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3538 Views 4 Replies Latest reply: Aug 25, 2011 6:56 PM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

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EIRP vs Antenna Gain

Aug 24, 2011 8:01 PM

Steven Williams 3,564 posts since
Jan 26, 2009

Ok I am struggling to know what the difference is between the two?


EIRP describes the amount of energy that is emitted from an antenna....isn't that considered gain as well?

  • ry 202 posts since
    Jun 22, 2010
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    1. Aug 25, 2011 12:15 PM (in response to Steven Williams)
    Re: EIRP vs Antenna Gain

    EIRP is the product of Tx power and Antenna gain (in watts), but if given in dB, then its the sum. And on the exam, if it mentions connector losses (in dB), then subtract the losses.

    It is the maximum amplitude, as a function of angle, of the antenna pattern.

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  • Pete Nugent 1,256 posts since
    Dec 8, 2008
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    2. Aug 25, 2011 1:22 PM (in response to ry)
    Re: EIRP vs Antenna Gain

    dB is logrithmic so it is alot easier to work with these as you only add and subtract dBs.


    Antenna gain will only ever be given in dB (as far as I know)  and only applies specifically to the antenna.


    EIRP relates to the total power emitted from the antenna which is


    Radio Tx power(dB) - cable loss (dB) + antenna gain (dB) = EIRP.


    The only bit you "should" need to convert will be the radio Tx power because that may be in mW.




    10dB = 10mW


    3dB is half or double power depending if its +/-.


    so 13dB is 20mW 16dB is 40mW etc.


    20dB is 100mW.


    You will be fine tomorrow mate

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  • Brian 2,971 posts since
    Aug 17, 2009
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    3. Aug 25, 2011 2:53 PM (in response to Steven Williams)
    Re: EIRP vs Antenna Gain

    No, e.i.r.p. is not the same as antenna Gain.   The term equivalent isotropic radiated power or e.i.r.p, is a function of the antenna transmit gain G(T) in dBi and the transmitted power P(T) in watts fed to the antenna.  It is written as e.i.r.p. when used in text and as EIRP when used in formulas.


    EIRP = G(T) + 10 Log P(T)        (in dBW)


    The value of P(T) is the sum of the transmitter output power and all the loses/gain in the path from the transmitter to the antenna.


    The gain of antenna is typically in dBi, while e.i.r.p is in dBW or watts.  In wireless WAP i believe they reference mW or Watts.



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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,427 posts since
    Oct 7, 2008
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    4. Aug 25, 2011 6:56 PM (in response to Steven Williams)
    Re: EIRP vs Antenna Gain

    And if you're talking about externally attached antenna, don't forget that it also accounts for any of the dB LOSS in the cable from AP (initial transmitter) to the antenna.



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