The 24dBi antennas in use have about a 7 degree beam width (3db down points). If refraction effects at 2.4 GHz suggest a deviation from line-of-sight of, say, 20 degrees, then a second pair of antennas would be useful. If the prediction is beam bending of 5 or 10 degrees, then pointing the primary antenna pair a few degrees off LOS would be fine.
I haven't done the math, but I would be surprised if the direction of signal arrival changed appreciably over a 5 mile path relative to a 7 degree beamwidth unless there was a massive statified change in the air mass. In this case, the signal might not be getting out of the valley at all. But, to calculate this sort of thing, you need a vertical profile of pressure, temperature and humidity. Are you sending up weather balloons to get this? You can try to find nearby NWS data, but you are likely to have your own microclimate given your description of the terrain. Do you have signal strength vs time data for the link? A plot of that would be most illuminating, especially correlated with weather ballow data from NWS, or even time of day. Yesterday, a D-Link Internet camera was mounted behind an optical telescope and pointed along the microwave beam path. I am hoping to correlate some visible phenomena with the RF link problems. Visible and RF have very different refraction behavior, so if for example you are getting severe ducting at 2.4GHz you probably won't see much different in the visible. (Note that with all the discussion of DX contacts via ducting no one in California is seeing Hawaii in the visble). The basic reason is that the index of refraction of air for visible is a not function of the water vapor content. For RF, water vapor content is very important. The "4/3" radio horizon rule of thumb is based on the refractive index lapse rate for a standard atmosphere (with moisture). There is good coverage of this subject in the RSGB Microwave Handbook, Vol II (I think - from memory). This seems to be out of print, replaced by a single-volume book called the "International Microwave Handbook": http://www2.arrl.org/catalog/?item=8739 -- Greg Troxel <[EMAIL PROTECTED]> -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
