Admittedly from a bit of a pessimist about manufacturers using the
least in products they could get away with instead of the best...
Any one of the components you mentioned could give the behavior you
quote. Capacitors WILL change value as they heat up. In truth
putting a real kW+ on a lot of "bought" antennas will stress the
components to change value and eventually fail.
Rated at 1.5 Kw probably means 700 watts at the antenna and that just
not causing flashovers or obvious heating.
Shortened vertical antennas need higher current to radiate the same power.
When I make stuff like your antenna parts for friends, I use things
like RG400, large powdered iron toroids, transmitting door knob caps
in parallel rated in total for TWICE the expected reactive current,
double-polyimide-coated wire plus teflon tubing for toroid windings,
and so forth.
Depending on how much you like the antenna when it's working, you will
find that just about all the components can be upgraded in some
manner. But if you don't go for them all then you risk having the
problem that the most stressed component which you find and upgrade
simply transfers the problem to the next most stressed.
The "jump" you mention sounds more like something shorting with heat,
but not carrying the entire current path. Could be a connection that
is not well bonded or a solder joint evaporated of solder from
lightning induction.
But the only thing that will work is the careful replacement and
redoing of joints and components.
If you like the antenna, go "class" with your replacements.
73, Guy.
On Sat, Sep 12, 2009 at 12:46 AM, Art Trampler <atrampler@att.net> wrote:
> 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.
>
>
>
> 73,
>
> Art Trampler, KØRO
>
>
>
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>
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