Amplifying signal strength of a u.fl antenna
Even if you are only downloading files, there needs to be both TX and RX, its not a one way communication like an AM radio. The router is probably using some sort of duplexer for TX and RX, you would basically need another duplexer and 2 amplifiers tuned at different frequencies, most likely an LNA and a PA, I'm sorry but there is no easy solution if you use that antenna.
Probably the easiest solution is to use an antenna with higher gain high gain antennas, the higher the dBi rating, the higher the gain of the antenna. Instead of hacking your wlan card, use it on the router.