OHR WM-2 Alignment

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.

In the table below you can see the power measured with the WM-2 and compared to voltage measurements on the Elecraft DL-1 dummy load. The difference between the two readings is within 10% and we must keep in mind that the voltage measurement on the DL-1 is not very precise. The table also show the number of bars on the FT-817 power meter as well as the input current at 13.5 volts DC.

Power [W]
10W scale 1W scale
 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

I would also like to mention that the fully assembled wattmeter feels very robust. It seems that the materials used for the mechanical parts are of high quality as well as the big instrument. It has been a great pleasure to build the wattmeter and I am sure it will be a great pleasure to use it, too.

I don’t know yet what the next home brew project will be, but I am thinking of playing with the AVR micro controllers and the AD98xx DDS chip. Or maybe I should just go and buy that damn K1 now and be able to sleep during the nights again.

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.