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Feeding PrimeStar

In the quest for more gain over my 60 cm dish, yesterday I located a PrimeStar dish, and picked it up for $20 (after locating several people who 
had thrown them in the trash some time ago - doh!).

It's complete with the wall-mounting hardware and original feed horn/LNA.

How have others mounted helix feeds onto this thing?  At first glance, it's not immediately obvious exactly where the focus is for the dish.  Is it 
located at the opening to the feed horn?  Partially into the horn?  At the back?

Exactly where the focus is may impact how my feed mount has to be mounted, because the boom that supports the feed may (or may not) 
interfere with the 12.5 cm reflector on my helix/Drake assembly.  If it does interfere, I'll have to chop the boom off some and build a more 
complicated mounting system (or cut off part of the reflector).  It's matte gray, so I can't use the sun to find the focus (unless I glue tiny mirrors 
all over it... hmmmm...).

Also, does the dish size impact the optimum helix size?  I've got a 2.25-turn helix that I built for the 60 cm dish - will those dimensions be good 
for the larger PrimeStar?

I'll need a big AMSAT logo to cover up the "PrimeStar" that's printed on the reflector. :^)

David, ky0dr

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