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Re: Tracking AO-40 w/ C band - observations


Interesting observations:

>From: "Rich Dailey, KA8OKH" <okh.npi@gte.net>
>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
>* Near perigee,  the elevation angle of a0-40 can be as much as
>25 degrees below the clarke belt, given the same azimuth angle.

## Not surprizing since satellite altitude is so low.  Operation near
perigee is probably not possible with a standard TVRO dish setup.

>* 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.

## I find your limited dish movement angle surprizing.  I have installed
over 100 professionally and can usually get about 120 degrees of movement
before the actuator arm limits.  I wonder if your arm is on the east side
of the mounting pole.  It should have been installed on the west side of
the dish for covering 180-250.  Of course you may have a short actuator
arm.  Standard up in Alaska are 24 inch ones.

>* 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.

## Declination for our latitude ~61 degrees North is 8.6 degrees.  Most
dishes are capable of about 9.5 degrees.  Offset is only down ward as there
is no need for an upward offset.  In the southern hemisphere the dish faces
north so the offset is still downward.
Most dishes either use a setting screw or shim plates for declination
adjustment since it is set once for your latitude and not ever changed.
Changing the declination offset would only be an option if the dish were
dedicated to ham use only.  It should not be changed if still used for TV.

## Spill over should not really be too much of a problem and could be
compensated by narrowing the feed beamwidth by the amount of off-axis
mounting.  You would lose some gain then since it would under-illuminate
the dish.  In reality, I don't think this is very critical.  Something
other than a dipole feed is probably required.  A 3-turn helix is about
right for feeding a TVRO dish [focus/diameter ratio varies from .31 to .38
on c-band residential dishes].  A circular "can" horn is probably OK.
Depends on how particular you are.
>* 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 -
>73 de Rich
>Rich Dailey, KA8OKH - Phyllis Dailey, KB4NPI
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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