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RE: Helix Calculations



Message text written by "Howard Long"
>
I've also had a question about why I used odd numbers of quarter turns.
This
is for two reasons:

a) Ed Krome on page 118 of Mode S - The Book (2001 edn) describes how the
return losses minimize at odd numbers of quarter turns;
<
---------------

Howard:
        You know, I recently repeated those experiments and got exactly the
same results; best return loss at odd numbers of quarter turns. Possibly an
explaination of why 1/4 and 3/4 work better (considerably) than 2/4 or 4/4:
the first 1/4 turn is not really part of the antenna at all, but acts
exclusively as an impedance matching transformer, as it was supposed to do;
it didn't occur to me at the time that it would be effectively removed from
the rest of the antenna. Apparently so. Then it stands to reason that the
rest of the antenna likes to be in multiples of 180 degrees. 
        One of the problems with testing these little buggers is that I
have found it possible to test either 1) pattern or 2) return loss but...
not both at once. Just too much gear to drag out into the backyard all at
once. So I don't know the real effect on the pattern/sidelobes/whatever as
related to the return loss. If any. But it certainly would be an
interesting study.
        Speaking of studies, recent pattern tests on helix's, with a more
or less automated fixture (which I had been thinking about for years and
finally put together), where azimuth plots on the X  axis and db plots on
the Y while the antenna rotates 360 degrees, showed surprisingly good
correlation between the predicted results of the various helix programs
(helix.bas and helical.bas) and the actual pattern -3db and -10db points.
Always amazes me when the nuts-and-bolts match the theory. :-)
        If you haven't seen it yet, G3RUH and ON6UG have cooked up a really
interesting circularly polarized patch dish feed for .3 to .5 f/d dishes.
Man, is it neat.

73,

Ed Krome K9EK
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