omnidirectional circularly polarized FM antenna

[Ric Werme]

Tech question - how do they work?

In an off-list exchange with Dan Strassberg, I managed to ask
about this and much to my surprise he didn't send back a wealth
of information, claiming he's mostly an AM/MW person.

He suggested I talk to the group, but also named names.  I'll leave it to
the group....  Pretty much what I asked Dan follows.  For the less
technical folk on the list, light and radio waves can be polarized, common
sources of polarized light is blue sky, especially 90 degrees away from the
sun or reflected light, e.g. off glass or roadways.  Typical polarization
is in a plane, but if you combine a vertically polarized source with the
same source horizontally polarized and delayed by a 1/4 wavelength you get
a circularly polarized signal where the engery spirals out instead of
vibrating up/down or back/forth.

Millstone Hill Observatory is part of Haystack Observatory and shares a
road with various sites operated by Lincoln Labs.  Driving up the hill is a
good way to see some remarkable antennae -
http://www.haystack.mit.edu/atm/mho/index.html . Bicycling is better, you
get a longer look and can wonder what you're being fried with.

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It's puzzled me a bit that FM often uses circular polarization.  I'm
familiar with CP light, and have seen helical antennae along the road
through Millstone Hill Observatory that I figured were CP VHF/UHF for
ionospheric experiments or static noise suppression for satellite
reception.

My guess was that CP on broadcast FM could work with whip receiving
antennae at any angle and maybe with set-top rabbit ears.  I'd also expect
to see CP receiving antennae at Radio Shack, well, maybe elsewhere!
Perhaps the fancier set-top boxes with the rotary switches include some
lag/lead networks to capture CP.

I did a Google search, and noted that there are some vertical and
horizontal FM stations.  I hadn't considered that CP would be good against
multi-path, well, even path, signals.  Clearly I've spent too much time
bouncing light back instead of forward!

I also came across a flat CP receiving antenna - it uses an "X" shape
where one diagonal is too long and the other is too short to be perfectly
tuned dipoles.  The result is a 90 degree phaseshift.  Cute.  See
http://users.tns.net/~bb/attic.htm for details.

http://www.astronwireless.com/polarization.html is good too.

Finally, what does an omnidirectional CP transmitter antenna look like,
anyway?  The vertical component is no problem, of course, but does the
horizontal part need some serious compromises?

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I answered the "look like" query, one commercial source has
links to pictures and darwings, but I'm still not sure how they
work!  http://www.shively.com/nav-fmantennas.php

       -Ric Werme