Re: [amsat-bb] 18" dish specs

[K5OE@xxxxxxx]

Lee,

<<  > 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.

Nice setup on the helix+DEM preamp+Drake!

This design works quite well, but I wanted to caution other prospective 
builders to follow the design just as you have, with a 1" PVC pipe core.  The 
dimensions used are smaller than an air-core helix as the PVC pipe provides 
"loading" (capacitive reactance) to the helix, thus electrically elongating 
its physical dimensions. 

If you want to just use an air-core, then the dimensions are as shown in this 
article:  http://members.aol.com/homebrewpics/mmds-feed/mmds-feed.htm
... or the direct link to the picture is:  
http://members.aol.com/homebrewpics/mmds-feed/figure10.jpg
... or the main web page:  http://members.aol.com/k5oe   (select "MMDS")

73,
Jerry, K5OE

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