Tuning a vertical antenna with the ocean tide

cornwall qthHere’s a new one that you wouldn’t read in the books.

I’m seriously hammering the 40m band here in Cornwall and have tuned the vertical antenna so that the whole of 40m SSB is almost 1:1 swr, certainly from 7.05 through to 7.1Mhz. Over the last few days though, I’ve noticed that the tuning might change over a few hours to give me some serious SWR headaches. Enough to force me at one point to fold back the top of the element by 70cms.

It took me a while to track down what was happening but it turns out that when the tide is in, the vertical is effectively closer to the salt water ground and requires to be shorter than when the tide has been out for a while, long enough for the salt water to drain out the sand and reduce the salt water table by probably 15 to 20 feet or so.

This is the same effect as lowering or raising a dipole. Until recently, I used to say that the higher an antenna is raised, the longer the resonant frequency becomes – and that’s true to a point, certainly on the low bands when you are dealing with antennas less than a half-wave above ground. I digress but it turns out that at about half a wavelength, that assumption is no longer correct. It starts to come back down again, just like the impedance of an antenna changes according to the height above ground.

Anyway, I cracked it. When the tide is out, the element needs to be longer and when the tide is in, the element needs to be shorter. If I operated near the shore permanently, I may fit a relay and flick in a couple of coil windings instead of having to venture outside to correct the tuning.

An amazing hobby,