I read on another forum that the focal distance on the Fortec 1.8m dish is actually incorrect and should be moved out more to take full advantage of the entire dish. I have not had the time to confirm this.
If this is true then the dish is being under illuminated. This will reduce gain but can help to reduce thermal noise.
The following is from Chaparral and should help you:
The f/D ratio and scalar rings - why it is important to set properly
Proper setting of f/D on the feedhorn allows the feedhorn to take advantage of all of the signal being reflected off of the dish, without receiving interfering ground noise or terrestrial interference.
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". If the dish is very deep(example: 10ft diameter dish that is 24 in. deep), having an f/D of .28 for example, then the focal distance is relatively short. When that is the case, the focal distance is often only a few inches greater than the depth of the dish. Therefore, the feed needs to be able to "see" nearly straight to the side of the opening in the throat.
Conversely, if the dish is very shallow (example: 10ft diameter dish that is 11 in. deep), the f/D ratio would be closer to .42 and the focal distance would be much longer. In that case, the feed would need to have an narrower field of view so it would "see" the whole dish, yet not see past the edge of the dish.
Formulas for calculating focal distance and f/D ratios
To calculate the focal distance, you have 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 the example, we will 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
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 (D) = 120; and the calculated focal distance (f) = 50
f / D = 50 / 120 = .416
f /D = .416 which you would round up to give you a setting of .42
The list below shows how far the throat is out from the scalar rings for different f/D settings.
EXAMPLE: A dish with a .42 f/D will have the throat about flush with the rings.
Inches -- f/D
.12 ------ .42
.32 ------ .40
.52 ------ .38
.72 ------ .36
.92 ------ .34
1.12 ---- .32
.
