Jim,
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" <jim@audiosystemsgroup.com>
To: "Sam Andrews" <sandrews@aristotle.net>
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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