MFJ 259 Analyzers & Tweaking the XM240
The Cushcraft XM240 was installed on Sunday evening. On Monday, Dave finished clamping the antenna to the mast and lowered the coax. That evening I tested the antenna and was a bit surprised at what I found… On my 
Icom 746, I checked the SWR (yeah, I know, SWR ain’t everything - but you’d expect a commercial resonant antenna to have decent SWR). I found that the SWR minimum was at about 7.180… I was hoping for 7.080 MHz. At the very low end of 40 meters (7.000 MHz) the SWR was between 5:1 and 10:1.
I was curious as to what the MFJ-259 would report for this antenna. Again, it found 7.180 MHz as the SWR minimum, but this time the minimum SWR was about 2:1. Ouch. I took the day off on Tuesday to get some more work done and ponder this puzzle. Why were the Icom and MFJ-259 reporting such different results?
With my troubleshooting hat on, I tried the following:
- Test the XM240 with my Icom 706 MKIIG (installed in my car - this required driving my car to the back yard)…. Results were the same as with the IC-746: a minimum of 1:1 SWR at roughly 7.180 MHz.
- Test the MFJ-259 into a 50 ohm dummy load. Results: 1:1
- Test the car’s High Sierra mobile screwdriver-type antenna. Results: a minimum SWR on the 40 meter band of about 2:1.
Hypothesis: MFJ 259 has been giving faulty readings. More testing required. So, Bill (KØKT) offered to loan me his MFJ-259B with filter. Next:
- Test the XM240 with Bill’s (KØKT) MFJ-259. Results: same as with my MFJ-259, a minimum SWR of about 2:1. Inserted Bill’s MFJ analyzer filter. Same results.
New hypothesis: MFJ 259 analyzers are getting swamped with RF from a nearby source… even more than their optional analyzer filter can handle.
- Finally, tested Bill’s MFJ-259 with my High Sierra. Again, same results, high SWR.
Conclusion… there’s something in my neighborhood that doesn’t agree with antenna analyzers. I may take my MFJ259 with me to work tomorrow and see if there’s a different result far away from my home QTH.
XM240 adjustments…
Knowing that we would need to lower, quickly work on and then raise the XM240 before we lost daylight, I sent a plea for help out to the SCARC group and a few buddies. My usual team of helpers (Dave, my father-in-law Duane, and my neighbor Chris) were there, but I was also joined by Arnold (AG3V), Jerry (KØCQ), and Bill (KØKT) from SCARC. We had another perfect Iowa autumn evening (low wind, clear skies and mild temps) and we were organized enough to get the antenna down, make the adjustments and get it up again in very good time.
I’d talked to Cushcraft about the problem. The only thing I was able to come up with was a plan for checking things once the antenna was on the ground:
- Check for DC continuity across the coils
- Check for continuity to the driven element from the balun
- Check measurements (lengths)
- If necessary, replace the balun
The SWR curve looks much better now…
An organized plan, some detective work (to discover this antenna analyzer oddity in my neighborhood) and some great helpers allowed me to get past this strange problem with minimal pain.
Thanks to all involved!
…-.-