Antenna meltdown.
Ok, not quite antenna meltdown but possibly not too far from that. You know, or at least have reason to suspect, that there’s something wrong with your antenna system if the SWR starts to vary during transmissions. I have noticed this in the past few weeks but thought little of it, other than to touch the ATU (SEM TranZmatch z-match) between transmissions to see if it was getting warm, which it wasn’t.
A few words by way of explaining my normal way of operating. Mindful of having a relatively weak signal I tend not to call CQ a lot unless I’m working PSK31. I find it more productive to work stations that I can hear, strong or weak, because I at least have some chance of a QSO if there is at least one way propagation.
In a bit of a break from my normal practise of replying to other peoples CQ calls I have recently taken to calling CQ on 20m RTTY, usually in the evenings between 20:00 UTC and 22:00 UTC and running between 70 and 80 watts output power, just to see who comes back to me. I don’t seem to be putting out a particularly big signal because those stations that do come back to me tend to be mostly close-in EU stations (Germany, Spain, France & Scandinavia accounting for 80% or more).
It is during these periods of calling CQ, and mostly on 20m, that I noticed the variable SWR. Not wildly fluctuating, just slowly creeping up as I transmit, and easily remedied by just tweaking the controls on the ATU. It was only when I happened to touch the antenna feedline itself did I realise what was wrong - the feedline was very warm! Now this feedline is only simple twinlead bell wire so it’s not designed for RF purposes and my suspicion is that a high SWR within that feedline (which the ATU matches quite happily) is creating that heat and in all likelihood changing the characteristic impedance of the feedline. Not good. It seems that short sharp exchanges in my usual search & pounce style doesn’t generate this heat and the SWR remains stable over long periods (although the high SWR is likely still there and the same losses are occurring).
I am now considering how best to erect a multiband antenna at my QTH that I can feed by another means and where I can design out this lossy bell wire feedline.
I’m open to suggestions…
Hi Dean,
Thanks for the email. Really enjoyed your Wet String Chronicles and log in regularly to read your blog. I also have problems with space in which to construct a Amateur Radio Antenna that will be effective and be able to punch just above “little pistol” weight. I live in a terraced row of houses and only have a garden of 7m wide.
I started off with the TGM MQ36SR minibeam this was a great start for me on 6m through to 20m. Then last year after using a Hexbeam for the first time while away doing an IOTA in Wales i decided that i was going to try one at home. Not looked back since!
So i do sympathise with you very much on lack of space for the antenna. I do hope that you can put up some sort of beam?
Please keep up the Chronicles! and good luck with the ‘tenna work.
Thanks for the comments Tim. I think even a HexBeam would be a bit too much where I am now but sure looks like a nice lightweight antenna. I think I’ll be relying on improved conditions more than improved antenna for the time being!!!