Amateur radio, space, technology and other geeky stuff by Alexandru Csete
The OHR WM-2 QRP wattmeter from Oak Hills Research is simple and easy to build. It measures forward and reflected power from 5 mW to 10 W and is indispensable in a QRP shack.Be sure to check my pictures of the WM-2!
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.
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Now that my OHW WM-2 is assembled, it is time to align it. The manual has detailed step by step instructions on this as well and the alignment procedure is relatively straight forward. On the other hand, adjusting the potentiometers can be a little difficult, partly because some of them are hidden below the wires and partly because R6 has to be adjusted with a precision of 1 mV while it has a full scale of several volts. It would have been very nice with a multi-turn potentiometer instead.
I’m continuing with the connectors switches and the instruments. I don’t quite understand why the assembly manual wants me to use the bottom row on the power switch instead of the top row. The switch is already mounted on the cabinet and thus the bottom row is much more difficult to access than the top row. I just ignore this step and use the top row.
I made further progress with the wattmeter tonight. It was time to prepare the wires, which consisted of cutting them to various lengths and removing the isolation at the ends. Here I faced the first difficulty in trying to follow the instructions of the manual.
I have now received the three 1N34A diodes that were missing from the OHR wattmeter kit and tonight I resumed the assembly.
I just finished mounting all the resistors when I noticed that the three 1N34 diodes are missing from the component bag. Well, I guess I won’t finish the OHR WM-2 this weekend 😛
Today I’ll start to assemble the OHR WM-2 QRP wattmeter from Oak Hills Research. I bought the wattmeter via the German QRP Project few weeks ago but I didn’t really have time to build it yet.