MFJ 1788 and humid weather

Large plate capacitor
Over the past few weeks I have been very busy with work and not really anytime for radio or anything else really. I was on the radio for a very short time last week and the weather outside was very humid and HOT! The antenna I have is the MFJ 1788 loop and I have always been able to get a decent SWR on most bands the antenna was designed for. But for some reason I was not able to get an SWR under 2.1:1 on some bands the best I could do was 3.1:1. It turns out that the Humidity can affect the MFJ 1788 antenna and my guess is since the antenna has a large moving plate capacitor the humid air affects the air gap between the plates? The other day the weather was back to cooler non humid conditions and I tried the antenna again and the great SWR was back again. The antenna had not moved and nothing had changed other than the weather.
Mike Weir, VE9KK, is a regular contributor to and writes from New Brunswick, Canada. Contact him at [email protected].

7 Responses to “MFJ 1788 and humid weather”

  • harry k7zov:

    Interesting and a little surprising. I live in the Mountains of AZ and we had a lot of rain this summer and snow is not far behind. I have had some SWR issues, but mostly when the wind is making the 1786 shake. I was planning on talking the loop down and tighten thing up, if that is possible. Now I will add looking for any moisture effects. Thank for the heads, up… BTW have you noticed SWR drifting up and down if the winds shakes you loop? Just curious if this is a common problem of just mine. We all know how good the quality control is at MFJ when it comes to catching cold solder joints and other things. I might have a cold older joint or a loose screw or nut. Good luck with your antenna. I still like my loop for PSK31, JT65 and WSPR where I can park it after tuning.
    73 Harry K7ZOV

  • GB KC5GB:

    Another data point for you, Mike. For the first time in the 5 yrs I’ve owned the 1786, I noticed a heat/humidity effect also(only on 20m). It’s been more humid this summer. I do think there is come correlation between two factors since I’ve never noticed the SWR swing during damp winter wx.
    Even with its touchiness, I still think it is the best balcony or deck antenna around.
    BTW, how is 40m? I want to get on 40 as the cycle decreases and I’ve never read anything pro or con about the low end of the 1788.

    I came to that conclusion since no other parameters have changed. Only on 20m though…
    For all its idiosyncraties, I still think its the best antenna for decks/balconies.

  • Peter ZL1PWD:

    I wonder if there is a buildup of dust between the fins which has an effect in high humidity?

  • GB KC5GB:

    Hi, Peter…
    I did the “200” screw cover removal last year, and the insides looked vy clean, so don’t think that’s it.

  • Mike VE3WDM:

    Good afternoon Harry and very nice to hear from you, it’s great that your antenna does not seem to be affected by the humid weather. We lived right on a lake so that may also add to the factor. I am thinking the humid air seems to affect the air gap almost creating a very high resistance through the very damp air. This seems to affect the air gap on the cap and thus the SWR. Not that the humid air has been gone I have had no problems at all. As for the wind I can see that being an issue as the plate gap may change. As for me my antenna is on a balcony and it really does not get that windy at all but next time if I am on the rig and it’s fairly windy out I will keep an eye on the SWR and see if it is affected.

  • Mike VE3WDM:

    Hello GB and thanks for the feedback, I have used the 1788 on 40m and have had no success at all. I even tried it during a contest when some U.S stations were booming in, because it was a contest their ears are listening for all calls. I was not heard at all and I have done some blog reading about the antenna on 40m and have found out it’s about as good as a wet noodle! Having said that I am still going to give it a try and see if I can get some RF on the noodle and make a contact on 40m.

  • Mike VE3WDM:

    Hello Peter, I have checked that and there was none at all. I could see how that would affect the plates if lots of dust was there.

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