On the maximum transmission distance

Hi,

I am sure someone has tried to find out the maximum transmission
distance
using USRP2. For example, I am interested in the maximum transmission
range
in the following setting:

USRP2 with WBX, 400MHz/900MHz, outdoor, los, static,
benchmark_tx/benchmark_rx with default except tx_gain=25 (maximum).

We are interested in extending the transmission range by adding external
amplifier module. If anyone can share the experience, it would be
greatly
appreciated.

Thanks,
An

By the way the VERT400 144MHz, 400 MHz, and 1200 MHz Tri-band 7-inch
vertical
antenna is used.