The scope of this question may sound weird.
Let's say there are two very giant weird-shaped antennas (imagine they are both 50 meters long): one for transmission and the other for receiving. On a flat surface, one antenna is placed towards the other one while the other is set point to the sky. In this case, how can I determine the distance R in Friis Equation? (Or how to find the distance of arbitrary antenna pairs in 3d space?)
The example picture is shown as below. Don't mind my drawing, as I just want to show an uncommon case where the tiny distance difference is not negligible.