Also, bear in mind that you must use both connectors to get diversity back. Use of a single antenna lead does not result in antenna diversity. The AP 1020 supports diversity on the integrated patch antennae but by a much argued method. In short, the 2 internal antennae are mounted opposite directions and when the AP chooses which side it hears the signal from, it broadcasts the reply from the same side. This can be a problem when the client is moving or in extreme, and I say extreme, cases of multipath.

I'm now in the implementation phase and am a little confused.

I have configured the APs to use the external antenna, but it needs the gain in 0.5 dBm units. The ANT5959 docs state I peak gain is 2 dBi. How to I work out the value to enter in this field?

Double the number when filling in this field. ie 2dBi = 4 in the field.

That was my initial thought, but the difference in units still concerns me i.e.

dBi == The dB power relative to an isotropic source

dBm == decibels with respect to one milliwatt

I have tried to find more information in the Cisco docs, but without success.

probably wont. dB is the functionality of adding together different units that otherwise wouldn't be addable. (That's a real word in the southeastern US).

The fact that it concerns you shows you're thinking.

google or wikipedia decibel for a better understanding but dennis is correct.