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Re: Circular Polarity



Hi All,

I've seen a few statements regarding the helix antennas on AO-40...  as far
as I know, there aren't any!

While AO-10 has a helix antenna for 1.2GHz, and AO-13 had one for 2.4GHz as
well, AO-40 uses an assortment of different antennas.  Take a look at the
gain antennas, clearly visible in this photo (my favourite) :
http://www.amsat-dl.org/launch/L_image011.jpg

Here's my interpretation of this photo, with assistance from the AMSAT
Phase 3D Antenna Design Review at
http://www.amsat.org/amsat/sats/phase3d/antennas.html

1.  The 2m antenna array is composed of 3 folded dipoles evenly arranged
around the satellite's +Z facet.  They can be seen in the photo, each
dipole mounted one three white stand-off insulators

2.  The 70cm antenna array is composed of 6 circular patch antennas,
operating in RHCP mode, arranged around the 400N motor's exhaust.

3.  The 1.2GHz antenna is a short back-fire antenna, turnstile feed (Large
silver antenna, furthest from camera)

4.  The 2.4GHz antenna is a 50cm parabolic dish fed by a turnstile antenna
(large, yellow, upper-most antenna)

5.  The 5.6GHz antenna is a 25cm parabolic dish, probably also fed by a
turnstile / reflector arrangement.  This is the small yellow antenna at the
bottom of the photo.

6.  There are two separate horn antennas for the 10GHz system;  one fed
from a solid-state amplifier, and the other from a Travelling Wave Tube
(TWT).  These can be seen at the bottom of the photo, below the 5.6GHz
antenna.  Red "remove before flight" covers protect them from ingress of
dirt,etc.

7.  The 24GHz horn antenna can be seen at the closest point to the camera,
and is also fitted with a red "remove before flight" cover.  You can
actually see the horn inside the satellite.

8.  The four "hot cross buns" (1.6GHz patch antennas) arranged around the
+Z facet are part of the GPS experiment.

Apart from the 70cm array, I couldn't find any explicit statement that all
the high-gain antennas are operating in RHCP, however I believe they are.
Also, Table P3D-4 in the Radio Amateur's Satellite Handbook (Davidoff)
states that they are RHCP.


In photo http://www.amsat-dl.org/launch/L_image039.jpg , you can see the
three quarter-wave whips on the -Z facet, which are the omnidirectional
antennas for 2m, 70cm and 1.2GHz.  (The red-capped device in the center of
the satellite is the 0.1N arc-jet exhaust).


Regards,



Chris


p.s  Ideally, an RHCP antenna on the ground should be used to receive a
RHCP antenna in space





At 16:56 29/12/2000 -0500, you wrote:
>Hi Gang,
>
>      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 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...
>
>      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!
>
>TNX and 73, Ken
>
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>
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