It has a maximum power rating of 10 watts and has three switch selected measurement ranges of 100 milliwatts, 1 watt and 10 watts full scale for both forward and reflected power. Note that the 100 milliwatt scale is calibrated down to 5 milliwatts for the QRPp operator! The meter movement is a good quality 3 inch meter and is easy to read.
The unit will operate from an external 12 volt source or an internal 9 volt battery. The current drain is extremely low, around 1 mA. My first battery lasted about nine months before it needed replacement, even when leaving it on overnight many times.
The RF input and output connectors are SO239 type. I have placed PL259 to BNC adapters on both of my connectors so that I can use BNC cables for connection to my various QRP transceivers.
Construction is easy and if you follow the instructions carefully, you will have a quality instrument that will last for years. Alignment consists of setting internal calibration potentiometers using a digital voltmeter. A calibrated RF source is not needed for alignment.
Note that there is a shunt diode to ground in the 12 volt input source for reverse polarity protection, but there is not any protection in the battery power circuit. I modified my unit by adding a 1N5817 Shottky diode in series with the on/off switch to protect the vital circuits from accidental reversal of either the 12 volt external source or the internal 9 volt battery.
I compared the WM-2 to a Bird Wattmeter (the standard power meter for the 2-way radio industry) and found that the WM-2's readings were very close to the Bird's reading when looking at 2 to 5 watt transmitters. Below 1 watt, the Bird meter hardly moves and the WM-2 is clearly superior with QRPp sources. Also, by reversing the input and output cables, I found that the WM-2 is balanced perfectly with exactly the same readings (except that forward becomes reverse and vice versa). The WM-2 is rated for 300 KHz to 54 MHz, although I found that it can be used for relative power indication (not calibrated) in the 2 meter band.
OHR WM-2 Links:
Updated November 29, 2003