I have an XBee module with a U.FL. connecter. Is it possible to solder an appropriate wire antenna to this module (without buying a new module with a wire antenna)? Since U.FL. has 2 connections and wire antenna needs just one, how do I do it? (My knowledge on antennas are low.)
Sure, this is usually a pretty easy task.
Likely, you have an XBee that looks something like this (at least in antenna configuration):
Now, what you really want is this (again, just looking at antenna configuration):
So, what you need to do should be pretty clear from here. De-solder the U.F.L. connector, then solder a short wire into the little hole on the top right corner of the old U.F.L connector location (not the big hole with a big pad around it, but the one above that). The wire length depends on what frequency your XBee runs at - for 2.4 GHz it's roughly 31mm for bare copper, and 25 for an insulated wire (for a quarter wave antenna, which is what they normally ship with).
An alternate approach to Zplizzi's solution would be to use a piece of coax cable with the U.FL connector, then strip the shielding all the way. The length should be roughly 8cm. This way you don't have to de-solder the connector.