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Re: [TowerTalk] Sigma 12

Subject: Re: [TowerTalk] Sigma 12
From: jim Jarvis <>
Date: Thu, 20 Nov 2008 18:37:34 -0500
List-post: <">>
K7lxc wrote:

     I recently installed a Force 12 80M Sigma.  It's like an H on  
it's side
and fed asymmetrically which means the feed is  off-set so you can  
reach it
from the ground. It's a shortened antenna so  it's got big loading  
coils at the

     I got the antenna to resonate at the proper  frequency (3.775 or  
so) with
decent SWR (1.3 or so to 1). But as soon as I  hook up some coax to  
it, the
resonance goes up 75 kHz or so and the SWR goes to  over 3:1. I tried  
2 coaxes
- 50-75 feet of RG-8X and RG-8. It seems like  the coax is transforming
something but I don't have any idea what's going on or  how to fix  
it. Any ideas are
appreciated. Help!

     We're going to contact F12 but don't think they'll  be able to  
help since
the new owner is not an antenna engineer, did not design  the antenna  
probably doesn't have any hands-on experience with the  antenna.

Steve,  Is it possible that your proximity to the lower 'leg' of this  
loaded vertical
dipole caused detuning?    One way to find out what's going on is to  
take an electrical
halfwave of coax and measure there.



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