[TowerTalk] New Cushcraft XM240 Trouble
cdoneal at comcast.net
Wed Apr 12 22:27:39 EDT 2006
While it was up in the air, did you run any F/B tests with a local station
to see if it was working? You should get an honest 12 to 15 dB. If you
did, it is working fine and I wouldn't worry about the SWR. SWR performance
is not an indicator of antenna performance. I have two of them and the
best I get is 1.4 : 1 or so. They work great.
SWR bridges are notorious for inaccurate readings, especially with slightly
reactive loads. If it reads correctly into a known 50 ohm load, it may not
read accurately into a slightly reactive load. Using a Bird watt meter is
----- Original Message -----
From: "Robert Pack (NX5M)" <nx5m at txcyber.com>
To: "towertalk" <towertalk at contesting.com>
Sent: Wednesday, April 12, 2006 8:42 PM
Subject: [TowerTalk] New Cushcraft XM240 Trouble
> I am out of ideas so let me probe the minds here. I have already been in
touch with CC on this issue.
> Bought a new XM240 a couple of months ago. Built it to specs for the
center of the band. The antenna did not present an swr any lower than
> This past weekend the antenna was taken down and up several times. The
coils and balun were checked but nothing seemed to be abnormal. All
connections are good. Measurements were triple-checked before the antenna
went up two months ago but we checked them again just to make sure. All are
correct. We even changed the tip sections to the upper part of the band
setting but still saw the same thing.......best swr 1.6 (only higher in the
band as it should be). The coax was tested and the antenna was even tested
with a different piece of coax. No luck at all. Finally the balun was
bypassed and the coax connected directly to the driven element but that did
not seem to make much difference.
> What I do find interesting and confusing is that at the lowest swr point,
or resonate spot, the analyzer is telling me that it is 29-30 ohms. This I
do not understand.....but it does explain the mismatch. I also do not
understand why the analyzer shows 50 ohms at another point in the band but
the swr is over a 3:1.
> Now CC wants me to send the coils back for testing which I guess I will do
since the antenna is on the ground anyway and somewhat useless to put back
into the air given the swr issue. All of this even after CC sent me two new
coils just in case I found a bad one........which to the best of my
knowledge none of them are bad.
> There are very few failure points on these antennas so I am really
confused and frustrated.
> Based on the 29-30 ohm reading I guess I could always feed the antenna
with two 130 foot runs of 75 ohm cable in parallel........just being silly.
But spending what I had to spend to buy the antenna seems the thing should
be better than this. The 40-2CD that is on the top of the tower works just
fine (flat swr at the design frequency).
> To me it has to be a coil or the balun.
> Anyone have any helpful ideas?
> Bob NX5M
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