MFJ HF/VHF SWR Analyzer Calibration

[AJ1G]

Picked up a nice condition MFJ-259 at Nearfest Friday.  It seems to track very well with SWR results I observe with the internal SWR scan/plot routines of my IC-7100 and I
IC-7300.  However, the indicated resistance at SWR of 1/1 seems low, about half of what I expected.  Looked into a mil spec Bird 500 watt 50 ohm dummy load, SWR near spot on 1:1 throughout the HF spectrum at all frequencies, slightly higher in VHF range no doubt due to effects of the short connecting coax candle and likely the internal arrangement of the dummy load.  However, indicated resistance at 1:1 SWR is only 1/2 what I would expect, right on 25 ohms. Is there a cal adjustment pot to correct this, haven?t found one on line so far but did see mentions of low indicated resistance at 1:1 SWR as a common problem.

Chris AJ1G
Stonington CT

[KD6VXI]

W8JI designed it.

https://www.w8ji.com/mfj-259b_calibration.htm

I had the dame problem years ago.  Mine was connected to my trucks antenna while a CBer keyed up a 3000 100 feet away.  Blew out the shottkey diodes.

3 homemade loads, a 12.5, 75 and 200 ohm, 4 new shottkey diodes and it's worked fine for another 10 plus years.

--Shane
WP2ASS / ex KD6VXI