Calibrating an MFJ-860 SWR Wattmeter

Now that I know the characteristics of my Cushcraft Ringo 6m vertical antenna, I wanted to see if my MFJ-860 SWR Wattmeter was any good. You see, I thought I could now run 6m without going through my MFJ-941E antenna tuner. My antenna is tuned for the SSB segment of the band, so I should be able to operate with the antenna connected directly to my Yaesu FT-450 rig.

I was keen to do this as it would allow me to free up a connection on my coax switch that I use to connect the rig to two antenna, a G5RV for general HF work and the 6m vertical. I can then use that connector to connect up a QRP rig as well as the FT-450 to the G5RV instead.

All this would only happen if I could rely on the MFJ-860 being accurate, as I do like the idea of being able to monitor power and SWR levels while operating.

I connected up the meter and keyed the rig up at 100W on 6m. Way off …

There was nothing for it but to recalibrate the unit.

This is remarkably easy to do. I opened up the unit by screwing off the lid of the enclosure  and was faced with internals that featured four variable pots. Two of these are for forward power  (two scale settings – 300W and 30W) and two for reflected power readings (60W and 6W).

These are the four pots.

Then it was a simple matter of connecting the rig to the transmitter connector and a dummy load to the antenna connector on the meter, choosing a power setting on the rig, keying down to produce a signal and seeing what the reading was. Then, with a small screwdriver, I adjusted the setting of the one pot (trial and error to see which one moved the needle on the meter) until it lined up with the 100W mark on the meter.

Calibrating the reflected scale was carried out in the same way, but with the antenna and transmitter connections reversed and remembering to reduce power (I chose 50W).

Then it was simply a matter of screwing back the enclosure and testing the meter. Final tests proved it to be more than acceptable. Now I have another useful piece of gear on the bench in the shack.


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