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# what is the difference between dbm and dbi

Posted on 2002-06-29
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does anybody gnow the difference between dbi and dbm. linksys insist on tellin me that the 2 antennas supplied with their wap11 are 15dbm, when everybody describe the output power in dbi. for example a corner reflectin antenna, is 9dbi and has a 75 degree beamwidth one way and 65 the other way.

gnow am i bein confused here deliberatly by linksys, cos they are very very stingy on technical details regardin the wap11. and i recall someone mentionin that the antennas wiv the wap11 are about 1.2 dbi each.

tia
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Question by:andru
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LVL 79

Accepted Solution

lrmoore earned 100 total points
ID: 7118970
System Gain (dBm) = total gain of the radio system w/o considering antennas/cables
= Tx Power - Rx Sensitivity

Rx Level (dBm) =
Tx Power - Cable/Connector Loss + Antenna Gain - FSPL + Antenna Gain - Cable/Connector Loss

Some antennas are specified in dBd
To convert from dBd to dBi add 2.
Example: 20 dBd = 22 dBi

Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). dBi is used to compare the power level of a given antenna to the theoretical isotropic antenna. The FCC in the U.S. uses dBi in its calculations. An isotropic antenna is said to have a power rating of 0 dB, for example, zero gain/loss when compared to itself.

Unlike isotropic antennas, dipole antennas are real antennas (dipole antennas are standard on Aironet Access Points, Base Stations and Workgroup Bridges). Dipole antennas have a different radiation pattern compared to isotropic antennas. The dipole radiation pattern is 360 degrees in the horizontal plane and 75 degrees in the vertical plane (assuming the dipole antenna is standing vertically) and resembles a donut in shape. Because the beam is "slightly" concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).

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Author Comment

ID: 7121267
not sure i really follow here. can dbi and dbm be compared? or are they different ways of describing the power output of a antenna.

linksys say their antenna that comes boxed wiv the wap11 is 13dbm. i have found a corner reflectin antenna, that is 9dbi. how can i compare these ratings?

what is the dbi of the antennas on the wap11?

tia
0

Author Comment

ID: 8198299
well, it looks like i'm gonna be waitin her till i get old, so i'll cut and paste your reply and ask someone else to explain it to me.

tia
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