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Re: Primestar (Gray - .9x.6m) Dish Characterized?




>Seems like a relatively long helix feed (6, 7 turns? I don't have the >turn 
>to f/d reference handy) is called for for 0.82? Since the
>fitting parabola has a f/d of 0.37, is this why K6CCC reported >maximum 
>signal at 2 3/4 turns, acknowledging K5OE's G/T comments? >Because he is 
>illuminating the full dish in width, but spilling over >the top and bottom? 
>And 6 or 7 turns will maximize the vertical >dimension, but not use the 
>"ears" of the dish, effectively turning it >into a 60cm offset dish?

Assuming this is a squashed (long, skinny) dish (ok thats in the title), 
with a hemispherical feed (helix) you will either underilluminate a portion 
of dish or overilluminate the other portion of the dish.  You can either go 
for maximum illumination, and spill over the other sides, or have no 
spillover and "waste" the large side of the dish.

You, for receive, probably would be better off with the better G/T and 
underillumination.  It is believeable that the effective f/d is 0.82 as the 
RCA DSS dishes are around 0.7 or so.  Design the feed for that.  And if you 
want, get the bandsaw out and chop 6 in from either side!

The "Squashed" dishes like the Primestars and the BBQ's have more gain and 
narrower beamwidth in one axis than in the other.  I suspect the Primestar 
Dishes were that shape to make the positioning of the dish simpler while 
still accomodating the narrower spacing between the medium-power Ku band 
satellites (as compared with high-power, wider spaced, DBS sats like DirecTV 
and Dish Network) -- Less gain in Elevation more in Azimuth.  Less windload 
that way too.   Primestar was linear (tilted) H/V polarization so axial 
ratio to the designers was meaningless....The BBQ's dipole feed wouldn't 
illuminate the missing portion anyway, so that is the trade off there...

Fred W0FMS


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