[TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

[NPAlex at aol.com]

Bob,
I think a more accurate description of several coiled turns of coaxial  
cable would be a RF Choke, serving to limit common mode currents on the outside 
 of the coax cable.
 
Norm W4QN
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In a message dated 12/29/2010 3:01:05 P.M. Eastern Standard Time,  
towertalk-request at contesting.com writes:
Message: 1
Date: Wed, 29 Dec 2010  06:00:35 -0600
From: Bob Alexander <realex at flash.net>
Subject: Re:  [TowerTalk] 1/4 wave coax  "Balun"?
To: _towertalk at contesting.com_ (mailto:towertalk at contesting.com) 

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Norm,
A 75 ohm coaxial 2:1 impedance transformer does perform a BALUN  function 
when wound
into a coil or when several turns are  wound on  a suitable toroid.
73, Bob, W5AH

============== original  ============================
 
On 12/28/2010 10:31 PM, W4QN wrote:
> Jim,
>
> The  use of an ODD electrical quarter-wave of coaxial cable as a BAL-UN 
is  not
> correct.  The correct description would be "series coaxial  transformer". 
 And
> there are certain other conditions to be  satisfied when constructing this
> impedance matching network.  An  odd quarter-wave of 75ohm coax is a 
special case
> of the Series  Transformer (which uses two different pieces of coax) and 
is
> nominally a  2:1 impedance transformer.  Its application for Cubical Quad
>  antennas was popular as often the loop impedance of a quad was around 
100  ohms,
> and required a 1:2 impedance change to match a 50 ohm  source.  It does 
not
> perform a BALUN function (Balance to  Unbalanced).  Coaxial baluns can be
> constructed using electrical  1/2w piece of coax (Same Z)and connected so 
that
> the shields are  together, the feed coax center to terminal 1 of the 
antenna and
> the 1/2w  piece center from T1 to the other terminal of the antenna - 
this also
> is  a 4:1 impedance transformer as well.
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