"Kevin Hastings" wrote in
:
I've spent 2 months or so of tweaking an older Cushcraft ATV-5 5-band
vertical with a friend's MFJ analyser and have had this thing exactly
where I want it several times now. After tightening things down and
putting vinyl electrical tape on all traps and section joints (where
it seemed to make sense) it would look fine for a few days to a week.
Then the resonant points of some bands would shift up to no-man's band
seemingly overnight and stay there. I suspected poor screw conections
at the traps, and so have been adding a stainless steel pipe-clamp
connection just above and below the trap for improved connection. Made
things better I thought - now 1-2 bands are just out of whack again.
There's always moisture inside the trap when I disassemble the thing,
so I'm wondering if maybe I shouldn't be trying to tape up the traps
at all - maybe they are retaining water or something that is affecting
the capacitance. Each trap has a drain hole at the bottom, and the
aliminium pole itself has drain holes every now and then that I'm
trying not to interfere with.
Is there a trick, or some practical experience here that you can
share?
I've had a lot of fun experimenting with this in any case, but
winter's coming...
Thanks,
Kevin VE9-XYZ
Kevin,
In addition to the other responses at this time...
I note you used an MFJ analyser, not further described. Assuming it is
the MFJ259 or like...
Did your VSWR readings with proper transmitter reconcile with the MFJ?
If not, I would favour the transmitter VSWR readings over the MFJ (the
wideband detector in such instruments is too easily disrupted by other RF
sources to depend on them absolutely).
Regarding tape, remember that any measure that you take to prevent
ingress of water is also likely to prevent egress. Worse, you may not
prevent vapour being drawn into the trap, but may prevent condensate
leaving and so build a water trap. Drain holes aren't always as effective
as we would like (blocking for various reasons), but I would not obstruct
drainholes. Indeed, period clearing of drainholes is a good idea if
convenient.
I see others have raised the issue of the reliability of internal
connections in the traps, another common problems with those antennas.
Owen