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Tracking AO-40 w/ C band - observations



Here are some observations regarding the use of a tvro polar
mounted dish to receive ao-40.  I used Instantrack and STSPlus
to calculate the az/el of most of the popular C band satellites programmed
into my tvro system,  and compared them with positions of ao-40
at various phases of it's orbit,  over a period of 3-4 revs.  Please note
that this info applies to my location - your mileage can/will vary.  And
also keep in mind that this info was gathered only from 3-4 revs of
ao-40.

* Near perigee,  the elevation angle of a0-40 can be as much as
25 degrees below the clarke belt, given the same azimuth angle.

* Near apogee, ao-40 typically rides 6-8 degrees above the clarke
belt,  and can increase to 11 degrees or so 3/4 of the way thru
the orbit.

* There are, of course, times when ao-40 passes right across the
clarke belt (as seen from the antenna).  I can't find much pattern
to these occurances,  but they do happen for very short periods
of time.

* Without the benefit of a horizon-horizon mount,  my dish will
only provide azimuth coverage from 180 to 250 degrees.  This
limits reception possibilities quite a bit.

* The declination offset of three different dishes (two 10', one 8')
only allows for declination adjustment plus-minus 5 degrees,  again
limiting possibilities if the feed is at the prime focus.  Offsetting the
S band feed would help somewhat,  but I guess this would introduce
a risk of illumination spill-over.

* The declination offset could be automated,  but most tvro dishes
are limited in azimuth coverage,  so in my opinion,  the az/el mount
still wins.  *However*,  it is something that is do-able,  given the
right set of circumstances.

I would encourage anyone that has a functional C band system that
is properly aligned to give it a try.  You can mount the s band feed a
little off-focus to compensate for the declination.  You may even have
a spare RG-6 cable already running to the house  (most direct burial
c band cable has two RG-6 cables,  although most of the time, only one
is used - unless you have a dual C-Ku system).  Some attenuation may
be required - I don't know.  Run off the az/el numbers of the C band
sats from your location,  and compare them with pass predictions of ao-40,
keeping in mind the azimuth limits of your dish.

Here are some links regarding the declination offset angle,  and
general alignment of a tvro system -

http://www.geo-orbit.org/sizepgs/tuningp4.html#anchor799066

http://pages.prodigy.net/nelpi/nelpageo.htm

73 de Rich


-------------
Rich Dailey, KA8OKH - Phyllis Dailey, KB4NPI
mailto:okh.npi@gte.net
http://home1.gte.net/web22jfw/
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