[TowerTalk] Cushcraft XM240 & Shunt fed tower

K4SAV RadioIR at charter.net
Mon Jun 12 20:09:16 EDT 2006

Wow! Very interesting info on your XM240 balun, Steve. Please describe 
your tower, height, and what you have on it at what height.  I would 
like to model this and see what the model tells me about the currents at 
the XM240 balun. Based on some crude experiments, I always thought this 
balun was pretty much of a non-balun at 160 meters. Perhaps it has just 
enough impedance to produce a lot of dissipation even with relatively 
small currents.

Jerry, K4SAV

VE6WZ_Steve wrote:

>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/
>TowerTalk mailing list
>TowerTalk at contesting.com

More information about the TowerTalk mailing list