How is the driven element constructed/insulated and matched?
The downward shift indicates increased capacitance across the feed
point when the antenna is wet. Look for moisture to be trapped in
the DE insulator ... for example, if you are using split PVC is the
split *UP* in one antenna and *DOWN* in the other? If you have a
rod of some kind in the element to stabilize the feedpoint is it
completely sealed to moisture? Are the balun feed points completely
sealed? If there is a support for the beta match (if one exists)
is it designed so it will not absorb water/allow water to stand on
it? If there is a coil across the feed point, does the construction
allow water to accumulate on supports, etc.? If the antenna is an
OWA design, check the insulator of the *FIRST DIRECTOR* as well as
that element has a significant impact on antenna tuning.
It only takes a very few picofarad change in shunt capacitance to
move the "resonant point" quite a bit.
73,
... Joe, W4TV
On 4/2/2016 8:21 AM, StellarCAT wrote:
I’m testing a yagi... I can raise it to ~37’ for testing and lower it
to about 8’ for adjustment ... it was made as a duplicate to another
one and testing showed it to be just that – a duplicate. Until it
rained! The SWR shifted downward. At 21425 it was ~1.35:1 ... and it
would go over 2:1. I’m at a loss as to what is causing this. It is a
used antenna reassembled... but I find it very difficult to believe
that water could get in to the mechanical joints and cause this kind
of a problem. The balun doesn’t appear to be taking on water – there
wasn’t a drain hole in the bottom – there is now – I went out early
this morning just after it rained – SWR up high – and there was
nothing coming out of the drain hole. I have checked all connections
– they’re tight. Oh – and the strange thing – it goes back to normal
after only about 4 – 5 hours without rain. It has done this 3 times
now. It is now – 45 minutes after checking it this morning where it
was at 2.33:1, its now down to 1.8:1 and continuing its decent. And
further note the SWR seems to be about the same – its just shifted
downward when wet. Also I hosed it down – the balun/coax connection,
hairpin for a good 5 minutes – no change. I hosed down the elements
at their joints – no change. It rains: changes! The fact that it
shifts seems to be independent of height – it does this if at 37’ or
at 8’.
Also: is Noalox the same as penatrox? It is the No AL OX sold by HD
... it says it is for increased conductivity as well as anti-seize...
perhaps IT is the problem at the joints ... unless someone knows it
to be fine (or KNOWS it to be bad).
thanks – I’m hoping someone has personally seen such a thing and has
an idea. Its holding up the testing of the other antennas.
Gary
ps: the first one had no problems when it was rained on.
_______________________________________________
_______________________________________________ TowerTalk mailing
list TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk