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Re: [TowerTalk] Balun question

To: "" <>
Subject: Re: [TowerTalk] Balun question
From: "Jim Brown" <>
Date: Wed, 13 Jun 2007 20:54:32 -0700
List-post: <>
On Wed, 13 Jun 2007 20:22:48 EDT, wrote:

>    Anyway, all comments appreciated. 

At the risk of sounding like a broken record, I will point you to the 
tutorial on my website, supplemented by subsequent measurements of some 
coaxial chokes.

The following are some internal communications I shared with other guys who 
were doing their own choke research. I haven't had time to finish the 
measurements (I've been on the road a lot) and revise the tutorial to 
reflect the new data.

When Kevin (K6TD, who worked with me on the first series of measurements 
documented above) put up a 40M beam last weekend (10 ft above the StepIR 
that you installed for him), he wound 4 turns through a stack of 7 #31's. 
He only did 4 turns because that's the best he could do without taking the 
connector off the coax. Two more turns would have been better. 

For 6M, I suggest 2-3 turns of RG8 through about 5 #31 toroids. The turns 
MUST be wide spaced outside the toroids. See the photos in the links above. 

To understand my recommendations, consider that R comes from the short 
length of the coax passing through the ferrite cores, not the coil. The 
coax outside the cores adds inductance and stray capacitance, both of which 
lower the resonant frequency and raise the Q. An ideal choke has lots of R 
and a very low Q (which makes it more broadband). 

So it is important to minimize the contribution of the coax that is outside 
the cores. That contribution will be least if the diameter is smallest and 
the turns are widely spaced. Increasing the diameter of the turns lowers 
the resonant frequency of the choke a lot if you allow the turns to be 
closely spaced outside the toroid stack, but not a lot if you do not. The 
MOST important thing about winding these chokes is to keep the turns widely 
spaced outside the stack. I recommend the smallest diameter that does not 
compromise the recommended bend radius of the coax. 


Jim Brown K9YC


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