Re: [amsat-bb] 18" dish specs

["Lee" <lee-fl@xxxxxxxxxx>]

> I'm finally building my downlink for AO40 and I'm trying to figure out the
> gain for a converted Dish Network dish (18" round) with a 5 1/4 turn helix
> feed.  Anyone got exact figures?

I measured 19 dbi using Sun noise on my 18 inch dish.  With a very good
preamp (<0.7 db NF) directly at the feed it is usable, but if you can swing
it, the extra 6db from a 3ft dish is well worth it.

> Also, which way (looking out from the
> feedpoint) should the helix turn, clockwise or counterclockwise?

I always manage to get it wrong when I try to describe it so here is a link
to a picture of my feed.
http://home.cfl.rr.com/lmclamb/dssfeed.jpg

The dimensions for the feed were shamelessly copied from K5OE's 16 turn
design with the winding direction reversed to use as a feed.  Basically the
feed is wound on 1 inch PVC.  The first 1/2 turn only 1/4 inch from the
reflector.  The remaining turns each have a spacing of 1 inch.

Good Luck,
Lee-KU4OS



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