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Re:Question on 1/4 wavelength matching section



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> 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.
> http://www.amsat.org/amsat/articles/w6shp/ant_tips.html
>
> Bob  W7CTW  AMSAT 21409
>
> Floyd Sense wrote:
>
>> I'm looking at ways to feed a pair of crossed 70cm yagis so I can switch
>> the
>> polarization for satellite work.  I've researched the Satellite Handbook
>> and
>> ARRL Antenna Book and these questions arise from those readings.
>>
>> First, the antenna in question consists of a pair of 10 element yagis on
>> one
>> 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
>> 1/4
>> 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
>> resistive.
>>
>> If you can shed any light on either or both of these situations, I'd be
>> very
>> grateful.
>>
>> K8AC
>> Floyd Sense - Angier, NC
>>
>>
>>
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>>
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