[amsat-bb] Polarization (attitude mostly)

Tue May 31 20:20:52 UTC 2016

Actually, I would tend to suggest the majority of polarization shift is
simply due to the always changing attitude of the spacecraft with respect to
the user.

The fact that all TVRO Satellite dishes worked perfectly well when switching
back and forth from horizontal to vertical polarization when changing
channels and once the dish was initially aligned then those vertical and
horizontal polarizations remained accurate across the entire sky and across
the many dozens of satellites, then would suggest th contribution due to
faraday rotation was small (at C band anyway)...

Yes, there is some faraday rotations at HF and at extremely low elevations,
but I think for one using an ARROW antenna, all of the polarity issues are
simply due to the instantaneous orientation of the satellite.  Not the
ionosphere...

But, just my humble opinion...

Bob
WB4APR

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Bob
Sent: Tuesday, May 31, 2016 1:52 PM
To: Doug Andrews
Cc: AMSAT-BB
Subject: Re: [amsat-bb] Polarization

"The antennas I see in the photos of satellites we work are whips. Is the
polarization becoming "circularized" as it re-enters earth's atmosphere or
something?"

Yep, that is exactly what is happening.  It is called Faraday Rotation, and
as the signal from the satellite passes through the ionosphere, all sorts of
polarity changes can and do happen. A linear polarized satellite antenna
(horizontal or vertical) can appear to be the opposite or somewhere in
between.  That's why folks rotate their Arrow or Elk antennas -- trying to
match the polarity.

Using a circular polarized antenna helps a bunch -- it doesn't matter what
the polarity of the linear satellite antenna happens to be at any moment in
time.

But there is no free lunch -- Even a circular polarized antenna might need
to be switched from Right Hand Circular Polarization (the default) to LHCP
from time to time depending on what nasty thing the ionosphere is doing at
any given moment.  Changing the polarity switch might bring a S0 signal up
to S5, a 30 dB improvement.  I had that happen to me during a recent ARISS
contact.

73, Bob, WB4SON

On Mon, May 30, 2016 at 8:25 PM, Doug Andrews <dougg27 at hotmail.com> wrote:

>
>
> I too have wondered about this.
> I have not had much trouble hitting SO-50 and some success on AO-85
> with a
> 5 watt handheld and arrow antenna without turning it. Worth a try.
> DougKG7UNU
>
>
>
> Sent from my Sprint Samsung Galaxy® Note 4.
>
> -------- Original message --------
> From: Ken Alexander <k.alexander at rogers.com>
> Date: 5/30/16  4:41 PM  (GMT-08:00)
> To: amsat-bb at amsat.org
> Subject: [amsat-bb] Polarization
>
> I clipped this from another message because I didn't want to drag the
> discussion off course.  It's a question I've been wondering about
> since getting into this a few short weeks ago.
>
> I've also read (but haven't tried yet) about the trick of rotating the
> antenna 90 degrees on transmit, once you've established the best
> receive orientation.
>
> 73 de Bill, KG5FQX
>
> So far, with SO-20 I have rotated my Arrow antenna for best reception
> of the downlink and don't think I've had too much trouble being heard.
> At the same time I have wondered whether I should twist the antenna
> when transmitting to orient the 2m elements to give the same
> polarization as in receive. I don't know if this is a good idea or
> not, and frankly I have enough trouble remembering calls and grids,
> tracking the satellite, adjusting frequency and switching back to the
> correct VFO to worry about one more thing.
>
> I've seen that some commercial OSCAR antennas use circular polarization.
> The antennas I see in the photos of satellites we work are whips. Is
> the polarization becoming "circularized" as it re-enters earth's
> atmosphere or something?
>
> Comments and observations would be most welcome!
>
> 73,
>
> Ken Alexander
> VE3HLS, FN03
>
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interested persons worldwide without requiring membership. Opinions
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Not an AMSAT-NA member? Join now to support the amateur satellite program!
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