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Community Member

Wireless power output

1) To conform to the standard, the total output from the AP cannot exceed 36dbm, it depends on the countries. The maximum power in an AP is 100mw, is about 20dbm, please correct me if I am wrong, does it independent of the antenna gain which I use? Does a 2.2dBi and 5.2dBi antenna produce the same dBm if the AP power output is 100mW?

Thanks.

3 REPLIES
Community Member

Re: Wireless power output

You definitely want to do your RF math in dB, not mW.

The U.S. version of the 350 series AP has a maximum Transmit power of 20dBm (decibels referenced to milliwatts). Using the 36dB EIRP max, you can add 16dB of gain with an antenna.

The gain from the antenna is consistent no matter the outpur power, so 15dB + 5.2dB = 20.2dB and 20dB + 5.2dB = 25.2dB. However, to convert from dBi to dB (actual power gain), subtract 2.15. So, a 5.2dB antenna on a 20dBm (100mW) AP yields the equivalent of 200mW (23dBm), not 400mW (25.2dBm).

FCC regulations are often interpretted differently by different people, but 20 + 16 = 36 is safe. If you want to use the '3 for 1 rule' to get up to 42dB output, you have to reduce Transmit power by 3dB (half) and add 9dB in antenna gain, so your max in that config is 17dBm from the AP and 25dB from the antenna (without amplifiers).

Matthew Wheeler

Chief Wireless Architect

www.BlueModal.com

Community Member

Re: Wireless power output

Thanks for your guidance.

I have another question, hope you can help me with this.

In one location, three AP can coexist, as there are 3 non overlapping channels can be used. If the location requires more than 3, take 10 APs. It is to provide sufficient bandwidth to each client, definitely the channel I select for the 4th AP will be overlapping (interference occurs) or same channel as one of three APs (CSMA/CA). Between the two options I have, I should go for which? What is the impact to use the same channel as CSMA/CA will happen compared to using overlapping channels as interference will cause retransmission.

Thanks for your advice.

Community Member

Re: Wireless power output

For any location where more than 3 802.11b APs (or bridges) will be installed, use careful antenna selection to limit the signal visible from one to another.

For example, 120 degree sectorized panels will let you put three channels in a tight circle (back-to-back). Using three more installed just above or below the first three, but turned 180 degrees will allow you to put six channels with minimal same-channel interference.

Of course, the better the antenna, the fewer headaches you will have. You would not want to use typical patch, grid or yagi antennas in this way. There are several well-isolated sector antennas available, as well as some circular/hemispherical antennas that can work well back-to-back.

Matthew Wheeler

Chief Wireless Architect

www.BlueModal.com

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