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Homework Help: Satellite relay system and transmission losses

  1. Jan 25, 2014 #1
    Below I have a problem I can't quite understand to the full:



    http://www.israup.net/images/6757f5c30cbcfe051295b1b1e4614489.png [Broken]

    And that is the suggested solution:


    Now my question is about that [itex]g_{amp}[/itex] calculation in the solution provided in the solution manual for the book I'm solving from.
    If [itex]P_{in}=35 dBW[/itex] and not 30 dBW as it is in the example, and also uplink frequency value changes from the example provided to the question, then how [itex]g_{amp}=18+144=162 dBW[/itex]? Shouldn't it be [itex]g_{amp}=18-103=-85 dBW[/itex]? -103dBW should reasonably be substracted (and not like 144 being added) since it stands for transmission loss, isn't it?
    But the in Psat (out) it seems like they do compensate for it because they do add 162dB...

    I have a strong feeling that I miss something here in the idea behind [itex]g_{amp}[/itex]. Could it gain the whole uplink loss in some way? I'm quite curious about that really... :uhh:

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 25, 2014 #2


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    Gold Member

    The big typo in example 3.3-1 is the statement: 35dBW + 55dB - 199.1dB + 20 dB = -144.1dBW.

    The result should be -89.1dBW.

    The idea behind gamp is that it is variable (within some practical limits) and is whatever it needs to be to produce 18dBW (input to the downlink transmitter antenna).
  4. Jan 27, 2014 #3
    Another question, same subject

    Well, first of all thanks for your quick response lewando :approve:
    I have another little question:


    I found this solution on Chegg.com:

    http://www.israup.net/images/06364d1120e02d09bb453073c3d4fb12.png [Broken]

    Everything is clear to me, apart from that multiplication in 6 when calculating the maximum antenna gain. Why 6?


    Do you suggest there's probably a mistake? Or maybe I misinterpret once more? :shy:

    Last edited by a moderator: May 6, 2017
  5. Jan 27, 2014 #4


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    Gold Member

    I don't know where that 6 came from. One possibility is that the chegg equation is for a specific antenna. Your working assumption that Ae (the effective aperature) = area is based on unity efficiency of the "antenna area". Perhaps the chegg antenna had less than unity efficiency.
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