I recently erected a cobweb antenna (covering 20m, 17m, 15m, 10m & 6m) on my 6m mast outside & have had extremely good results with it over the last few weeks.
It was tuned to be resonant around the SSB portion of the band (which typically means a slightly high SWR requiring tweaking with a tuner if I want to run FT8 at the bottom of the band).
I’ve been away with work for a few days & got home to find that the SWR is now sky high.
I put the antenna analyser on it & the SWR curves look completely normal but the resonant frequency has now dropped by around 200-300KHz ON EVERY BAND!
A few days ago it was resonant on 20m at around 14.250…now its great for FT8 & if I want to operate illegally at around 13.9MHz (no, I’m not going to do that)!
Similar story for all of the other bands.
The antenna looks intact. I can’t see any obvious damage and we haven’t really had any significant bad weather over the last few days, so I don’t think the antenna would have been damaged.
I’ve disconnected the coax at the base of the mast & checked with the antenna analyser there. The SWR readings are identical at the base of the mast as inside the shack, so I don’t think there’s anything strange happening in the coax run between the shack & the mast.
As far as I can see, nothing has changed or been tampered with in my absence over the last few days.
It has rained fairly heavily this morning & the antenna is wet. I’ve heard of weather conditions affecting the SWR but I wouldn’t have thought that it would make this much difference.
Anybody else running a cobweb antenna had similar problems?
Any antenna experts out there got any suggestions?
I’m very reluctant to shorten elements in an attempt to re-tune it without understanding exactly what’s changed.
It could be as simple as this. The weather forecast for Swindon suggests you should have some dry weather over the next few days so let it dry out and see if that makes a difference.
Thanks. I’ll tilt the mast over & check inside the balun box at the feed point when it stops raining long enough for me to do it without getting drenched!
Westflex 103. When I tilt the mast over I’m going to try swapping this out for another coax to see if it makes any difference.
Copper wire will stretch under tension (I think a cobweb keeps the wire taut) it might be worth measuring the wires while it is down, especially if you have a record of the original lengths…
If you can fit a small dummy load to the antenna end of the coax and check the SWR in the shack it would prove whether the cable is shot or not. Maybe it doesn’t plug in but is a wired connection so that may not be so easy. I’m a firm believer in using heat shrink sleeving for every outside connection to keep water out. I wouldn’t be without my heatgun and assorted sizes of shrink sleeving. It’s rare to get any ingress of moisture in any connections at G4OBK, but maybe you have proofed everything well and its something elsewhere that has gone U/S. I have no experience of using a cobweb antenna myself.
Hope you fix it and get back on the air ASAP!
73 Phil
PS Having reread your message could the SWR drop in frequency be caused by rain on the elements: Quite possible I would say with the type of antenna the Cobweb is. Its been pouring down in Yorkshire since 9.00am, maybe you have the same?
I wonder if the ground conditions might be to blame? The article linked to below suggests you have to make an antenna a bit shorter if the ground conductivity increases which it might have done if it’s now flooded.
Absolutely, or at the very least self-amalgamating tape with a protective overlayer of electrical tape (more for UV protection). When I used to do serious contesting, the group always used self-amalg, even just for a weekend. Better than having to lower 4 x 17el in the middle of the night.
Some years ago I had a genuine Cobwebb antenna on a 20 ft pole in the garden. I drove across to Malton to collect the antenna from Steve Webb G3TPW and we had a brew and a nice chat.
My Cobwebb did the same with the resonant frequency dropping significantly after rain fall then rising again after a dry spell.
Interesting. I think that’s what’s happening here. I have just checked the SWR on 20m & it’s currently resonant at around 14.148. Slightly lower than I would like but closer to where it was before.
It’s a similar story on the other bands…everyday is a school day in this hobby!!!
It’s annoying but I might have to stick a tuner in-line to make it behave on rainy days & bypass the tuner on dry days!
I guess that’s possible. I don’t have measurements for the elements before because it came pre-tuned from Paul in Poland (sells them on ebay). It seemed to be resonant where I wanted it to be when I put it up, so I didn’t think any more of it.
I’m not much of a cobweb antenna expert, but on my old EFLW, when it had rained and drops were on the wire, this would knock my antenna SWR out quite a bit! A good 400w pushed through it for a few minutes sorted it out
I hear some folks saying you need to clean the flies, months and other bugs of the cobweb regularly. Having a spider at the centre may help.
Any antenna wire made from ordinary copper will stretch. My original 80 m dipole went from 3.6 MHz down to 3.4 MHz over 5 years. Then it broke.
One end was in a tree which moved a bit in strong winds and although I thought I had enough sag to cover that the regular tension variations did it in the end. 12 ga magnet wire from memory.
The effect should be smaller on a cobweb but maybe still noticeable.
As noted water detunes antennas as does adhering snow and ice.
