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

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.

Sounds reasonable. Incidentally, the FM broadcast "cloverleaf"
antenna was invented by Philip Smith, the inventor of the Smith Chart:
http://www.ieeeghn.org/wiki/index.php/Philip_H._Smith_Oral_History#FM_Broadcasting_and_t he_Cloverleaf_Antenna

I blundered cross this page on a 6/2 meter CP antenna design. Looks
workable but very narrow band:
http://www.wa7x.com/cycloid_info.html

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.

With my limited abilities, it will probably take more than a "short
while" with an antenna modeling program. I've never done any
polarization studies. I'll take your word for this, but it would seem
that there would be a gradual transition from total reversal in sense
from a perpendicular reflector, to conversion to linear polarization
with shallow reflection angles. I'll see if I can find some research
on this. (I'm curious).

I once did some crude experimentation on the degree of isolation
provided by a reflective sense reversal. I just happen to have two
big 2.4Ghz helical antennas, both RH CP. I separated them by about 15
meters and measured the received signal level. I then placed an
obstruction (corner of building) along the line of sight, and supplied
a flat plate reflector. I didn't think to try varying angles of
incidence and reflection and just ran it at 45 degrees from the plane
of the flat plate reflector. The signal dropped about -15dB which I
guess is about all I could expect in an uncontrolled environment. The
loss would indicate that the signal was still substantially circularly
polarized at 45 degrees. I still have the helix antennas and can
repeat the test if necessary (and if I can find the time).

Thanks for the clarification.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558