So, remove SIM, remove two small screws and CAREFULLY prise apart case and check that you have the pcb mounted socket. Obtain a CRC9 connector to RP-SMA adapter or a 3G aerial with CRC9 connector, there are plenty on eBay. I used a 3G aerial + 3metres cable + CRC9 connector for £7.49p inc postage. http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item...
Mark out and drill suitable hole, refit pcb, screws and SIM.
Obviously you do this at your own risk!
So in difficult areas, a USB extension cable and the above and you should get a better signal. At 3G frequencies RG174 coax has a high attenuation. You can get CRC9 adapters and use a lower loss coax if you are looking for either a longer coax or lower losses.
"As I hurtled through space towards re-entry at twice the speed of sound the only thought in my mind was that this craft was entirely built by the lowest bidder!” John Glen, Astronaut
Andrews & Arnold
DrayTek Vigor 2800V