I am just Cut and Pasting from the web on this... Hope there is some information in here that helps...
The focal length of a (normal prime focus) dish
can be calculated from the diameter and depth of the dish surface.
f (focal length) = D squared divided by 16 times C
or D x D / 16 x C
where D is the diameter of the dish surface and C is the depth.
Find the depth by placing a long straight stick or rod across the face of the dish and
measuring down to the centre of the dish. Then do the above calculations.
The f/d ratio (focal length to diameter ratio) is just that. Divide D into f from the
formula and it should come out to 0.3 - 0.4 - 0.5 or whatever. The importance of this
figure is in the way the feed apparatus illuminates the dish surface. The larger the f/d
ratio, the further away the feed will be and the characteristics of the feed will need to
be changed accordingly so as to correctly illuminate the dish with not too much
"over-spray" as this can degrade the noise figure of the system as a whole. Similarly, a
feed with too narrow a beam will waste some of the dish surface due to under-illumination.
A feed with a lobe which is 10db down at the edge of the dish is considered to be about
right. You can see how the f/d ratio will affect this figure.
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.
I have just cut and pasted this information for you to digest. The best that I can get out of it all myself is that the F/D ratio is simply and obviously the ratio between the focal depth of the dish vs the dish diameter. The resultant figure tells you how much of the dish surface is "illuminated" by the LNBF. Which, in my opinion is a sort of misnomer as it is not a function of the LNBF, it is a function of the dish geometry. In essence, it is function of how much of the dish surface area that it being fully utilized and how much it it is concentrating the signal upon the LNBF. Or, how much of the signal from the satellite is being captured, amplified and reflected to the LNBF.
I am not sure if a higher or lower F/D ratio is more positive. However, you may now understand where it is developed from.
RADAR
