EIRP = TX power + antenna gain.
The Cisco Aironet Bridge transmitter power is 20dBm, which is 10dBm lower than maximum. This then allows the use of antennas up to 10dB over the initial 6dBi limit, or 16dBi.
In Point-to-Point systems for 2.4GHz systems, using directional antennas, the rules have changed. Because a high gain antenna has a narrow beamwidth, and therefore the likelihood is high that it will cause interference to other area users. Under the rule change, for every dB the transmitter is reduced below 30dBm, the antenna may be increased from the initial 6dBi, by 3dB. (29dB Transmitter means 9dBi antenna, 28dB transmitter means 12dBi antenna). Because we are operating at 20dBm, which is 10dB below the 30dBm level, we can increase the out antenna by 30dB. However Cisco has never tested, and therefore is not certified with any antenna larger than 21dBi.
