I have always noticed a slight change in the swr with hf yagis when it
is raining. Just a
minimal change though. Nothing like what Gary, K9RX reported.
Bob
K6UJ
On 4/12/16 7:36 PM, Gary Huber wrote:
So if the SWR returns to normal with a towel, what would a gentle
shaking of the antenna using the rotor to rock the antenna back and
forth do? I'm sure some of those rain drops would drop....
73 ES DX,
Gary - AB9M
On 4/12/2016 7:02 PM, StellarCAT wrote:
An update to my post on rain and connections for those that might be
interested...
I’ve had beams since 1978... I was off the air from 1990 through 2005
and when I returned it was in Arizona where rain is rare and when it
comes, generally during the summer monsoon, EVERYTHING is
disconnected. I don’t remember seeing the sort of effects I’ve seen
with both the 15 and the 20 yagis here. And that I remember no one in
their replies said “that’s normal” ... except one private email I
received from an antenna manufacturer that asked to be unnamed...
he said indeed it IS normal and it is caused by the dielectric change
with the rain acting as a covering (impartial and uneven thickness)
and thus changes the tubings velocity factor and in essence lowers
the antenna resonant frequency. That is what I saw on the 15 – not
what I saw, or at least I wasn’t able to confirm it since my rig (and
my license) won’t allow operation lower than 14.0 which is where I
saw the SWR’s worse change on the 20... I found it difficult to
believe ...
so ... now I have the 20 on the test tower and the 15, now with a run
of new coax (common suggestion – replace the coax, even though coax
can’t change properties off and on in a matter of a few hours) on saw
horses... it rained last night and BOTH antennas indeed saw the same
increase in SWR high in the band. And both after a number of hours
(about 5 again) returned to normal.
So it rained again a little while ago and the 15 went up again ... so
this time I went out and I dried all the elements with a towel.
Before drying it I noticed that all of the elements had water beading
along their bottom along the whole element length. Didn’t do a great
deal – just ran a towel over the elements. And lo-n-behold the SWR
returned to normal! This over a matter of 4 minutes from the test
showing it was high to the test showing it was back to ‘normal’ after
I had dried the elements.
So ... that to me is proof positive it is normal. I’m not sure why I
don’t remember seeing it before – or why no one else mentioned it
(many comments with many yagis/antennas) ... but now I know
definitively that there is a direct relationship between water on the
elements and high(er) SWR. [note – the SWR shifts downward so indeed
it might lower the SWR on the low end of the band thus in that case
being beneficial]
Thanks again to all that replied. (And my apologies for doubting! )
Gary
K9RX
From: StellarCAT
Sent: Saturday, April 2, 2016 5:03 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] frustrating rain and connections... also
Noalox?
Thanks for the comments – it is a hairpin design. It is an M2 6
element 15 that was built using their 15M6-125 antenna and converted
to their 15M6DX ... so I refer to it now as a 15M6DX-125. Anyway it
has a fiberglass center insulator with thick donuts to ensure there
isn’t any creepage along the fiberglass... and if it were anything
related to that it should have done it when I hosed it down (lowered
to 8’)... I couldn’t really get it to change with a hose.
Its off the tower now – had to move on (take advantage of clear days)
to testing the 20M5LGS made from a 20M5 ... its now on the tower at
8’ up with high winds so its not going up any higher until the hills
are calm.
I’ll continue to test the 15 on saw horses. What I was seeing I’m
sure will show even at that height. And once again: whatever design
wise might be suggested it has to be remembered this is a near exact
duplicate of another one that didn’t have this problem. So I’d guess
it has to be either related to construction or possibly a deficiency
caused by being used. The latter shouldn’t be a problem – I’ve dealt
with used antennas before and never had a problem... but it could be.
Please write direct if you have any other thoughts that come to mind.
Gary
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