On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote:

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

Sorry, my original reply to this comment was screwed up thanks to me
talking on the phone while writing.

Yeah, the problem with turnstiles CP is at the horizon. At the
horizon, one element of the turnstile would be roughly perpendicular
to me, thus acting as a simple linear dipole. The other crossed
element would be seen from the end, resulting in no radiation in my
direction. So, at the horizon, a turnstile is mostly linear
polarization.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

If you really want a turnstile GPS antenna:
www.arrl.org/tis/info/pdf/0210036.pdf

As for the choke ring, you've probably seen them in the center of
C-band DBS/TVRO dish antennas.

Links:
http://www.javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html
http://www.trimble.com/infrastructure/gnss-choke-ring-antenna.aspx?dtID=overview
http://www.gpsworld.com/survey/news/trimble-choke-ring-antenna-uses-dorne-and-margolin-dipole-3620

More detail:
www.novatel.com/Documents/Papers/3D_choke_ring.pdf
If you remove the radome in the center, it's a "pinwheel" antenna,
with which I'm totally unfamiliar. Note the comments on "low
elevation tracking", which is what I was mumbling about for improving
the performance at the horizon. The above article don't show it, but
the choke ring does maintain some semblance of CP near the horizon.

Patent on the dual frequency (L1 and L2 for GPS) choke ring:
http://www.google.com/patents/about?id=EiwIAAAAEBAJ&dq=6278407