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Thread: LNB to Dish, Length of arms on Prime focus dish

  1. #1

    Lightbulb LNB to Dish, Length of arms on Prime focus dish

    I have being offered a prime focus 1.5m dish, it is new, however it does not have the 3 struts to hold the clamp to the dish, I can make some out of threaded rod bar but need to know what length to place the clamp from the dish.

    Is there a method or does someone have a prime focus 1.5m dish that can measure the struts for me, it will have a normal 40mm collar for the lnb, not a c120 mount

    Many thanks in advance

  2. #2

    Re: LNB to Dish, Length of arms on Prime focus dish

    what is the depth of your dish?

  3. #3

    Re: LNB to Dish, Length of arms on Prime focus dish

    Sorry I dont have access to the dish so can not measure the depth, is there some calculation based on the depth of the dish ?

  4. #4
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    Re: LNB to Dish, Length of arms on Prime focus dish

    Of course there is some calculation based on the depth of the dish.
    Assuming to be the diameter of the dish, the depth and 'f' the focus then the focal length divided by the diameter equals the depth (f/d=h). It therefore follows that f=d*h from simple mathematics.
    I hope it helps.

  5. #5

    Re: LNB to Dish, Length of arms on Prime focus dish

    Thanks for that, however just tried this equation on a 110 cm prime focus dish i have spare, diamater 110cm, depth 14cm, lnb to centre of dish is 46cm or 50 cm to collar, so if i follow your first f/d=h 46 / 110 = 0.41 which is known to be wrong as the depth is 14cm

    Best

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    Re: LNB to Dish, Length of arms on Prime focus dish

    I think that prime focus not so good that ofset

  7. #7
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    Re: LNB to Dish, Length of arms on Prime focus dish

    Quote Originally Posted by boxerman23 View Post
    Thanks for that, however just tried this equation on a 110 cm prime focus dish i have spare, diamater 110cm, depth 14cm, lnb to centre of dish is 46cm or 50 cm to collar, so if i follow your first f/d=h 46 / 110 = 0.41 which is known to be wrong as the depth is 14cm

    Best
    Sorry, I erred in my earlier submission. Here is the correct procedure:
    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
    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".

  8. #8

    Re: LNB to Dish, Length of arms on Prime focus dish

    Thanks for that, just tried the calculation on my spare 110 dish and it works ok, interesting though, the focal distance is measured to the lnb clamp and not the face of the feedhorn. Just an observation but now i can get the 1.5m dish up and running, Thanks again

  9. #9

    Re: LNB to Dish, Length of arms on Prime focus dish

    Quote Originally Posted by golongo View Post
    I think that prime focus not so good that ofset
    I agree, but once you get above 1.2m it is difficult to find offset dishes, and certainly where i live its difficult to find a satellite shop ! lol

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