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[Towertalk] Cushcraft melted Balun

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Subject: [Towertalk] Cushcraft melted Balun
From: (VE6WZ)
Date: Tue, 11 Jun 2002 13:48:33 -0700
The Balun on my XM-240, 2 el 40m Yagi has been severely overheating.  This
is the Cushcraft MNXM 1:1 balun which is a stack of three, 1.5" torroids
wound with about 6 turns of teflon coax.  The plastic box has melted and the
outer jacket on the Teflon coax has cracked in numerous places. I have
posted a few pictures here:

I can't imagine that this heating is from 40m current on the shield.

My conclusion is that shunt feeding the tower on 160m has caused the
problem.  The XM-240 split driver is insulated from the boom of coarse, and
the yagi wants to be part of the action on 160 : )).
I recall a post recently about the need to ground insulated yagi elements on
shunt fed towers......maybe this is another reason.  My tower loaded fine on
160, so I assumed there was sufficient capacitive coupling to the 40m
yagi......I wonder how many 160m QSO's I've missed because of heating up
that balun  ; )
A new 4" air core balun will replace the CC unit.

Maybe something for others to check this summer while you are up the tower.

de steve VE6WZ.

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