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Re: [TowerTalk] short multiband dipole and 'isolation coils'

To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] short multiband dipole and 'isolation coils'
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 14 Dec 2008 19:22:20 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Sun, 14 Dec 2008 14:32:09 -0600, K4SAV wrote:

>Isolation coils is a term invented by the company.  I think what they 
>are doing is using a self resonant coil but not operating it at the self 
>resonant frequency.  This requires a little explanation. 

>Looking at their coils, it is obvious that these are not high Q coils, 
>yet they claim they will handle full legal power.  It's not likely you 
>can do that with low Q coils operating at their self resonant frequency 
>in a trap dipole. 

I have MEASURED the coils on two of Barry's antennas and put them in an NEC 
model. They are NOT traps, they are not "isolation" coils, they are simply 
loading coils. The significant results of the model are in the PPT that I 
listed in my previous email. This design fooled me -- before I measured the 
coils I also thought they were traps. They are not -- indeed, I never found 
a resonance below about 15 MHz!  

I'm not sure what you'd call "high Q," but I've used two different models of 
Barry's antennas running legal power for lots of contesting (heavy duty 
cycles, so lots of power stress) with no ill effects. After several years of 
this, I can see no signs of heating, and their actual Q is higher than I can 
measure with any accuracy.  

The antennas I've used and tested are the long 80/40 and the long 160/80, to 
which I've added a 40M fan element. One of my buddies has a multi-band 
version that's pretty short. I helped him put it in his trees. It's not 
nearly so well behaved, and I wouldn't recommend it. 

73,

Jim Brown K9YC 


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