[amsat-bb] Re: Cupping a patch feed

[ae517@xxxxxxxxxxxxxxxxxxx (Russ Renaud)]

>Hopefully the patch experts will reply.  What you prpose sounds reasonable
>from an optical standpoint.  The questions that occur to me is whether ther
>is near-fiedl interaction at the edge of the patch with the "cup" wall?
>How deep will the cup need to be to effectively truncate the pattern?

Well, I'm not a patch expert, and I don't even play one on TV.  But I've 
wondered what the purpose of the "cup" wall is.  Having read W1GHZ's mcirowave
antenna book more than once, it would seem to me to be either:

	a 'choke-flange' similar to that of a VE4MA feed to reduce edge 
	currents; or, 

	a short cylindrical horn fed with a patch instead of a monopole.

I too would like to hear from an expert.  Certainly it's something that the
intrepid experimenter could play with.
	

>PS:  I have a question about patch feeds (actually two of them):
>1)  How broadband are they...I have the need for a circular pol dish feed
>on 1268-1296-1420 MHz!  (f/d=0.4)
>2)  Are there designs for two feedpoint patches (one RHCP and the other
LHCP)?

Well, for CP patch feeds with a single feedpoint, my understanding is that 
the patch is purposely made to be non-resonant at the frequency of
interest so that the feedpoint impedance is such that the resistive 
component is the same as the reactive component ie 50 ohms - j50,  then the
current phase will either lead or lag the applied voltage by 45 degrees. Then,
you change the geometry of the patch by adding a tuning screw or cutting off
some corners to excite a second mode in the patch.  This second mode should
have the opposite sign as the first mode ie 50 ohms + j50, so the current 
either lags or leads the applied voltage by 45 degrees.  The 'two'
feedpoints are in parallel, so the -j cancel out the +j reactance in terms
of feedpoint impedance, but the current still leads or lags the voltage.

This produces the necessary 90 degree phase shift for circular polarization.
I would suspect that going off-frequency would change the phase relationship
slowly going from circular to elliptical to linear polarization.

I *think* a design for a two feedpoint patch would be very similar to feeding
a CP yagi that doesn't use a 1/4 wave offset to produce quadrature, but uses
uses a 1/4 wave feedline to introduce the necessary delay. 

How's that for a lot of conjecture and obligatory hand-waving?

--
73 de va3rr/aa8lu
http://www.qsl.net/va3rr
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