Thursday, November 6, 2014

Weather VS MFJ 1788 loop

Outcome of my damp MFJ 1788
The weather has been up and down up this way from snow and minus 2C then up to plus 14C it's the top end of the temp scale that I enjoy but that will soon come to an end as winter is on it's way! While on the subject of the warmer weather also comes showers and damp weather, for some reason this has an affect on my MFJ 1788 isoloop. In past I have found my antenna to be SWR sensitive to the damp and or humid weather. I had posted on some of the mag loop user groups to see if  others had the same issue. I was not able to get lots of responses but the consensus was that weather really had no effect on others loop! I ended up taking my loop apart and just checking out the insides and all looked good.
The work of the internal tuner
Once the damp and or humid weather changed all was well with the loop. The effect on the antenna is the SWR will not go below 2.7:1 on any given band but any other time the SWR would be either flat or close to it. I posted about this very problem this past summer on my blog and it really was not a humid summer so the problem did not arise that often. Now that it has happened again my own conclusion is the damp weather has an effect on the dielectric characteristics of the air that separates the capacitor plates in the antenna. This by no means is a scientific conclusion but as for simple old me it's the conclusion I am leaning toward. I do have an internal antenna tuner in the Elecraft K3 and my Elecraft KX3,
The SWR of the antenna
this allows me to lower the SWR so the rig does not see the high SWR.



  1. Hello Mike, I think there is only one solution. But it is difficult.... Replace the variable (air)capacitor for a vacuum one. Although I think you better think of building your own magnetic loop from scratch with parts you think are being the best avoiding such problems like humidity. As long as I've been a ham radioamateur I had only one commercial (Diamond CP-6) antenna, it didn't last long. After that I made my own antennas with much more succes, satisfaction and better results. Good luck, 73, Bas

    1. Mike, I mean I had only one commercial antenna for HF. For VHF/UHF I still use a commercial W2000. Although I had to modify it as well.....replaced guess.....50MHz capacitor for a high voltage one. Bas

  2. Hi Mike, I know some amateurs make their MFJ loop weather sealed. To avoid too much water in it. 73 Paul PC4T

  3. Good evening Bas, it's fantastic that you build most of your antennas I must say that I have made maybe one antenna and thats it! Thanks for the suggestions and have a great weekend.

  4. Good evening Paul very nice to hear from you, I will do some internet searching to see who and how they found to be the best way to seal the housing with.