EIRP Rule!!

darrenj · ‎05-25-2007

Hello all, hopefully someone out there can help me!

A WLAN is deployed in England and is therefore subject to ETSI rules. The EIRP (Effective Isotropic Radiated Power) rule for ETSI countries, stipulates a maximum of 20dBm EIRP. This is further broken down as a maximum of 17dBm transmitter power with a 3dBi gain from antenna and cable combination.

The information below is from my WLAN Controller(4404):

(Cisco Controller) show>ap config 802.11b Civic_Floor2_AP2

Tx Power

Num Of Supported Power Levels ............. 6

Tx Power Level 1 .......................... 17 dBm

Tx Power Level 2 .......................... 15 dBm

Tx Power Level 3 .......................... 13 dBm

Tx Power Level 4 .......................... 10 dBm

Tx Power Level 5 .......................... 7 dBm

Tx Power Level 6 .......................... 0 dBm

Tx Power Configuration .................... AUTOMATIC

Current Tx Power Level .................... 1

Phy OFDM parameters

Configuration ............................. AUTOMATIC

Current Channel ........................... 6

Extension Channel ......................... NONE

Channel Width.............................. 20 Mhz

Allowed Channel List....................... 1,2,3,4,5,6,7,8,9,10,11,12,

......................................... 13

TI Threshold .............................. -50

Antenna Type............................... EXTERNAL_ANTENNA

External Antenna Gain (in .5 dBm units).... 5

Diversity.................................. DIVERSITY_ENABLED

Tx Power Level 1 = 17dBm which makes sense as this is the maximum transmit power for EIRP in ETSI countries - as discussed above. However, I am using a 5.2dBi antenna which, if used with a transmit power of 17dBm breaks the EIRP rule (max 20dbm, mine would be 22.2dBm (17dBm+5.2dBi).

I assumed by configuring the antenna gain on the WLAN Controller for this AP, that the transmitter power would decrease in line with EIRP rules - however, it doesn't work as I think. If my antenna is 5.2dBi, then the transmit power should be 14.8dBm or less. Transmit power level 3 = 13dBm so this would be perfect. However, I configure the antenna gain to be 5 (in 0.5dBm units) which equals 2.5. This added to the 3dBi already included in the calculations gives 5.5dBi.

The problem is the transmit power doesn't drop to power level 3 (13dBm). Any ideas why?

Another question, why is the antenna gain specifed in 0.5dBm and not 0.5dBi?!?

Hopefully, this will make sense to someone, and I will be really grateful for some clarification!

Darren

scottmac · ‎05-25-2007

You didn't mention how much cabling, or of what type, you are using between your AP and the antenna.

Check the cabling spec and include the attenuation in your EIRP calculations.

I'm "pretty sure" that the antenna gain indicated on the label does not include the cable lead (the pigtail attached to some of the antennas).

The dBm / dBi thing is probably a typo, since you'd need to have the absolute input power and the absolute antenna gain to figure out what the dBm output of the antenna is (it is measurable with a field strength meter, but they are not common within your typical business environment).

Off to vacation ....

Good Luck

Scott

darrenj · ‎05-25-2007

Im using AIR-ANT1728 antennas. They are directly connected to the APs with no added cabling and I assumed any loss from the built in cable would have been taken into account when it is specified as a 5.2dBi antenna by Cisco???

I asusmed the dBm/dBi was a typo but wanted to make sure I wasn't overlooking something.

Cannot believe you are off on vacation!! LUCKY YOU!! Have a great time, and relax your brain ;-)

Darren