I have a question, I've been thinking of extending the range of a Dlink Airpremire access point. It has 2 high gain antennas which i would like to move off the actual access point. Using a coaxial
extention cable would I be able to run it around 25 metres to increase the range of the coverage. As we have a L shape building and the signal doesn't quite get thought to all the class rooms. We don't have many wireless devices as yet so just looking for a cheap alternative as to buying another AP (these AP's are bloody expensive)

Any insight, help, opinions is greatly appreciated and I thank you in advance,

Shayne

Even high-quality cable will be lossy at that length, possibly requiring a signal booster as expensive as just adding another AP.

Here's how I'd approach the problem:
First, I'd make sure the AP is diagonally in the corner of the "L", if it isn't there already. This might mean putting a shelf up in the corner of the hallway, and running some ethernet cable through the ceiling. Maybe your modem could fit there, too.
Test it. If it's enough, stop.

Second, I'd try parabolic reflectors to focus the antennas, each down one of the two corridors, assuming that's how your building is arranged. They're easy to build from thin cardboard and aluminum foil. http://www.freeantennas.com/projects/template/
Test it. If it's enough, stop.

Third, I'd replace the antennas with something more powerful: bigger dipoles or collinears with parabolic reflectors, or homemade biquads.
Test it. If it's enough, stop.

Fourth, I'd add some reflection at the very end of the corridors, in the path of the transmission beams -- some sheet metal bowed slightly outward.
If that doesn't do it, just go ahead and add another AP.