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# dBm  vs dBi

Posted on 2011-02-23
Medium Priority
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Hi all

I am speccing up a site at the moment which will need wifi access.

I am comparing 2 devices in terms of signal strength - a firewall with built in wifi, and a CIsco Aironet 1131.

The Cisco has a 3 dBi gain antenna. The other device according to the manufacturer has -27dBm.

Question is, how to I convert these 2 into the same format and determine which is going to be better?
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Question by:hongedit
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Expert Comment

ID: 34963611
The Cisco has a 3 dBi gain antenna. The other device according to the manufacturer has -27dBm.
Question is, how to I convert these 2 into the same format and determine which is going to be better?

the two are unrelated and cannot be converted, the unit dBi is used to compare the gain of an antenna compare to a "Isotropic" antenna; this antenna by the way does not exists, and therefore is theorectical. The unit dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). dB values are easier to work with when is comes to large and small decimal numbers. The dBm is referrenced to devices that output power via a RF amplifier of a transmitter. Typical transmitters will have an output of 100mW to 1W of output power and manufactures will specific this power in dBm 10^27/10 = 500mW of power. Fed with a transmission line (Coax[transmission lines incorperate loss]) and an antenna (Antenna will have a gain) you will have a much stronger signal assuming the loss is lower than the gain of the antenna.

AP=500mW (27dBm)
Coax=-1db
Antenna=3db
29dBm of total output power (27 + -1 +3)

Billy
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Author Comment

ID: 34963767
Sorry, I typo'ed - it's -17 not -27.

I still dont fully understand it though - put blunty, which is "more powerful"?
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Accepted Solution

rfc1180 earned 2000 total points
ID: 34964087
well, I highly doubt that the max output power of the AP is -17, 0dBm is 1mW, anything below 0dBm (Expressed in negative values) will be lower than 0dBm; -17dBm is about .02mW, 17dBm is about 50mW of output power.

>I still dont fully understand it though
it is something you can't learn on one thread. Just note, that you can not referrence and/or compare a antenna with the output power of an AP. You comapre antenna gain (dBi) with other antenna gains (dBi); additionally, you compare AP output power (dBm) with other AP output power factors (dBm).

Here is a senario, you have 2 AP unit from 2 different manufactures

AP 1 from Vendor 1
Max Output power = 17dBm
Integrated antenna (Built-in) gain is 6dbi

AP 2 from vendor 2
Max Output power = 30dBm (1watt)
Integrated antenna gain is 3dBi

Based on these values AP2 will have a stronger signal than AP1
AP2 will have about 2 watts of output power

In some cases. AP1 will be the choice to use in a office area as it has a larger gain than
AP2; the idea in a office area is to not increase power to be heard as the weaker link will always be the client. You want to match or lower the power of the AP to that of the client and have a higher gain antenna to pull in weaker siginals.

Billy

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Author Closing Comment

ID: 34964178
Thank you Billy.

The 17dBm number was directly from the manufacturer, so I'll call them tomorrow to double check.

Explained beautifully
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