Focal Length Calculation of a Satellite Antenna

This calculation works for prime focus antennas and helps check that the dish is constructed correctly for maximum efficiency and that the RF feed horn is in precisely 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) (shown in green). One method is to use some fishing line across the face of the antenna and then measure from the line to the center of the antenna.
3. Remember to keep all the measurement units the same.
4. Use the below calculation to check the distance from the center of the dish’s surface to the middle of the RF feed horn.

Focal Point Calculation

FL = D² / 16d

Focal Length = Diameter / 16 x depth

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