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Thread: F/D Ratio Question for 12 Ft Dish

  1. #1
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    F/D Ratio Question for 12 Ft Dish

    Calculated F/D Ratio Versus Actual or what works:
    I measured my 12 Foot Dish Diameter last November and it was 145 7/8 inches (145.875 inches). My dish depth = 23 3/4 inches. Using the Formula:
    F/D Ratio = D*D/ 16 X d I came up with: 12.15625 X 12.15625/ 16 X 23.75 =
    147.77 / 380 = 0.3888. But when I was adjusting my Corotor II LNB I found out that when I adjusted F/D Ratio for 0.38 I couldn't get a signal. When I set F/D back to 0.36 I was receiving the Sat Signals. At that time my declination Angle was at 10 degrees when it should be at 6.61 degrees for my location.

    My Question: Now that I have re adjusted my Declination Angle should I try to re adjust the F/D Ratio to 0.38 to get a better Sat Signal?? Any replies would be appreciated.


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  3. #2
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    There are several factors related to the proper positioning of a lnb. F/D ratio is only one. You need to make sure the distance between the lnb and the dish face is set as calculated. You also need to make sure the lnb is pointing directly to the middle of the dish and parallel to the dish face. It sounds like the lnb distance to the dish face isn't set properly. Declination and F/D ratio have no relationship. You should set the declination once to the proper degrees and then forget about it. Good luck.

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    Thread Starter
    Quote Originally Posted by Hermitman View Post
    There are several factors related to the proper positioning of a lnb. F/D ratio is only one. You need to make sure the distance between the lnb and the dish face is set as calculated. You also need to make sure the lnb is pointing directly to the middle of the dish and parallel to the dish face. It sounds like the lnb distance to the dish face isn't set properly. Declination and F/D ratio have no relationship. You should set the declination once to the proper degrees and then forget about it. Good luck.
    Thanks very much for the reply Hermitman. When you say "It sounds like the lnb distance to the dish face isn't set properly". - I'm not sure what you mean, that is, do you mean the LNB assembly may NOT be parallel or needs to be set closer to the center of the Dish?? With my setup you can reposition the Depth of where the LNB is sitting in the Scalar Ring if my terminology is correct.

    On Friday carefully standing on a ladder in the snow with my Trimax Satellite meter (with the Strap around my neck) I checked a couple of Tp's on G25 - 97.0W. I also tried to readjust and re position the entire Corotor II LNB but nothing changed much. I still couldn't pickup Tp's 12166, and 12155 which is a strong Tp. There was one spot where when I set the LNB a bit deeper - the signal seemed to get a bit better, but rotating the whole LNB away from 11 and 5 o'clock didn't seem to help much. Maybe I'll take some closup pictures of the LNB today. Any replies would be much appreciated.

  5. #4
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    I think I've found a couple of problems with your math. First, the formula you used is for the focal distance. And the units in the formula need to all be the same (all inches or all feet).

    Focal Distance = (DxD)/(16xd)
    f/D Ratio = Focal Distance/Dish Diameter

    Using your numbers, focal distance = (145.875x145.875)/(16x23.75) = 55.998 inches

    f/D Ratio = 55.998/145.875 = .3838

    The f/D ratio is the number you set your lnb to in the scalar ring. This is important when it comes to antenna match, gain, cross-polarization and pattern symmetry.

    The focal distance is measured from the deepest part of the dish to 1/4 inch inside the lnb throat. This is also important because if you're off by as little as 1/2 inch, your signal strength can be reduced to the lnb by 50%.

    Beside these measurements, you should make sure the lnb is pointed directly toward the center of the dish and the scalar ring face is parallel with the dish rim. More info can be found at the below webpage. Good luck.


    C-band Satellite Installation





  6. #5
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    Thread Starter
    Quote Originally Posted by Hermitman View Post
    I think I've found a couple of problems with your math. First, the formula you used is for the focal distance. And the units in the formula need to all be the same (all inches or all feet).

    Focal Distance = (DxD)/(16xd)
    f/D Ratio = Focal Distance/Dish Diameter

    Using your numbers, focal distance = (145.875x145.875)/(16x23.75) = 55.998 inches

    f/D Ratio = 55.998/145.875 = .3838

    The f/D ratio is the number you set your lnb to in the scalar ring. This is important when it comes to antenna match, gain, cross-polarization and pattern symmetry.

    The focal distance is measured from the deepest part of the dish to 1/4 inch inside the lnb throat. This is also important because if you're off by as little as 1/2 inch, your signal strength can be reduced to the lnb by 50%.

    Beside these measurements, you should make sure the lnb is pointed directly toward the center of the dish and the scalar ring face is parallel with the dish rim. More info can be found at the below webpage. Good luck.


    C-band Satellite Installation


    Thanks a lot for the correction Hermitman that is a great webpage. What I don't understand is when I adjusted the Corotor II LNB to the mark at 0.38 previously I couldn't get a signal at all!! But when I set it for 0.36 I got Satellite Signals. Maybe I need to try it again at 0.38 and re adjust the LNB Depth in the Scalar Ring???


  7. #6
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    Here's some of the theory. When you place the lnb in the scalar ring at the .3838 setting and lock it down, the lnb and scalar ring become one electronic unit. This unit is now ready to receive the satellite signals reflected off the dish and most effectively and efficiently guide those signals down the throat to the lnb probe. The dish is designed to reflect satellite signals to a single focal point about the size of a golf ball. And this focal point on your dish was calculated to be 55.998 inches. The idea is to now move the lnb/scalar ring unit so the focal point is just inside the entrance of the lnb opening. The signals are then guided down to the lnb probe and then to your receiver. Its quite possible that when you moved the lnb from .38 to .36, you actually moved the lnb throat to a position that now is closer to the focal point and signals are now going down the lnb throat to the probe. In my opinion, I would try setting the lnb to .3838, the focal distance to 55.998 inches and see what happens. To check for a better signal, the first thing I would change is the focal distance slightly in or out while leaving the lnb/scalar ring setting unchanged. Good luck.

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