I respectfully disagree with dismissal of moisture as a cause. It normally
takes months for coax to dry out (assuming low humidity) when moisture has
seeped into it (and when it does, it often goes several feet into it). Art
didn't say where he lives, so I'm assuming a "normal" weather pattern of
rain at least once per month.
Stephen W9SK
-----Original Message-----
From: antennaware-bounces@contesting.com
[mailto:antennaware-bounces@contesting.com] On Behalf Of DAVID CUTHBERT
Sent: Monday, July 13, 2009 3:57 AM
To: atrampler@att.net
Cc: antennaware@contesting.com
Subject: Re: [Antennaware] symptoms of balun saturation?
Art,
since it happens when it is not raining or damp we can discard the moisture
'evidence.' It certainly sounds like the core is simply heating. When it
reaches its curie temperature the relative permeability drops sharply. As
the flux swings in the core there is loss which heats the core.
The solution? Reducing power is the simple fix. I don't know about modifying
the "matching unit." Is it sealed? Another fix is to replace the AV640 with
a different antenna.
Dave WX7G
On Sun, Jul 12, 2009 at 2:48 PM, Art Trampler <atrampler@att.net> wrote:
> Hi All,
>
>
>
> I have an AV640 vertical from Hy-Gain, which is about 18' off the
> ground at the feed point. Overall I have been satisfied with its
> performance, which has generally met or exceeded (apparently realistic)
expectations.
>
>
>
> My chief complaint comes with respect to QRO. When running a KW or
> more into it (fed with LMR400, about 125'), my SWR will initially stay
> flat. I am a CW operator and after two or three ragchew exchanges the
> SWR will begin to increase a bit, and then suddenly go high (8:1 or
> better). It seems that this happens more frequently if it is damp/has
> been raining.
>
>
>
> Can you help me understand what is likely going on? Is this likely a
> symptom of balun core saturation which is somehow related to heat,
> rather than simply the power/magnetic flux?
>
>
>
> Or is it more likely this has something to do with the RF choke from
> the vertical element used to give a discharge path for static electricity?
> That
> choke is 470uh, .3 amp. Is there any reason not to replace it with
> something of greater current handling capability, and is the
> inductance critical (would 500uh work just as well?).
>
>
>
>
>
> Thanks for your replies.
>
>
>
> Art, K0RO
>
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> Antennaware@contesting.com
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>
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