Arthur Harris wrote:


The Free Space Path Loss Equation is:

Path Loss (dB) = 36.6 + 20 Log F + 20 Log D

Where F is freq in MHz and D is distance in miles. This assumes isotropic
antennas as both ends. The "path loss" represents the portion of the
transmitted signal that is NOT captured by the receiving antenna. It does
not include absorptive losses (which should be negligible in free space).

Some will argue that this is not a true dissipative loss, and that is
correct. But by knowing the "path loss" you can determine your needs as to
transmit power, receiver sensitivity, and antenna gain in order to assure
successful communication.

The key here is a theorem that shows that the "capture area" of a
perfectly efficient isotropic antenna is (wavelength)^2/(4 Pi).

-jpd