[TowerTalk] Cushcraft XM240 & Shunt fed tower

[Kelly Jones]

Steve,

Thanks for the great information.  I wonder if heating will be an issue
with a coax balun?  I did have the MFJ BN76 (or whatever the model # is)
balun on the antenna up until about a month ago.  At that time I took it
down and tore everything apart looking for an intermittent.  When I put
it back on the tower I replaced the "boxed" balun with a coax one (I've
become a fan of coax baluns).  Would there still be heating in a coax
balun?

73
Kelly - N0VD

PS - sorry for the "double" post.  I think my first message got held for
approval since I inadvertently sent from a non-subscribed address.  It
must have recently been released by the moderator.

 

-----Original Message-----
From: VE6WZ_Steve [mailto:ve6wz at shaw.ca] 
Sent: Monday, June 12, 2006 5:35 PM
To: Kelly Jones; TowerTalk at contesting.com
Subject: RE: [TowerTalk] Cushcraft XM240 & Shunt fed tower

Kelly:

Don't forget about the driven element.
That driven element may have a DC ground through the coax shield, but
the current balun can get real hot carrying the 160m RF !!

A WORD OF CAUTION when shunt feeding a tower with an XM-240 on top:
There is a very good chance you will "cook" the balun unless you short
the driven element to the boom when TX on 160m.  Here is a photo of what
my XM-240 CC balun looked like after a year of shuntfeeding the tower on
160 without shorting the current balun:
http://www.qsl.net/ve6wz/CCbalun.htm  I came to suspect this problem
when I could see the 160m SWR slowly drift with KW key down as the balun
heated up !!!

It's likely each shunt-fed tower setup may act differently depending on
the top loading configuration (other Yagis etc.) but you may want to put
a small relay at the driver.  I just use a small 12v relay that is
powered in parallel with my tower mounted switch box.

BTW...before I grounded my XM-240 reflector I would here the same
"tick"-"tick" sound, but not only when electrical activity was nearby,
but also on warm breezy sunny days too.  Wind static ???

de steve ve6wz http://www.qsl.net/ve6wz/