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[TowerTalk] Fw: book Tribander comparison test

To: "towertalk" <>
Subject: [TowerTalk] Fw: book Tribander comparison test
From: "Sam Andrews" <>
Date: Fri, 4 Mar 2011 15:56:58 -0600
List-post: <">>
Tnx.  Makes a lot of sense.  Several pfs on each side of the choke at 30 mhz 
is a negligible amount.

73, Sam  AE5L

----- Original Message ----- 
From: "Jim Brown" <>
To: "Sam Andrews" <>
Sent: Thursday, March 03, 2011 12:50 PM
Subject: Re: [TowerTalk] book Tribander comparison test

> On 3/2/2011 5:09 PM, Sam Andrews wrote:
>> Jim,
>> I have read your chokes paper cover to cover and found it very 
>> instructive - thanks for making it available to us.  I also enjoy reading 
>> your posts to many of the threads.
>> I will be using RG8X/Big Clamp On cores  in my spring rebuilding project. 
>> One question though - to keep the chokes very near to the feed points, 
>> they will have to be attached to the beam boom,  and in the case of the 
>> tower supported vees, to the metal outrigger from the tower supporting 
>> them.  If done in that fashion, does coupling to the metal parts degrade 
>> performance? Or if I need to avoid such coupling, how much separation 
>> from the metal parts?
> Hi Sam,
> Your question is a very good one.  Obviously, we must be careful about 
> rigging the choke so that it is safe from mechanical damage. Taping the 
> coax to the boom results in capacitive coupling to the boom, which can 
> change the tuning of the choke a bit (lowering the resonant frequency). I 
> have not measured this (it would be rather difficult to do). My advice is 
> to secure the coax to the boom in such a way that capacitance is 
> minimized -- for example, secure it at one or more points, but let it loop 
> down away from the boom at least an inch or so for the rest of the path. 
> If you think about it, you have two small capacitors in series -- 
> the capacitance on one side of the choke to the boom, and the capacitance 
> on the other side of the choke to the boom -- plus the electrical length 
> of the boom in between, which is also capacitive if it's shorter than a 
> quarter wave.  Bottom line -- it's likely to be a fairly small effect if 
> you follow these guidelines.
> 73, Jim K9YC 


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