[amsat-bb] Antenna Polarization Question

Mon Mar 19 19:09:39 UTC 2018

ANy antenna that launches an RHCP wave out the front emits and LHCP wave
out the back.   That is why all real-world antrennas for gain have a
reflector so that the LHCP out the back gets a 180 phase shift and then
also goes out the fron RHCP.

But on a small satellite, there is not enough realestate for a huge
reflector.

Hence at one orientation, the wave may appear to be RHCP and the people on
the other side will see LHCP.

But then this effect is minimal if the antennas are linear on the
satellite which most are.  BUT if there are dual monopoles on the
sateliilte to make it cross polarized, then you do get RHCP and LHCP at
the same time. And so there can be nulls.  If you use exclusively one or
the other on the ground and the satellite orientation changes.

Bob
WB4APR

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Burns
Fisher
Sent: Monday, March 19, 2018 2:33 PM
To: Jean Marc Momple <jean.marc.momple at gmail.com>
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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> >>
> >>
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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.
> Not an AMSAT-NA member? Join now to support the amateur satellite
program!
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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.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb