[TowerTalk] 1/4 wave coax "Balun"?

[Bob Alexander]

On 12/28/2010 10:31 PM, Norman Alexander 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,
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