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question about ant. gain and radiated ..

  • This topic has 4 replies, 1 voice, and was last updated 13 years ago by rf fan.
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  • #54718 Reply
    rf fan

    Hi all,
    For example we have Transmiter+combiner+spliter+feeder+antena.

    After losses computation we have +17dBm at the input of antenna. Antenna have 2 dBi gain. How to compute radiated output power from antenna ? How many dBm can i expect, input power is +17 dBm ?

    #54719 Reply

    17dBm + 2dBi = 19 dBm
    your EIRP (radiated power) = 19 dBm

    it’s as simple as that ๐Ÿ™‚

    I assume it is an omni directional antenna ? in this case, in all 360 degrees, there will be a TX power of 19 dBm.

    In case of panel antennas, only in the main direction of the lobe will you have 19 dBm. In other directions, you must include a certain attenuation (check the antenna pattern).


    #54720 Reply
    rf fan

    Yes ant is omni for example, but i don’t understand how can be used addition or substraction on two different quantities

    #54721 Reply

    decibels are a weird unit ๐Ÿ™‚ but very convenient !

    dBm are absolute power, they correspond to a certain amount of mW. For instance 0dBm = 1mW, 33dBm = 2W.

    dBi are relative power: they indicate how much gain is given to an input signal. It’s like the “dB”.
    For instance :
    0dBm + 3dB = 3dBm = 2mW
    3dBm + 3dB = 6dBm = 4mW
    6dBm + 3dB = 9dBm = 8mW
    9dBm + 3dB = 12dBm = 16mW

    As you see, 3dB is not equal to xx dBm. It is a factor which means the power (in watts) is multiplied by 2.

    An antenna “multiplies” the input power by a certain factor. It’s “relative power”.

    #54722 Reply
    rf fan

    Wow, it’s clear now ๐Ÿ™‚ ๐Ÿ™‚
    Thank you very much for detailed and usefull explanation !

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