I was having a discussion with someone far more expert than I and for whom I have a lot of respect. He was telling me that the path loss of a radio signal from A to B is not necessarily the same as it is from B to A. One of the reasons he mentioned is diffraction over a knife edge which in radio could be the edge created where a highway cuts through a hill. In radio as well as optics, the diffraction produces a series of light and dark lines on the far side of the edge. My position is that the path loss is symmetrical and all the effects that affect path loss such as diffraction, absorption, multipath, the Fresnel Effect, which is really the same thing as multipath, and shadowing are all symmetrical with respect to the direction of propagation. With respect to diffraction, if the transmitter is a A and the receiver is at B, with the knife edge closer to B, B may be in a bright line or dark line but either way the path loss will be the same in both directions. Can anyone think of any reason this should not be so?