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: http://www.qsl.net/ve6wz/CCbalun.htm
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.