Re: [amsat-bb] Using C band polar mounts w/ ao-40

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

John,

Good suggestions as far as you went.  Remember that TV satellite dishes
have an additional vertical offset called "declination".  So in fact the
dish is pointing a few degrees below the equatorial plane.  This is to
correct for latitude when observing near-earth objects in a geostationary
orbit.  So you would need to add the offset angle to the 5 degrees to have
pointing correct for apogee [in other words the feed position will be 5 + 7
degrees for mid-latitude northern hemisphere locations].  

The dish will still not track AO-40 over the entire orbit since it is on
essentially an equatorial mount.  AO-40 will appear to vary from +5 degrees
from the equatorial plane to some angle below it depending on MA.  I do not
offer these comments to discourage you from using your TV dish, just to
point out that it is a little more complicated.  If AO-40's orbit had a
zero inclination angle then it would work perfectly; all you would need do
would be to place your feed to correct for the declination offset.

So it may be just as effective to find the az-el positions of several TV
satellites and see when AO-40 crosses near their position.  By moving the
dish east-west along the "Clark-belt" you may have a good match for an
interval of time [not sure how long..maybe twenty minutes?].  If you attach
your 2.4G feed on the side of the c-band feed it will have a small angle
offset in any case.  My 2.5m dish is fed with a ku-band wave guide feed for
VSAT [transmitting].  I merely attached a 3-turn helix onto the side of the
main feed for my radio astronomy work on 1.42G.  There is about a 3 degree
offset from doing this.

Ten years of installing TV-sat dishes and you get to know them well.  I
hope others are able to follow this.

Ed

>From: John Wright <g4dmf@AMSAT.Org>
>Due to a brain malfunction, I sent this direct instead of to the BB, 
>however, as I think this
>may be a viable way for users to employ domestic TV dishes for AO40 as well 
>, I'll put it on the BB
>
>>During a good portion of the orbit, AO40 will be inclined about 5 degrees 
>>to the standard geostationary arc.
>   So if you've got a dish on a polar mount, you may be able to use it on 
>AO40 without upsetting TV reception.
>  ( unless the other half wants to watch a channel during a good pass!! )
>
>If you mount a feed 5 degs offset from the focal point, you should generate 
>a beam offset by that amount in the opposite
>direction. So by mounting your S band feed 5 deg below the TV feed  then 
>your pointing angle will be 5 deg ABOVE where the
>dish is really pointing,  a 5 deg squint in feed point will only have 
>minimal loss in overall dish gain, so it should be right for
>both TV and AO40 ?? ... Don't forget that if using circular, the dish will 
>reverse the sense of rotation.
>
>Just an idea, won't upset the use for TV either!
 



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