Ugango,
According to experts in geometry, one needs to first calculate the offset angle theta; after that the "correct" position for the LNB (and length of LNB arm) can be estimated.
The offset angle theta can be calculated using the formula:
sin(theta) = 4p/d(cos(theta)) ; also, Angle theta = (1/2)*asin(8p/d)
where:
d = length of the chord across the dish from top to bottom (or Height);
p = maximum perpendicular distance from the chord to the dish (or Depth).
In simpler terms, and
referring to the attached picture:
Offset Angle = asin [(AF - BF) / AB]
Where:
AB = distance across dish from top to bottom
BF = distance of bottom of dish to focal point
AF = distance of top of dish to focal point
Another simpler formula for length of LNB arm is:
Focal Length = Width[SUP]3[/SUP] / (16 x depth x height)
Example: suppose we have an Off-set dish with:
Length = 500mm
Width = 460mm
Depth = 43 mm
And the maximum depth of 43 mm is at a point 228 mm up the chord from the bottom edge.
The calculated offset angle = 21.7 degrees and hence a complement of 68.3 degrees.
Focal length = 460[SUP]3[/SUP] / (16 x 43 x 500) = 282.95 mm [
which is the estimated length of LNB arm]
In summary, just measure the Length, Width and Depth of your dish and do the calculation.
You also need to estimate the offset angle so that you can incline the LNB arm accordingly
A typical 1.8 meter offset communications antenna has a 22.6 degrees offset angle.
All the best.
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