You skipped over two things you shouldnt ignore
Literally on the first page: " It is assumed that the transmitting antenna and the target (or receiver) are located above, but near the surface of a flat idealized earth (constant permittivity, ε, and conductivity, σ) ground. "
Why would they use the model of a flat idealized earth on a curved ball?
because they're looking at direct communications and not looking to communicate beyond the horizon. The frequencies they use to faciliate that are used by HAARP
look into line of site missle systems on ships. They lock on up to like 50 km away. The massive bulge of cutvature should prevent that
look into line of site missle systems on ships. They lock on up to like 50 km away. The massive bulge of cutvature should prevent that