802.11b has to be line of site, I have a 12db gain omni directional antenna with a legal limit amplifier/preamplifier, if you can get this high enough and don't have too much foliage obstructing the portables it will work fine.

<edit> The antenna, amplifier, coax, and hardware cost over $1400.00 US around March 2001, the radio (Cisco Aireonet ) was another $1300.00, it is all just collecting dust right now </edit>