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Thread: Offset Antennas

  1. #1
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    Default Offset Antennas with C band


    Dear Friends
    I have got some question regarding the offset antennas. Your valueable comments will be appreciated.

    1- Can anyone tell me which diameter is to be considered for offset antenna....height or width.
    I have got an offset antenna having the dimensions of 2.05m (height) and 185cm (width). I am trying to understand which of the lengths is to be considered as the size of antenna. Is this antenna a 2.05m offset antenna or 1.85cm offset antenna.

    2- How can i calculate the focal point of this antenna. Some friends have given some complicated formulas but i dont understand them. Prime focus has a simple formula....what about offset?

    3- I am using a c-band LNB (17K) with an orthomode chapparel feed (Dual band). Unable to focus the transponder 3864 H on Intelsat 10 at 68.3 E (ARY digital, Hum Tv). The signal strength is 67% and signal quality is only 20% (not increasing). I get the channels but with many sparkles.
    Some friends said use conical scalar ring for offset...but it does not work. Some said to change the LNB fro 17K to 15K or less....for this i am still waiting for the new ones to come...at that time i might tell you guys if i succeeded or not....
    Meanwhile pls advise me solutions......

    Looking forward for your replies.....
    Rgds
    Nasir
    Last edited by nasirsyed0; 31st August 2007 at 10:41. Reason: misspell

  2. #2
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    Default

    Quote Originally Posted by nasirsyed0 View Post
    Dear Friends
    I have got some question regarding the offset antennas. Your valueable comments will be appreciated.

    1- Can anyone tell me which diameter is to be considered for offset antenna....height or width.
    I have got an offset antenna having the dimensions of 2.05m (height) and 185cm (width). I am trying to understand which of the lengths is to be considered as the size of antenna. Is this antenna a 2.05m offset antenna or 1.85cm offset antenna.

    2- How can i calculate the focal point of this antenna. Some friends have given some complicated formulas but i dont understand them. Prime focus has a simple formula....what about offset?

    3- I am using a c-band LNB (17K) with an orthomode chapparel feed (Dual band). Unable to focus the transponder 3864 H on Intelsat 10 at 68.3 E (ARY digital, Hum Tv). The signal strength is 67% and signal quality is only 20% (not increasing). I get the channels but with many sparkles.
    Some friends said use conical scalar ring for offset...but it does not work. Some said to change the LNB fro 17K to 15K or less....for this i am still waiting for the new ones to come...at that time i might tell you guys if i succeeded or not....
    Meanwhile pls advise me solutions......

    Looking forward for your replies.....
    Rgds
    Nasir
    with dishes you go by the width so if its 1.85 cm dish


    as to focal point
    with prime focus its easy but with off set ive never been able to find the formular so carnt help you there
    if you do find it can you please let me know


    the chaparral duel band feed horn for ku and c band lnbs arnt they made for prime focus dishes and not off set ones changing your c band lnb from a 17 k to a 15 k wont make that much diferance go for a bigger dish the bigger the better
    your dish is just a reflector so the bigger the dish the more signal is reflected into the lnb the stronger the signal
    changing the lnb wont make the signal any stronger only increase the signal quality as the dish is still reflecting the same amout of signal
    ABA

  3. #3
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    Default

    This has been discussed before, why do you keep repeating the same question, We've supplied the formula, and you where told it was a complex thing, but unfortunatly, that's the way it's done. Unless you want to simply hold the LNB in front of the dish until you get a signal then make up a bracket to suit. That's the simply way, sometimes it works, but mostly it dosen't.
    We have also discussed the correct scalar ring, and for an off-set dish, you need an off-set scalar ring , It's a case of Horses for Courses

    Really, you would be better of disgarding your off-set dish and getting a 2.4 solid Prime Focus dish. Then it will work.. Your probem is, your not using the correct combination if equipment, and regardless of LNB
    db, it will not change that fact.. Changeing the LNB will not alter the fact that your focal length and Scalar ring are wrong.
    regards from OZ bassett

  4. #4
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    Hi friend Bassett
    I know we discussed this issue before but, as per your advise i purchased the right scalar ring which is the conical scalar ring for offset. I found it somehow, added to the feedhorn but did not get any signals. before when i had the normal scalar ring at least some signals were there but after changing i did not get any.....i know i am doing something wrong....can you help me in getting the exact focal point from the formula you gave.
    The dish size is 2.05m height and 1.85m width. My new LNBs with 13K are on the way also.
    By the way the advise you gave me regarding the purchase of new dish is not possible at all since i purchased this dish very recently and paid a hell of a money to it.
    Waiting for your comments and also advices from all friends as well.
    Rgds
    Nasir

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