On 12/29/2010 9:35 PM, NPAlex@aol.com wrote:
> 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
> ======================================
>
>
> In a message dated 12/29/2010 3:01:05 P.M. Eastern Standard Time,
> towertalk-request@contesting.com writes:
> Message: 1
> Date: Wed, 29 Dec 2010 06:00:35 -0600
> From: Bob Alexander<realex@flash.net>
> Subject: Re: [TowerTalk] 1/4 wave coax "Balun"?
> To: _towertalk@contesting.com_ (mailto:towertalk@contesting.com)
>
> =======================================
> 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.
> ===========================
>
>
Norm,
Correct, it is an RF choke. A BALUN works as an RF choke in series
with the outside of the coax shield
Bob
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