[TowerTalk] tower resonances

[Jim Brown]

On Wed, 11 Feb 2009 09:47:16 -0800, Rudy Severns wrote:

>My interest is driven by a recent practical problem.  

Rudy,

I'm convinced that this is a VERY common problem with ground-mounted 
verticals, but that few hams are aware that it is. My main TX 
antenna on 160M is a T vertical. I've also used a vertical on 80M. 
My TX antennas for other bands are all high dipoles, typical coax 
lengths 130-150 ft. 

When I first moved to this QTH and was just getting antennas up and 
observing how well they worked, I noticed that the 80M vertical 
"stopped working" when I added a couple of the high dipoles. I added 
big ferrite chokes halfway down the feedlines of the high dipoles 
and the 80M vertical started working again. I subsequently 
constructed an NEC model of the vertical and those feedlines that 
showed considerable pattern distortion of the 80M vertical without 
the chokes, and the interaction going away when the chokes were 
added. 

73,

Jim Brown K9YC