פורום הלווינים והטכנולוגיות הישראלי

A Word About Focal Depth

The f/D ratio is the focal distance of the dish (f), divided by the diameter (D). When dealing with most prime focus antennas, the number should come out between .28 and .42. If you notice, most of those numbers are also on scale on the side of the feedhorn. You simply set the top edge of the scalar ring even with the line that corresponds to your correct f/D setting. What this adjustment actually does is determines how wide of an angle the feedhorn can "see".

To calculate the focal distance, you need to measure the diameter (D) and the depth (d) of the dish. Measurements should be in like units (you can't use feet for the diameter and inches for depth). For this example, let's say we have a dish that is 120 inches in diameter (D) and 18 inches deep (d). Focal distance (f) equals the diameter squared (D x D) divided by 16 times the depth (16 x d) or:

D x D = 120 x 120 = 14400

16 x d = 16 x 18 = 288

D x D/16 x d = 14400/288 = 50

Therefore focal distance f = 50 inches

After you have calculated the focal distance (f), you can use that figure to calculate the f/D ratio of your dish. In this case, using the same diameter of (D) = 120; and the calculated focal distance (f) = 50

f / D = 50 / 120 = .416

f /D = .416

And round up to give a setting of .42.

All of this information will be provided in the dish manufacturer’s instructions. It is important to read the directions and understand the type of equipment you are working with. Often times you will be working on a system that was previously installed.

19.2E - Dish
Elevation:48.7°
Azimuth: 207.6°
LNB skew: 30.6°

16E - Dish
Elevation:47.2°
Azimuth: 212.6°
LNB skew: 27.0°

13E - Dish
Elevation:45.5°
Azimuth: 216.9°
LNB skew: 30.4°

5E - Dish
Elevation:40.5°
Azimuth: 227.0°
LNB skew: 38.2°

4W - Dish
Elevation:33.9°
Azimuth: 236.4°
LNB skew: 44.7°

7W - Dish
Elevation:31.6°
Azimuth: 239.2°
LNB skew: 46.5°