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dBm vs dBi

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?

rfc1180

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

hongedit

ASKER

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

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

ASKER CERTIFIED SOLUTION

rfc1180

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hongedit

ASKER

Thank you Billy.

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

Explained beautifully