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Thread: KU LNB - Offset Vs Prime Focus

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    Premium Member nfnovice's Avatar
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    Default KU LNB - Offset Vs Prime Focus

    Is there any difference in the lnb on a Offset vs Prime Focus Dish
    both are KU but physically could the same lnb could go on either ?
    is there in fact two different KU 10700 lnb's - one for offset dish and one for prime focus - who knows ?
    Last edited by nfnovice; 06-06-10 at 03:12 PM.
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    Quote Originally Posted by nfnovice View Post
    Is there any difference in the lnb on a Offset vs Prime Focus Dish
    both are KU but physically could the same lnb could go on either ?
    Both are designed to operate at optimum performance on a dish they were designed for.

    It's possible to interchange them, but there is likely to be a reduction in performance.

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    Senior Member irdeto2engineer's Avatar
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    Quote Originally Posted by mtv View Post
    Both are designed to operate at optimum performance on a dish they were designed for. It's possible to interchange them, but there is likely to be a reduction in performance.
    OK here I have a setup for 124E and 128E for a customer in Asia. He cannot have more dishes and the other issue wanted multiple decoders.

    Sky Perfect Japan broadcasts from 2 satellites. Normally you would use an 85cm. But here we have a 1.8m dish with a mount for 2 Prime Focus LNB's. Also on a mount is an OFFSET LNB with a V & H output. This offst LNB on a prime focus only works as the 1.8m has so much attentuation that I still max out the decoder on signals. Using an offset LNB on a prime focus dish not recommended as yes lots of loss. I prefer prime focus dishes so I can have a KU and Cband LNB on the same dish on different satellites. Bottom pic a 3m on Thaicom 5 KU band at 78.5E with another Cband LNB on 68.5E IS10






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    The answer is yes and no.

    A prime focus lnb is designed to work on a prime focus dish (and offset lnb on offset dish) for maximum gain, however a offset dish is a section of a prime focus dish.

    You can use a offset lnb on a prime focus dish if your pointing to one side of the prime focus dish, in effect taking the signal from a offset dish area.

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    Senior Member Farmsky's Avatar
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    Perhaps two simple pictures are worth a thousand words.

    Basically ... the focal length of prime and offset dishes are radically different ... meaning that the beam angle (f/d) of prime and offset LNBs are radically different ... with results as irdeto2engineer, godzilla and mtv describe.

    The f/d of an offset lnb is typically 0.6 while that of a prime focus lnb is typically 0.3.

    The use of offset lnbs on larger solid dishes, is a practical solution for mixed C/Ku installations on the same dish. In irdeto2engineer's case, if the focal length is measured; using the formula f=d squared/16d; the 'effective diameter' of the parabolic reflective surface may be calculated.

    In very simple terms; if the focal length of the prime focus dish installation is larger than that of say an 80cm reference offset dish; then the performance of the prime focus dish with an offset lnb is theoretically better.
    Note: There are some additional losses to take into account when using a prime focus dish with an offset lnb, as the lnb's 'shade' a percentage of the reflective surface; so an effective diameter, some 25% larger may be required to match the reference dishes performance.



    Note that if the prime focus has a central hole, the performance of an offset lnb is further degraded.

    How f/D is calculated.

    Last edited by Farmsky; 07-06-10 at 10:20 AM.

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    I cant thank you guys enough
    again I understand how little I know - and in my terms that means I am learning something
    cheers and thanks
    Dm500, DM5620, DM600 x2, DM7000 x1, DM7020, DM7025, DM800, VU+DUO and a partridge in a pear tree
    All it takes for evil to prevail is for good men to do nothing

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