I use a lot of fibreglass poles with wires strewn up the sides, normally in a lazy helical fashion so that they don’t flap about too much.
I have great success with these poles as regular readers know however I have noticed that certainly wet weather can detune them ever so slightly. Effectively, they become slightly longer and I fathom that as the fibreglass supports become wet, I am achieving some slight inductance with the water that drips from the surface and gets between the wires and the poles. As soon as they are dry again, the tuning comes back to normal.
This year next to the ocean, I noticed the same effect only on a huge scale. The wind is currently blowing on-shore and there is a gradual build up of salt crust forming on the fibreglass. When it’s completely dry, it seems to have little effect but when the tide is in and extremely close to the antenna, the salt builds and it remains wet.
I happened to work this out last night when all of a sudden we had a massive downpour of rain. I had been noticing the SWR creep away and my 40m element which tunes at 7.1 Mhz was now tuning at nearly 6.6 Mhz, that’s around 80 cms in length.
I connected the MFJ analyser and literally watched the SWR come back to normal over a period of 2 minutes as the wet salt was washed off the elements.
So now I can assure you that my previous conclusion that the tide changed the tuning wasn’t quite right. Indeed, there was more salt spray (when the tide was in) which is damp – but as the tide went back out, the salt dried off and the SWR became normal.
How do I correct this? Probably I need to build some spreaders, to physically remove the bulk of the wire away from the fibreglass pole. I will experiment.