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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