[TowerTalk] Cushcraft XM240 & Shunt fed tower

Kelly Jones kjones at chefscatalog.com
Mon Jun 12 11:39:14 EDT 2006

Hello TTs,
As some of you may recall, I'm in the process of shunt feeding my shorty
tower for 80/160.  On top of this tower is a Cushcraft XM240.  Some have
suggested that I may need to ground the reflector in order to get enough
top hat loading.   Some have also suggested this in order to prevent
arcing at high power from the boom to element as well.
Well, I had an interesting event take place yesterday afternoon.  We had
what was essentially a "dry" thunderstorm come through.  The air was
very "charged".  You could just feel it.  I happened to be in the back
yard and I kept hearing this "tick, tick, tick" noise.  After a little
investigation, this noise appeared to be emanating from the back of the
XM240.  Then I happened to notice that every time there was a
cloud-to-cloud lightning strike, the "tick" would become a "crackling"
noise.  The intensity of the "crackle" would depend on how close the
lightning strike was.  On a couple of occasions, I noticed the area
around the rear of the XM240 light up with white light when it would
"crackle".  This caused an extremely uneasy feeling!
I was not brave (or dumb enough) to walk out underneath the antenna to
see if I could visually see where it was arcing, but my gut tells me it
was between the reflector and boom.  So my question is has anyone put a
ground strap between the reflector and boom?  Would this change the
characteristics of the antenna?  I tried contacting CC to bounce the
idea off of them, but trying to get to a real person doesn't seem
possible.  I know that the older 402CD did use a ground strap but for
whatever reason the XM240 does not do this.
I am pretty certain this will change the my shunt feed project as well,
hopefully for the better, but I obviously have an issue that needs
addressed.  I suspect if I do nothing it's just a matter of time before
the smell of ozone is in the air (and in my shack).
Kelly - N0VD

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