The choke I've described IS a common mode choke, and far better than
anything you can buy.
73,
Jim K9YC
On Sun, 15 Feb 2009 20:43:31 -0800 (PST), Edward Sylvester wrote:
>Jim:
>Will a common mode filter be as effective?
>--- On Sun, 2/15/09, Jim Brown <jim@audiosystemsgroup.com> wrote:
>> From: Jim Brown <jim@audiosystemsgroup.com>
>> Subject: Re: [TowerTalk] To Choke or Not to Choke?
>> To: "towertalk@contesting.com" <towertalk@contesting.com>
>> Date: Sunday, February 15, 2009, 8:14 PM
>> On Sun, 15 Feb 2009 17:58:56 -0800 (PST), Edward Sylvester
>> wrote:
>>
>> >I have a LW/Inv L that I will feed thru an auto tuner
>> at the base,
>> >with radials. Was wondering if I should bother with
>> forming a
>> >coaxial choke (12-15 turns at 6" diameter) before
>> it enters the
>> >shack 30 feet away.
>>
>> I coil of coax with no ferrite is a waste of coax -- it
>> will look
>> inductive, the rest of the line will look capacitive, and
>> the result
>> will be lots of feedline current. You DO want a choke on
>> the coax
>> side of the tuner, but it needs to be 7 turns through five
>> Fair-Rite
>> #31 1.4-inch i.d. cores (commonly advertised as FT-240-31
>> by those
>> mark them up 4X their cost and resell them to hams). If
>> you're
>> feeding with RG8, you will need to wind the choke before
>> you put the
>> coax connector on -- you can fit 7 turns through those
>> cores, but
>> only with no connector attached.
>>
>> For a tutorial on all the technical background on this see
>> http://audiosystemsgroup.com/RFI-Ham.pdf
>>
>> 73,
>>
>> Jim Brown K9YC
>>
>>
>>
>> _______________________________________________
>>
>>
>>
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>
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