f/d ratio of the prime focus dish.

[SENTHIL KUMAR]

f / d ratio of the prime focus dish.
​

f / d ratio is one of the important number in prime focus dish.

This indicate which is the correct position to fit your LNB on scalar ring.

Calculation. ( Examble : 6 feet prime focus dish )

Diameter = 180 cm. ( D )

Depth = 30 cm ( d )

so, Focul length ( F ) = D*D / 16*d

F = 180*180 / 16*30 = 67.5 cm.

Now calculate the f/d ratio.

f/d ratio = F / D * 100 = 67.5 / 180* 100

f/d ratio = 37.5 . Take it us 38. ( It is the range in between 34 to 42 , depend upon your dish size and depth.)


You see the lines with reading 34 to 42 on LNB top body.

You simply set the top edge of the scalar ring in the line of correct f/D setting of LNB.