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Re: Satellite Antenna Polarization Data



Ok, so if you don't have the  ability to switch polarization on a crossed 
yagi design then you are better off going with a high gain linear polarized 
antenna. Got that. Now the question is, how much gain do I need on that 
linear antenna to be able to hear well enough? Can anyone give me a ball 
park figure?
 The reason I ask is that I constructed a cross  yagi antenna that was not 
going to be switchable. It was RHCP only. I got frustrated with figuring out 
the feed harness however, so now I'm back to square one for my 70cm antenna. 
Seems like in my situation that linear is the way to go anyway.
73,
Michael, W4HIJ
AMSAT # 36017
----- Original Message ----- 
From: <k5oe@aol.com>
To: <amsat-bb@amsat.org>
Sent: Thursday, March 16, 2006 3:29 AM
Subject: Re: [amsat-bb] 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
----
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