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



I did some real "back of the envelope" calculations this morning, and
I wanted to offer them up for comment.

Per Paul Wade's Ch. 5, I laid out my Primestar dish on the kitchen floor and
filled it with water (wife is on travel).

When filled to the shorter vertical dimension, I get 60cm vertical, about
57cm
horizontal, and about 4.5cm depth max about 28cm from the bottom (ballpark,
since water started to leak around the four screws in the center --
where's the dish towel?).

HDL_3B3 reports at 2401MHz:

"Focal length = 426.7  mm.
This offset reflector is a section of a full parabola with a diameter
of 1138.4  mm.  whose vertex is at the bottom edge of the offset reflector.
The full parabola has an f/D = 0.37, which determines criticality of focal
length.
The focal point of the dish is 426.74  mm.  from the bottom edge of the
reflector and 616.54  mm.  from the top edge of the reflector.
For operation with the main beam on the horizon with the feed at the bottom,
the dish must be tilted forward so that the large axis is 71.6 degrees
above horizontal.
Illumination angle for feed = 67.4 degrees on the large axis and 68.2
degrees
on the small axis.  A feedhorn with a 3 dB beamwidth of 38.9 degrees is
needed,equivalent to the feed for a conventional dish with f/D = 0.82
Gain at 50% efficiency = 20.1 dBi
If you do really well, you might get 60% efficiency for a gain = 20.9 dBi
To design a feedhorn, use f/D = 0.82 in Menu option F"
(end)

I cut some string to the focal point dimensions given, and I get a point
just in front of where the old LNB horn cover was.

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?

Am I even close with this? :-)

Bill
w1pa



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