Jeff Liebermann wrote:
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

The "skew planar" looked like a cloverleaf antenna with each "petal"
rotated 45 degrees. The other was a copy of a broadcast antenna
advertised to be circularly polarized. I used a simple hand held dipole
and field strength meter to judge polarization. I know now it was
subject to a number of shortcomings, but I feel it did a reasonable job
of indicating circularity. Both antennas were reasonably circular.

. . .

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

. . .

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.

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.

. . .

Roy Lewallen, W7EL