I have an odd issue - SWR swings wildly, like a loose connection (there are no loose connections), on 17 and 15 M, only mid-afternoon local. I have made contacts as far as Japan (from central NC) at other times of the day. Also, the SWR issue is intermittent; many days I can work 17 and 15 mid-afternoon with no issues.

It happens more often on hot days - maybe related? But, shack temperature doesn't fluctuate. I have tried turning off AC, in case that was interfering.

The radio shuts down transmit when this happens.

Any suggestions/ideas?

Call: KN4OJN
Radio: Icom IC_718
Tuner: MFJ-969
Antenna: dipole trimmed to 80 meter band
Feedline: 400 ohm window line

EDIT: Additional info - The problem goes away if I bypass the tuner, but, if I use the dummy load in the tuner, I still have the erratic SWR.


3 Answers 3


If it acts like a loose connection, it probably is one, just not an obvious one.

If it fluctuates from day to day, especially with a pattern, it could be water intrusion. If it works worse on hot days, maybe the water is actually helping -- maybe you have wire fatigue, and the water is bridging the gap.

Similarly, if the SWR goes up (or down) as you first start transmitting, and does so faster at higher power (try qrp?), you may be heating up (and evaporating) water in the antenna or feed line.

If the SWR bounces around rapidly while transmitting, you may have a loose or corroded connection that is arcing.

As pointed out in the other answer, the problem doesn't have to be in your antenna -- it could be in anything from the radio to the antenna, including feed line and tuner. Potentially there could even be arcing at mount points where the antenna is supported.

I had an antenna that was doing this, and I found that ants had built a nest in my feed point and were shorting it out at high power. A loose connection isn't necessarily an intentional connection.


SWR swings wildly, like a loose connection (there are no loose connections)

SWR should not change in a properly operating system (though SWR meters may show changing SWR when the transmit power changes, so check with a mode other than SSB to be sure).

Are you sure that there's no loose connections? You've checked all the connections that you made, but what about the ones someone else did?

The first place I'd look is the inside of your tuner. MFJ is infamous for quality control issues, and I myself had a MFJ Travel Tuner with an obvious “cold” solder joint when I opened it up to look, after I couldn't get it to tune properly.

Another problem I remember encountering, though not where, is a SO-239 socket whose chassis mounting screws were loose, so the shield continuity was bad even though the cable was tight.


Try testing with a different feed line and with a dummy load at both the transmitter and antenna end. That many isolate the issue down to a smaller set of potential locations (intermittent connector(s), bad feedline (moisture, crack), tuner internals, or a cracked/corroded antenna element issue). Check the transformer or filters on the transmitter finals for issues. Also test continuity with a dummy load and an Ohmmeter while someone jiggles the feedline, antenna, and etc.

  • $\begingroup$ Thanks. You reminded me of another useful bit of info. The problem goes away if I bypass the tuner, but is there f I use the dummy load in the tuner. $\endgroup$ Jul 31, 2021 at 16:53

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