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[Towertalk] Cushcraft melted Balun

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Subject: [Towertalk] Cushcraft melted Balun
From: (VE6WZ)
Date: Tue, 11 Jun 2002 15:25:50 -0700

> Steve, as I understand it the issue is voltage, not current -- the induced
> voltage at the top of a shunt fed tower can reach several thousand volts
> with high power, and a surprising amount even with low power.  Perhaps it
> found a way to ground through the insulation on the balun windings...

Interesting...the damage really looks like appears like the core
stack just got VERY hot.
I can't see any evidence of arcing or other high voltage effects.  Unlike
some others, I have never
noticed arcing or sudden SWR changes indicating same when loading the tower
on 160.
I run 1 KW.  Maybe it is not the shunt feed that is the problem.
Could there be enough common mode current on 40m to generate this much heat?
I will "push" the beam during the SSB dx tests and xmit split very high in
the SSB band
with a high SWR.  The yagi is tuned for around 7.030.

> Does the XM-240 use some sort of a hairpin coil to get a match to 50
> ohms?  In that case, it might be a simple fix to ground the shield of the
> coax to the boom close to the feedpoint, so that the driven element's hot
> side is at the same RF potential (at 160 M) as the boom.

NO...the yagi is a 1:1 match to 50 ohms.  The benefit of those small lossy
coils : (

These 40m elements will be mounted on a boom together with a shortened 80m
2el Yagi,
and my plan includes relays to ground all elements during 160m xmit.

de steve VE6WZ.

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