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#7
August 25th 07, 12:02 AM
posted to rec.radio.amateur.antenna
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Cushcraft Vertical Trap Consistency - Is there a trick guys?
"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 |
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