[amsat-bb] Antenna Polarization Question

Tue Mar 20 03:34:51 UTC 2018

Hi Burns, Jean Marc and Satellite Fans,

Here is my explanation of why Antenna Polarisation varies:
If the satellite is transmitting random linear polarisation then a CP 
antenna should pick up most of the signal,
however remember that a CP antenna uses phasing between the Vertical and 
Horizontal antennas to determine
the sense (RHCP or LHCP) of the circular polarisation.
If you consider the satellite to also have Vertical and Horizontal 
components to its polarisation then how these
components change in phase when passing through the ionosphere determines 
whether RHCP or LHCP will work better.

In addition there is the problem of reflections (ground and adjacent 
objects) with the receiving antenna.
You can see this by looking a plots of the Vertical radiation pattern of 
antennas at various heights.
At the low heights of typical satellite antennas any Horizontal component of 
the received signal will vary
depending on the elevation angle of the satellite. Any Vertical component 
will be pretty constant.
As an example AO-91 seems to work better with Vertical polarisation at low 
elevation angles and changes to
Horizontal at high elevation angles (height above ground 1.5 metres).

So a satellite receiving antenna needs to be able to vary the polarisation 
to be able to avoid fading at some elevation angles.

73,
Terry Osborne ZL2BAC

-----Original Message----- 
From: Burns Fisher
Sent: Tuesday, March 20, 2018 7:33 AM
To: Jean Marc Momple
Cc: amsat-bb at amsat.org
Subject: Re: [amsat-bb] Antenna Polarization Question

I'd really like to understand this better, but it is still not making sense
to me.  I believe that if you are transmitting with a CP antenna, the E and
M waves actually go through an entire circle in one carrier cycle and a CP
antenna is able to "follow" that.  Surely a satellite is not spinning at
anywhere close to 145 or 450 million revs per second, so I don't get
"spinning satellite" as an explanation for why an LHP or RHP antenna might
work better at different times.

What I do get is that a CP antenna can receive linearly polarized waves at
any angle equally.  But this should be true whether the antenna is LHP or
RHP, and I would not think which direction should matter if the signal is
linear in the first place, even if the signal is spinning slowly.

That all said, I have definitely heard people say that they can get better
reception by changing from LHP to RHP.  I'm not saying this is not true.
Just that I don't understand it.

73,

Burns WB1FJ

On Mon, Mar 19, 2018 at 1:57 PM, Jean Marc Momple <
jean.marc.momple at gmail.com> wrote:

> Guys,
>
> Long debate for not much. I have followed this stream and believe that I
> need to intervene.
>
> It is very simple, most HAM birds does spin (sometimes a lot) and there is
> no way one can anticipate as different from one bird to others. It is just
> operator skills (on the spot) to determine/switch from LHCP or RCCP based
> on what  is the best received signal strength. It works both on the Uplink
> and Downlink. There is no miracle formulae and it is a just operator 
> skills
> as mentioned before.
>
> For commercial birds it is totally different game and should not be
> compared with our humble Ham birds, they have much more means to do things
> that we cannot afford to do, except if all HAM worldwide donate to AMSAT,
> say $10 we then may be able to match some of the features commercially
> available.
>
> Just a suggestion and food for thought.
>
>
> 73
>
>
> Jean Marc (3B8DU)
>
>
> > On Mar 19, 2018, at 8:54 PM, Franklin Antonio <antonio at qti.qualcomm.com>
> wrote:
> >
> > When you say a satellite "has" RHCP, we have to be careful that we're
> communicating clearly about what "has" means.  Every satellite "has" both,
> by virtue of he way antennas work.
> >
> > I suspect that you mean that the satellite has an antenna which
> transmits RHCP in its main lobe.  It is important to realize that the
> signal from such an antenna is only RHCP in its main lobe.  The sidelobes
> are gonna be LHCP.  In between, you can get anything in between.  So if a
> satellite is oriented so that its antenna points right at you, and they
> designed it to be RHCP, then that's what you're gonna get, but if it is
> pointing off to the side, then you get something else.
> >
> > This means that there are situations in which you might get a stronger
> received signal if you switch to LHCP, or maybe even to linear.  In the
> commercial satellite biz, they design satellites so that their antennas
> point at the users.  Hams build cheaper satellites, which typically don't
> have sophisticated attitude control, so sometimes they point away from
> you.  Also, hams try to use the things even when conditions aren't the
> best.  If that's your aim, then most folks have found that polarization
> switching sometimes helps.
> >
> > ________________________________________
> > From: AMSAT-BB <amsat-bb-bounces at amsat.org> on behalf of Jordan Trewitt
> <jmtrewitt at gmail.com>
> > Sent: Sunday, March 18, 2018 2:43 PM
> > To: Eduardo PY2RN
> > Cc: amsat-bb at amsat.org
> > Subject: Re: [amsat-bb] Antenna Polarization Question
> >
> > Maybe I'm not understanding it, but why does one need to switch between
> > both, unless a particular satellite has LHCP or RHCP?
> > Jordan
> > KF5COQ
> >
> > On Sun, Mar 18, 2018, 16:34 Eduardo PY2RN <py2rn at arrl.net> wrote:
> >
> >> Hi Brian,
> >> Both. You are going to need to switch between RHCP and LHCP often
> during a
> >> sat pass. The same happens if using linear polarization (V/H) but in
> this
> >> case the switch between V and H will happen much more often than in CP.
> >> 73
> >> Ed
> >> PY2RN
> >>
> >>      From: Brian via AMSAT-BB <amsat-bb at amsat.org>
> >> To: "amsat-bb at amsat.org" <amsat-bb at amsat.org>
> >> Sent: Sunday, March 18, 2018 6:21 PM
> >> Subject: [amsat-bb] Antenna Polarization Question
> >>
> >>
> >>
> >> Should I be using LHCP or RHCP when setting up the 2 meter and 440
> yagi's
> >> to work the LEO's.
> >>
> >> Thank you
> >>
> >> Brian, KG8CO
> >>
> >> _______________________________________________
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> >> of
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> >>
> >>
> >> _______________________________________________
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> >> to all interested persons worldwide without requiring membership.
> Opinions
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> > _______________________________________________
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> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions
> expressed
> are solely those of the author, and do not reflect the official views of
> AMSAT-NA.
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_______________________________________________
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to all interested persons worldwide without requiring membership. Opinions 
expressed
are solely those of the author, and do not reflect the official views of 
AMSAT-NA.
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Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb 