Well I have a QRP Transceiver and wanted an appropriate SWR meter for it. For my non-radio friends, QRP is the “Q” code for lower power. Now days it is understood to be a transceiver with 5 watts or less output. SWR is Standing Wave Ratio; per Wikipedia it is”the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.” Basically how well matched your transmitter is to your antenna. And if you are working with only 5 watts, it is important.
So I wanted one. I could buy QRP SWR meter but they are a bit pricey. So I bought a used MFJ-860 SWR meter and converted it to a QRP SWR meter. The MFJ-860 however has two settings, one is for 30 watts and the other is for 300 watts. Definitely not QRP.
MFJ makes a QRP meter (MFJ-813) but it is not a cross-needle like the the MFJ-860. Hence it is a two step process to read SWR as opposed to a single step like on the MFJ-860. MFJ does make a QRP tuner with SWR meter built-in (MFJ-971). But it is $119. And not too available on the used market. however it uses the same movement as the MFJ-860. The lower range on the scale is 0 to 6 watts. Why the MFJ-860 has the same meter movement, who knows. Anyway for the 0-6 watt forward power setting and the 0 to 1.2 watt reverse power setting, the MFJ-971 uses adjustable 10K ohm resisters in the final stage. The MFJ-860 uses adjustable 100K ohm resisters in its final stage for the 300 watt range. Put a 10K ohm fixed resister across the 100K ohm resister and you get 9K ohm. Close enough, and now after calibration, I have a SWR meter with a 6 watt and 30 watt power settings for alot less.
MFJ-860 Meter movement:
I used my Yaesu FT-897D with a power setting of 6 watts to adjust the for forward setting and my Yaesu VX-7R on 6 meter AM (1 watt) for the reverse setting.