[Antennaware] AV-640 SWR Creep

Art Trampler atrampler at att.net
Fri Sep 11 21:46:44 PDT 2009

Hey Gang,


I have a Hy-Gain AV-640 multi-band vertical, which in most respects performs
admirably.  It is mounted so that the feed is about 18 to 20 feet off the
ground.  I have had it almost two years and it is the second I owned; the
first survived an Ozarks ice storm, but its mounting pipe did not!


I am a CW operator, primarily a rag-chewer, and when I operate QRO and have
been chatting for a while the SWR will begin to climb rapidly.  Going
barefoot corrects the problem quickly, and in a few minutes I can QRO again.
This happens if I am running more than about 1100 watts into it.  Typically
I run 800 watts out and if I am trying to get that new country will let the
Titan put out its 1500 watts, as I know I won’t be rag-chewing.


I have generally assumed it is one of the two baluns overheating and then
saturating, despite the unit’s 1500 watt rating.  There is a 1:1 current
balun followed by a 4:1 voltage balun.  However, there are two other
components and I wanted to see if you think these might be the more likely
culprits.  However, wouldn’t the saturation be a function of the current,
and therefore occur relatively quickly, rather than temperature built up
over a QSO?


There is an RF Choke to ground from the radiator, and a DC blocking
capacitor between the voltage balun and the radiator.  I have read that some
have had failure of the RF choke, but is there reason to be suspicious of
the blocking capacitor? When the RF choke is compromised it is shot and
there is a low impedance path to ground for the RF.  I am wondering if a
stressed capacitor could change the impedance of the circuit.


The SWR jumps at first to roughly 2:1 and will quickly go up from there if I
don’t back off (obviously I back off).


I’ll be taking it down to inspect/prepare for another Ozarks winter in a few
weeks and wanted to solicit your learned opinions first.



Art Trampler, KØRO


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