Fresnel zone and path clearance
First Fresnel zone radius, earth bulge, the clearance a link needs under ITU-R P.530, and the diffraction loss an obstruction causes.
How this is calculated
First Fresnel zone radius r1 = 17.32 x sqrt(d1 x d2 / (fGHz x (d1+d2))) in metres, with d1 and d2 in kilometres to the obstacle - the full expression, not the mid-path shortcut, because an obstacle away from mid-path needs it. Earth bulge b = d1 x d2 / (12.75 x k); the worst-month factor is ke = 157 / (144 + 2670/D). ITU-R P.530 wants the full first zone at the median k, and 0.6 F1 over an isolated obstruction or 0.3 F1 over an extended one in temperate climates (0.6 F1 in the tropics) at the worst-month ke - take the higher of the two. Knife-edge diffraction uses nu = h x sqrt(2(d1+d2)/(lambda x d1 x d2)) with the ITU-R P.526 loss L = 6.9 + 20 log10(sqrt((nu-0.1)^2+1) + nu - 0.1). Antenna heights are measured above ground and the obstacle height is measured from the same reference. Terrain elevation is not included - this is a geometry check on the values you enter.