P3D Re: polarization

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Bob,

> After I posted that, I got to thinking as to whether it was
> correct or not, ie. you get a 3db loss in
> 
> circular -> linear   mismatch
> or
> circular -> opposite circular mismatch

I'll stick to my first notion. Signal type and antenna type matched
results in no loss. Circular signal into circular antenna of opposite
handedness results in very high loss. Linear signal into crossed linear
antenna results in very high loss. Linear signal into circular antenna
results in 3dB or 50% loss. Circular signal into linear antenna results
in 3dB loss. 

A circularly polarized signal can be thought of as a pair of linearly
polarized signals, crossed at 90 degrees and out of phase by 1/4 cycle.
Each component carries half the power. One of these linearly polarized
components is parallel to a linearly polarized antenna. The other is
perpendicular to the antenna. The result is that the component that is
parallel to the antenna is passed with no loss while the component that
is perpendicular to the antenna is completely lost. Thus, only 50% of a
circularly polarized signal will be passed by a linearly polarized
antenna.

A linearly polarized signal can be thought of as a pair of circularly
polarized signals. One is right handed while the other is left handed.
Each of these two components carries half the power. If you have a right
hand circularly polarized antenna, then the RHCP component is passed
with no loss while the LHCP component is completely lost. Thus, half the
power of the original signal is detected.
 
> I think I may have been confusing this with the discussions that
> occurred at the time that FM radio broadcasting first became
> widespread (yeah, I'm that old). At the time most broadcast
> band FM radios where horizonally polarized on the tower because
> they figured most people were listening at home. Since most
> auto antennas are vertical, you took a hit in signal strength.
> I've not heard about that for several years. I wonder if normal
> BC/FM transmitting antennas are still horizontal?

Broadcast radio FM is still horizontally polarized. Same for TV sound
signal which is FM. Broadcast radio AM and TV vieo which is AM are
vertically polarized. Have you noticed that the windshield antenna on
cars is T shaped. The vertical part is for AM and the horizontal part is
for FM. My guess is that it does not either job very well. Probably
significant losses for either AM or FM but it is a cheap compromise that
gets enough signal for most folks. 

GPS is circularly polarized. It often comes from high in the sky. I
don't recall which handedness.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design



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