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

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 ?

pix
Guest

17dBm + 2dBi = 19 dBm

it’s as simple as that ๐

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).

Regards
pix

rf fan
Guest

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

pix
Guest

decibels are a weird unit ๐ 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”.

rf fan
Guest

Wow, it’s clear now ๐ ๐
Thank you very much for detailed and usefull explanation !

Viewing 5 posts - 1 through 5 (of 5 total)
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