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Re: Question on 1/4 wavelength transformer

Hi Floyd.  Go to this AMSAT web page and go down to C.  Article 
"Circular Polarization, Antenna Matching".  About half way down the page 
is a article on it.

Bob  W7CTW  AMSAT 21409

Floyd Sense wrote:

> I'm looking at ways to feed a pair of crossed 70cm yagis so I can switch
> polarization for satellite work.  I've researched the Satellite Handbook
> ARRL Antenna Book and these questions arise from those readings.
> First, the antenna in question consists of a pair of 10 element yagis on
>boom, with a 1 inch offset between the vertical and horizontal elements (NOT
>a 90 degree spacing!).  The yagi driven elements are gamma matched and I'll
>assume for now that they're supposed to be adjusted for a 50 ohm match.
>While this antenna is believed to be Cushcraft, there's no documentation
>available, although I have the manual for the Cushcraft A144-20T Twist,
>which is identical in construction, but for 2 meters instead of 70cm.
> Now the questions:
> 1. Forgetting the requirement for switchable circular polarization for the
>moment, the Cushcraft literature shows, for "Axial Radiation", connecting
>the 50 ohm feedline to a "Tee" and then running an unknown length of RG-59U
>to the two gamma matches.  I assume that this length is an odd multiple of
>wavelength at the operating frequency (won't odd multiples of 1/4
>wavelength perform the same transformation as 1/4 wave?).  I don't
>understand why matching section is 72 ohm line.   With a single antenna and
>a feedpoint impedance of around 110 ohms, the 72 ohms would make sense to
>transform the impedance to that of the 50 ohm feedline.  Perhaps the gamma
>matches on these driven elements are actually adjusted for a 110 ohm match
>and then it would make sense to me.  What do you think?  Does the fact that
>the two antennas are being connected to a single feedline factor into the
>impedance chosen for the matching sections?
> 2. On page 10-10 of the Satellite handbook, Figure 10.11:  The figure shows
>feed systems for crossed yagis and again the 72 ohm transformation section
>appears, so, same question as above.  I want to use system (A) in that
>diagram, since the (B) and (C) apply to crossed yagis with a 90 degree
>offset.  In (A), I'm puzzled by why the 1/4 wave delay line is 95 ohms.  By
>the way, in these three systems the yagis are assumed to be 50 ohms
> If you can shed any light on either or both of these situations, I'd be
> K8AC
>Floyd Sense - Angier, NC
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