The well know Friis's Equation gives the free space loss from one isotropic antenna to another with variables being the frequency, wave length, and distance between antennas.
When the transmitting or receiving or both antennas are directional, then the gain of the antennas (in db) is subtracted from the Friis free space loss.
However, for high gain antennas, Friis's equation someti9mes gives impossible results.
For example, if frequency = 2 GHz, distance = 0.5 miles (about 800 meters),
and the gain of each antenna is 30 db, then the equation predicts a free space gain
instead of a free space loss. This is impossible, of course, since you can not get more power out of the receiving antenna than you put into the transmitting antenna.
The likely soure of the anomalous result is that Friis' equation is for the "far field" and does not take into account "near field" phenomena.
One wold think that half a mile separation would be far enough to be able to ignore the near field., but it apparently is not.
Does anybody know of a Free Space Attentuation formula that considers the "near field" in a simple enough way to aid in hand calculations. Or maybe I'm barking up the wrong tree.
Thanks for your help.