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900MHz Cable Feeds

Discussion in 'Electronic Design' started by Joe G \(Home\), Jun 18, 2006.

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  1. Hi All,

    I have a 900MHz radio system and I want extend 50ohm coax cable from 3m to
    20metres.
    (well.... I mean - replace the 3m with a complete 20metre section with
    proper termination coax connectors etc)

    What losses and reflections should I watch out for?

    Eg if the cable is a multiple of the 900Mhz wave length or 1/4 wave length
    etc... is there refection effects.

    Obviously there will be losses rated in db per metre (foot) etc....


    What else should I pay attention to?


    ** PS the antenna is std 50ohm termination and the transmitter is 50ohm
    termination and the cable is 50ohm coax style - already.

    ** Transmission is spreadspecturm **


    ???? Is this the right NG? If not - please advise which NGs might be more
    appropriate.

    Thanks in advance.

    JG
     
  2. Guest

    The obvious question is: how much loss can you *stand* in this
    application?
    (X-posted to more appropriate groups)
     
  3. gravity

    gravity Guest

    hardline might work. quite expensive.

    i'm not sure what the losses from the connectors would be in a hardline
    system.

    Gravity
     
  4. Joerg

    Joerg Guest

    Hello Joe,

    If both ends (or at least one) are terminated with line impedance there
    should be no reflections. Losses will be listed in the cables specs.
    There should be a graph, losses per meter versus frequency, or at least
    a table that contains losses at 1GHz. If frequencies that high aren't
    listed you need a better cable. If there are no loss specs at all don't
    even consider buying the cable.

    Let's say you arrive at 3dB which would be kind of high for 20 meters.
    That would tell you that half the transmitted energy doesn't arrive at
    the other end and that your SNR would be 3dB worse than if the receiver
    had been directly connected to the antenna.
     
  5. Thanks,

    Why is the SNR 3dB worse?

    JG
     
  6. Joerg

    Joerg Guest

    Hello Joe,
    When the cable loss is 3dB then the signal received at the antenna will
    drop by 3dB before it reaches the receiver. The receiver's noise
    performance, however, will remain the same unless it's mismatched.

    If the other station had a similar cable added you'd lose a total of 6dB
    because its transmit energy would have dropped by 3dB before reaching
    its antenna.
     
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