Focal Length Calculation of a Satellite Antenna

This calculation works for prime focus antennas is useful for checking that the dish is constructed correctly for maximum efficiency and the RF feed horn is in exactly the correct position.

The Focal Length (FL) is the distance from the feed horn to the centre of the antenna.

  1. Measure the diameter of the antenna (D) (indicated in red)
  2. Measure the depth of the dish (d) (indicated in green). One method is to use some fishing line across the face of the antenna and then measure from the line to the centre of the antenna.
  3. Remember to keep all the measurement units the same.
  4. Use the below calculation to check the distance from the centre of the surface of the dish to the middle of the RF feed horn.

Focal Point Calculation

How to find the focal point of a VSAT dish
Antenna Focal Distance Calculation

FL = D² / 16d

Focal Length = Diameter / 16 x depth

D = Antenna diameter
d = Antenna depth
FL = Antenna focal length (prime focus)


Satellite Dish Focal Length Calculator

Satellite Dish Diameter (D): cm
Satellite Dish Depth (d): cm

Focal Length: cm

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