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Re: Cross Yagi with folded dipoles.



Check out the theory of NEC2 and related programs.  It may not model 
things like J-poles and folded-dipoles that well.  My hand-held LPVA
(also used on FO-20/29) was designed by computer-aid experimentation 
with NEC2, and i am very pleased with the result.  The actual antenna 
does differ somewhat dimensionally from the model, but that it to be 
expected.  I, too, find that a higher SWR at 2m tends to desense the
70cm side.

With respect to polarization, i think the most important thing is to
have polarization diversity.  So using a linear polarization on the
ground with a circularized satellite, or visa versa, while not really
optimal, does seem to work fairly well.  Having a right/left mismatch
in circular is not much fun, and i heard lots of complaints about
polarization shifts on one of the Fuji birds this weekend; while i
was getting fairly consistent results with the hybrid approach.  On
the other hand, i could use a few dB more of downlink, so it really
is a trade-off.  Fortunately, i don't think elliptical vs. exact
circular is that big a difference.  Just another opinion, of course.

Conceptually, i much prefer a folded-dipole over a gamma match.  The
former is wider band and i can sort of understand it.  I find the
J-pole hard to deal with, but i built a homebrew colinear using that
as a feed mechanism (and managed make it operate dual-band, albeit i 
have no idea about the pattern at the 3d harmonic).  So i guess 
sometimes it's a matter of "whatever works".  I wish i could model 
that stuff better with NEC2, but that's another matter for another 
day.  My own design is direct feed at 50 ohms in both bands, so that 
kind of matching is not something i have to worry about.

It's good to hear about all the experimentation out here!

                        -- KD6PAG  (Networking Old-Timer, "RF newbie")
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