Focal distance for Elliptical dish

W

wium78

Regular Member
My Satellite Setup
fixed,polar mount and C band dish.
My Location
south africa
I am trying to ascertain the correct focal distance for an elliptcal dish with the following measurements.
Wide 900mm Height 670mm Depth 105mm.My present FD is 490mm, but i am unable to calculate the FD due to not having a formula.Can a member please do a calculation and advise the formula to be used.
 
The typical oval-shaped offset dish will be a paraboloid cross-section, and then indeed the FD can be calculated using height, width and depth.

Your dish is not shaped like that. It was a cross-section first, but which the upper- and/or lower parts removed!
Since we don't know exactly how much upper/lower part exactly is removed, I don't think the FD can be calculated so easily.
 
I copied this for you from another forum.
Hope it helps.

"In the off set dish ,there are two dimensions, ie antenna height (vertical) and there is antenna width (effective diameter).
the formula for off set angle= Arc cos(antenna width/antenna height)
the Antenna height = effective diameter/Cos(offset angle)
effective diameter = Antenna height X cos (off set angle)
for example the following are the dimensions of an off set Dish
1.Height on vertical axis (W) = 500 mm
2.Effective diameter on Horizontal axis (D)= 460 mm
3.Depth of the dish (h) = 43 mm
Focal length = (D*D*D)/ 16*h*W
= (460x460x460)/(16x43x500)
=282.95 or 283 mm
F/D ratio = 283/460 = 0.61
Hope this will help you."

There are more calcultors here

**http://www.satellite-calculations.com/
 
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