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Re: Satellite Antenna Polarization Data



Many of the sats have a predetermined polarization (at the satellite antenna), but that does not guarantee that polarization will be constant at the ground station.  From 4 years of working thousands of contacts on various LEOs, I can absolutely state the ground station "view" of polarization is practically random.  This is due to many factors, including spin of the spacecraft, angle to the groundstation, approach versus retreat of the spacecraft in relation to the groundstation, and Faraday rotation.  A typical LEO satellite like AO-27 will change it's "apparent" polarization several times during a pass.  This is why having switchable polarization on circularly polarized antennas is popular.  I don't want to start the argument over linear versus circular polarization, but I will state several lessons learned:
- the "correct" circular polarization is always best, but that means being able to switch
- linear is simpler and easier, but requires significantly more gain to hear as well
- a low gain antenna at the wrong polarization will be deaf to LEOs, thus the common "fading" effect
 
In short, selecting a RHCP antenna for a specific LEO satellite will not be an optimum solution all the time, only some of the time.  If you want to hear well all the time, either go for switchable CP or high-gain linear antennas.  One of the reasons modest hand-held antennas work so well for LEOs is because your wrist is connected to your ears via your brain... and you automatically adjust for best polarization.
 
73,
Jerry, M0MOE
 
Hard to tell for sure from this but:

>From http://www.amsat.org/amsat/sats/n7hpr/ao27.html

"The uplink antenna is the linear polarized whip on the top face of the
spacecraft and is shared with the commercial payload's receivers.  The
downlink antenna is a 1/4 wave whip mounted on the bottom face of the
spacecraft.  Polarization is nominally linear, the rotation and
revolution of the spacecraft and propagation effects will cause the
actual signal polarization at a ground station to vary widely during a
pass."

73's

Don WoodwardKD4APP



On Wed, 2006-03-15 at 21:13 -0500, Andrew Glasbrenner wrote:
> I'm almost positive AO-27 is circular on the downlink, which direction I 
>can't say. Emailing one of the control ops thru www.ao27.org should settle 
>it....
> 
> 73, Drew KO4MA
> 
> -----Original Message-----
> >From: Alan WA4SCA <WA4SCA@amsat.org>
> >Sent: Mar 15, 2006 9:14 AM
> >To: AMSAT-BB <amsat-bb@amsat.org>
> >Subject: [amsat-bb] Satellite Antenna Polarization Data
> >
> >Hi,
> >
> >So far, with the help of work by OZ1MY, WD0E, WA4SXM, and many fragmentary
> >references from various internet sources, I have come up with the following
> >tentative list of satellite antenna data.  It doesn't cover birds no longer
> >operative.  Please take a look and pass me any corrections.  At the end of
> >the list is the proposed format, chosen for brevity, with AO-51 used as an
> >example.
> >
> >
> >Alan
> >WA4SCA
> >
> >
> >============================
> >
> >
> >AO-16
> >
> >U: RC 437.050 MHz RHCP 
> >   PSK 437.025 MHz LHCP 
> >
> >AO-27
> >
> >V:  Linear
> >U:  Linear
> >
> >FO-29
> >
> >V:  RHCP
> >U:  RHCP
> >
> >SO-50
> >
> >V:  Linear
> >U:  Linear
> >
> >AO-51
> >
> >T: Linear
> >V: Linear
> >U: TX A (usually digital) LHCP
> >   TX B (usually analog) RHCP
> >L: Linear
> >S: Linear
> >
> >VO-52
> >
> >V: LHCP
> >U: RHCP
> >
> >ISS
> >
> >V: Linear
> >U: Linear
> >
> >
> >
> >=====
> >AO-51 ECHO
> >Catalog number: 28375
> >Launch date: June 29, 2004
> >
> >Status: Testing
> >Curent Modes: FM Repeater, V/U-High power
> >Digipeater - OFF
> >
> >Analog voice downlink: 435.300 MHz FM
> >                    435.150 MHz FM
> >Analog voice uplink: 145.880 MHZ FM
> >                  145.880 MHz USB
> >                  145.920 MHz FM 67Hz PL tone
> >                 1268.700 MHz FM 67Hz PL tone
> >
> >Digital Downlinks: 435.150 MHz FM, 38k4 Digital, PBP, 1 watt output
> >               2401.200 MHz FM 38k4 bps, AX.25
> >Digital Uplink: 145.860 MHz FM, 9k6 Digital, Pacsat Broadcast
> >             Protocol (PBP)
> >
> >Mode and Antenna Polarization:  
> >T:Linear, V:Linear, U:TX A (usually digital) LHCP, TX B (usually analog)
> >RHCP, L:Linear, S:Linear
> >
> >Broadcast: PECHO-11
> >BBS: PECHO-12
> >
> >Official Webpage: http://www.amsat.org/amsat-new/echo/
> >==============================================
> >----
> >Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> >Not an AMSAT member? Join now to support the amateur satellite program!
> >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



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