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Re: Circular Polarity



Ken wrote:
>From: Ken Freedman <n1qqv@cshore.com>
>      Today I bit the bullet and ordered an S-band down converter/preamp 
>that has really nice specs.  That was the easy part.  Next I spent a couple 
>of hours trying to figure out the polarity of the S-band hi-gain antenna on 
>AO-40 so that I can start shopping for an antenna.  I  could find no 
>polarity info for any antenna on AO-40 except the 70 cm arrangement which 
>is RHCP.  I did find an article by G3RUH about how to build an S-band dish 
>(got that tucked away on the hard drive!) for AO-13, in which he uses a 
>helix at the feed, wound "opposite to a normal screw thread".

I think you will find that all the circular polarized antennas are set up
in RHCP on the satellite, but don't take my word for it, check with amsat.
The helix feed for a dish that G3RUH had in his article was probably LHCP,
because the polarization is reversed by the dish.  By using a LHCP feed, he
has set up the dish to receive RHCP signals.
 
>      I believe that a signal that leaves the bird from an  RHCP antenna 
>would want to see an LHCP antenna on the ground, unless a dish were in use 
>which would reverse the polarity on reflection to the focal point.  I
think...

NO.  You have it backwards.  If the antenna on the bird is RHCP then your
antenna on the ground must also be RHCP.  If your antenna is a dish then
the feed will have to be LHCP to receive RHCP signals.  

Here is a little exercise that will allow you to visualize this:  Take two
ordinary bolts {with right-hand threads}.  To turn a nut onto the bolt, you
must roate the nut in a right-hand or clockwise direction looking from the
nut toward the bolt.  To visualize how two CP antennas work, place a bolt
that has a nut threaded on and place it end-to-end with and identical bolt.
 If you turn the nut off the first bolt, it will thread right onto the
other bolt by continuing to turn the nut in the same direction.  Both bolts
have the same sense thread.  Think of the two CP antennas as the two bolts
facing each other.  The signal continues to "corkscrew" in the same
direction as it leaves one and enters the other.

Always determine the sense of polarization of an antenna from its back-end
looking in the direction of radiation.  RHCP twists in a clockwise
direction, and LHCP in a counter clockwise direction.  Don't feel bad if
this seems confusing.  I always have to sweat over phasing cross yagis for
CP...everytime ;-)

>      There must be someone out there who knows the S-band antenna 
>characteristics for AO-40.  Any help directing me toward the path of 
>righteousness would be appreciated.  Especially at these prices!

I defer to others on what sense AO-40 CP antennas are.  I'm at work and my
books are home ;-)

 Ed

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