we have several 2602 access points with external antennas, and some 1552 aps, i wonder if i have to set explicitly the Antenna Gain parameter in the configuration, or if i can leave the default value of 0 dBi.
I have one customer with a AIR-CAP2602E using the omni-directional 4dBi dual-band antenna (AIR-ANT2544V4M-R) and setting the parameter to 8x0.5dBi did help increase the signal-strength for 802.11g/n and improved the coverage in our environment.
I don't think APs with external antenna ports can detect what antenna was connected, or am I mistaken? I always manually set this parameter to the recommended gain value. It's just a little difficult to find, if you are using the GUI. It's in the Configure (or Details?) submenu of the AP list. You have to do this both for 802.11b/g/n and 802.11a/n.
In the Antenna Gain text box, enter a number to specify an external antenna’s ability to direct or focus radio energy over a region of space. High-gain antennas have a more focused radiation pattern in a specific direction. The antenna gain is measured in 0.5 dBi units, and the default value is 7 times 0.5 dBi, or 3.5 dBi. If you have a high-gain antenna, enter a value that is twice the actual dBi value (see Cisco Aironet Antenna Reference Guide for antenna dBi values). Otherwise, enter 0. For example, if your antenna has a 4.4-dBi gain, multiply the 4.4 dBi by 2 to get 8.8 and then round down to enter only the whole number (8). The Cisco WLC reduces the actual equivalent isotropic radiated power (EIRP) to make sure that the antenna does not violate your country’s regulations.