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prime focus or offset antenna

  • 16-04-2013 1:41am
    #1
    bossdrum
    Registered Users, Registered Users 2 Posts: 355 ✭✭


    Hi, This is probably very basic stuff but i can't find an answer.

    I'm looking to buy a new satellite dish to receive multiple satellites.
    I've seen two Triax TD110.
    They both look the same but one is 'prime focus' & the other is 'off set'.

    Does this matter? Whats the difference?
    Will both be able to receive signals using a multi feed bracket?


    Thanks


Comments

  • #2
    excollier
    Closed Accounts Posts: 1,576 ✭✭✭excollier


    A Triax TD110 is an offset dish, period. If you saw a prime focus dish, it's most definitely not a TD 110.
    A prime focus dish will be circular, the lnb will be centrally mounted on three support arms, spaced at 120deg and it will be pointing decidedly skyward, while an offset dish will have (in the case of a TD110) the lnb on one arm coming from the bottom of the dish, and the dish will be far less elevated, and it will be taller than it is wide, i.e. elliptical in shape.
    A prime focus dish is a relatively rare thing, whereas offset dishes are the norm.
    I'm not even sure that Triax make prime focus dishes anyway, I could be wrong.
    Google prime focus and offset dishes to see the difference.
    http://wsidigital.com/dish.htm.

    For 99% of domestic usage an offset dish will suffice, a prime focus dish tends tobe above the 1.4m size and is way more expensive and specialist in intention. For most uses, the extra expense will not offer any significant advantages.


  • #3
    bossdrum
    Registered Users, Registered Users 2 Posts: 355 ✭✭bossdrum


    Thanks.

    It's definitely a triax. The seller is just displaying the wrong information so.


  • #4
    watty
    Registered Users, Registered Users 2 Posts: 32,417 ✭✭✭✭watty


    Strictly speaking BOTH types are using "prime focus" unless you have and offset 2nd LNB.

    An offset dish is an off cut of a larger hypothetical symmetrical dish. In both cases you are using the Prime Focus unless you have a multi-lnb bar.

    Reasons for an offset dish (it's not Offset focus) rather than symmetrical is that snow slides off more easily and the LNB arm can more easily be a single beam that puts less "shadow" on the dish.

    Larger than 1.2m the mechanical stability of the dish is easier to achieve with symmetrical non-offset dish (not offset LNB) and the three arms are smaller percentage shadow.

    The LNB only seems offset. It's the shape of the dish is offset from the hypothetical symmetrical dish.


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