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Re: Question on 1/4 wavelength matching section



Floyd,

Regarding question 1, I believe that the Cushcraft literature is assuming 
that both antennas provide a 50-ohm resistive load *at the antenna*, once 
you properly adjust the gamma matches.  The 1/4 wave sections of 72-ohm 
coax then transform the 50-ohm impedance of each antenna UP to 
approximately 100 ohms at the end of each piece of coax.  When you connect 
the two coaxes in parallel at the Tee, you are essentially paralleling two 
100 ohm resistors to get the proper 50 ohm impedance that your coax and 
radio want to see.  An ideal choice of coax would be 75-ohm coax (if it 
existed), but 72-ohm is "close enough".

Regarding question 2 and circular polarization in general, CP is generated 
when waves leave the two driven elements 90 degrees out of phase, relative 
to the observer (satellite).  This means that there are two ways you can 
achieve CP:

1) Feed two antennas in phase, but shift one antenna "forward" of the other 
by 1/4 wavelength (90 degrees).  In this case, the only thing you are doing 
with your coax harnesses is doing impedance transformation.  Figure 
10.11(B) & (C) in the Satellite Handbook is for this configuration, as you 
accurately state.

2) Have two antenna where the driven elements are "lined up", or 
essentially the exact same distance from the observer (satellite).  To 
achieve the 90 degrees shift in this case, you would  need to feed one of 
the antennas 90 degrees before or after the other antenna.  You would do 
this by adding the additional line length in one of the coaxes in the harness.

The diagram in (A) is a bit more confusing, as the designers are doing 
something clever: they are achieving the impedance transformation AND the 
necessary phase shift via that one piece of 95-ohm coax and the 
relay.  Here is how to think it through:

First, note that BOTH antennas are at the end of 1/4 75-ohm impedance 
transformers, so BOTH antennas present 100-ohm impedances at their 
respective T connection (at the relay).  The diagram only clearly indicates 
that ONE of the coaxes are 75-ohm, but I believe they both are.

Now put the coax switch in the position indicated, connected to the top 
T.  The 50-ohm
feedline sees two things in parallel: the (transformed) 100 ohms from the 
top A antenna, and whatever is at the near end of that 95-ohm coax.  The 95 
ohm coax is itself a transformer, transforming the 100 ohms it sees at the 
end of the 75-ohm section from the B antenna, to about 90 ohms at the other 
end (the end nearest the relay).  So, the top T then sees 100 ohms in 
parallel with 90 ohms, which is close enough to the 50-ohm match you 
want.  I suspect that if 100-ohm coax were readily available, that would be 
an even better choice than the RG-133/U indicated in the drawing.

Now flip the relay the other way, and notice you get the same results 
(impedance wise), you just reverse which antenna is fed first and last in 
phase.  This is what provides the switchable circular polarization.

In your specific case, you say that the physical offset between your 
antennas is only 1 inch.  That means there is SOME phase shift between them 
(as seen by the observer) but not enough.  It seems to me that you will 
need to introduce the additional shift via your coax lines, such that you 
get the two driven elements creating waves that are 90 degrees apart as 
seen by the observer.

DISCLAIMER: I just made my first satellite QSOs about a month ago (RS-12) 
and my first mode J QSOs yesterday.  Therefore, I am a rank amateur (no pun 
intended).  I am going only on my hazy memories of electronic engineering 
school years ago for these comments!  :-)

At 12:34 PM 09/25/01 -0700, you 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.

73,

Eric, KU6J
CM99


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