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Note:
The above assumes that the dish is designed such that the bottom edge
of the dish is approximately at
the center of the full parabola. This is a pretty good
assumption, since the intent of the offset design is to
keep the shadow of the lnbf just off the dish surface, since you lose
signal when the dish is blocked by the
lnbf shadow, and you lose signal with increasing offset angle.
It is also assumed that the same units are used for
both focal length and dish height, and the
resulting answer will also be in these units. I used cm for both
when testing the program.
The measurement of the focal length to the bottom
edge of the dish is a good assumption,
even if the actual center of the parabola is off the bottom, however it
is not clear where on
the lnbf the measurement should be made, since the front of the lnbf is
covered in plastic.
I assume that the actual focal point should be somewhat inside the
plastic, however in my experiment,
using the front surface of the lnbf gave fair results.
To use the results of this calculator, you can make a
string with a knot in it, and connect one end
of the string to the top edge of the dish at a distance of "Distance to
Top" from the knot, and
connect to the bottom edge of the dish at a distance of "Focal
Length". Stretch out the string,
and the knot will be at the theoretical position of the lnbf.
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