Wednesday, September 10, 2014

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.

4 comments:

  1. Hi mike. Very interesting. A factor to be considered before purchase. Especially when operating in hot and humid parts of the world. 73 Dick

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    1. Good afternoon Dick, it was very interesting to find out this info and since then I have had some email's from other hams who own the same antenna, they tell me of the same affect regarding humidity.
      Mike

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  2. Hello Mike, I don't really see any serious corrosion on the plates. That certainly would happen if they get wet. Just my thought: try to spray the plates, moving parts and contacts with some WD40. It is the ultimate thing against water and protects against corrosion. Besides that the cap will move much better and the servo doesn't get hurt that way. If the problem comes back after a while (considering it helps) you know it's time to spray it again... 73, Bas

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    1. Hello Bas very nice to hear from you, the plates are covered by a plastic cover but just 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.
      Mike

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