View attachment 362930
HOW TO FIND LENGTH OF LNB ARM OF A PRIME FOCUS
D = Diametre
F = Focal length (length from centre of dish to the LNB)
d = Depth (of the dish)
D x D = F
16 x d
R = Radius of dish
r = Radius of LNB holder
L = Length of LNB arm
R – r = R1
F – d = H
(R1 x R1) + (H x H) = L squared
L = square root of L squaed
For example, a 6ft(72inches) prime focus dish has a depth of 1ft(12 inches)
The radius of dish = 36 inches , and of lnb holder = 3.25 inches. Calculate :
A. Focal length
B. Length of lnb arm
D = 72
F = ?
d = 12
D x D = F
16 x d
72 x 72 = 27
16 x 12
F = 27
R = 36
r = 3.25
L = ?
R1 = R – r
= 36 – 3.25
= 32.75
H = F – d
= 27 -12
= 15
( R1 x R1) + ( H x H) = L squared
= (32.75 x 32.75) + (15 x 15)
= 1073 + 225
= 1298
L = square root of 1298
L = 36.03 inches