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Re: circular polarization sense

on 10/9/00 10:00 PM, MERRETT, David at david.merrett@baesystems.com wrote:

> Can someone help me understand this?
> Ive been a bit confused about how much this affects signal strength if you
> get it 'wrong'.
> My first problem is I dont know which Sats actually transmit in circular
> pol.

ALL of them.  The satellites are typically stabilized (from what I
understand) by being spun.

Theoretically, if you have the sense of the polarization wrong, you will get
ZERO signal.  However, typically, due to distortions in the atmosphere and
what not, you will end up with an elliptical signal with an E-field vector
component greater in one direction than the other.

> Where can I find out what this info? does anyone already have a compiled
> summary of these kind of specs? (Im also interested in SAT Antenna, BeamW,
> pol, power etc).

Check the archives of this list over the last couple of months.  I and
others have posted our observations on CP and what happens during orbits.
If it wasn't 2:30 in the morning, I'd dig up the post myself for you!

> An article I read recently claimed that polarization will reverse sense
> after passing overhead (IE SAT goes from approaching to receeding). Implying
> that if an operator has a (say LH.) circular polarized RX ant, then he will
> only have good sigs on half of the pass
> To me this is counter intuitive if the sat is already transmitting in
> circular polarization.

Not at all really.  You see at the horizon, you have different effects going
on than when overhead.  While the satellite is on the horizon, it basically
appears like any other "terrestrial" station.  What I mean by this is you
will end up with multi-path reflections off objects, the earth, etc.  All of
this affects the resultant polarization you receive.  Keep in mind that when
the wave is reflected off an object, the sense of the polarization reverses.

When overhead, you have a straight shot to the bird with little in the way
to interfere with the signal.  You may get some distortion of the rotation
though due to potential E-layer clouds, tropo ducts, back scatter from the
ground beneath your antenna, Faraday rotation through the atmosphere, etc.

All in all, it makes up for much less than an "ideal" situation.

> how much does the atmosphere affect polarization?

See above paragraph.

> The way I understand it is that atmospheric effects cause a (slight?) random
> rotation over space or time or both.
> So a fixed pol antenna should have a good chance of getting a signal in its
> vector.

Not necessarily.  One of the reasons for CP is that it is really the most
forgiving polarization.  If you have a linear antenna, horizontally mounted
you will receive a perfect CP signal 3 dB below someone with a CP beam.
Now, in reality, that's not too bad considering that's about 1/2 an S unit.
But if things change elliptically, then we have a problem.  If the major
vector component is in the horizontal plane, then you will get a LARGER
signal than the CP antenna.  However, if the major vector component is in
the vertical plane, you'll hear virtually nothing while the CP antenna is
still working fine.  It makes a big, big difference.

You may be interested to know that TV stations (and I think FM Radio as
well) transmit in elliptical polarization (for TVs this is in the horizontal
plane).  I believe they found that the elliptical polarization helped to
combat a lot of the fading and multi-path interference that would happen
with a linear antenna.  There's also other reasons that escape me right now.

> But the other thing Id like to know is *if* a SAT is designed to transmit
> only one polarization, even if we elminiate atmosphere, is it ALWAYS this
> polarization? (IE can we be sure there is no small angular velocity on an
> axis radial to earth which would alter this over time? )

I think that's a risky proposition at best.  Keep in mind the bird is always
moving (even geostationary birds are moving - just at the same rate as the
earth turns).  You would have to guarantee that it would never change
position around its orbit in order to use linear antennas.  And anyhow, like
I said most birds are spin stabilized so the resulting signal out of the
bird is CP anyhow.

I hope this helps.



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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