Fixed Terrestial Wireless

=== [http://en.wikipedia.org/wiki/Fresnel_zone Fresnel Zone Clearance] ===

The remaining issue is one of ground clearance, commonly known as [http://www.afar.net/fresnel-zone-calculator/ fresnel zone clearance].  Fresnel zone clearance gives you an indication of what effect the ground along the path will have. If the path is very close to the ground, then you don't have much clearance, and will lose a few dB.  But the path should be stable.  If the path is high above the ground, and crosses over a flat area along the path that can be seen from the antennas at both ends, then you have the possibility of signal reflections off the ground arriving at the receive antenna delayed and out of phase with the direct path, and the two signals can cancel.  It is an unfortunate fact of life that the reflectivity of the ground can change drastically.  If you have a rough surface, or a nice crop of corn, it will attenuate any signal that would otherwise be reflected.  But at another time of year, the ground can be bare, maybe have a nice coating of dew or water, and be very reflective.  The solution to this type of problem normally is to alter the height of one antenna and change the reflection from a cancelling mode to an additive mode.  I suspect this is seldom done for this type of service.  Maybe this whole paragraph should be deleted.

The only good thing to know about this fresnel zone topic is that the path will normally be short enough you don't have to worry about [http://en.wikipedia.org/wiki/K-factor K] changing the fresnel zone clearance.