The Effects of Ground, irregular local terrain can have a profound effect on the launch of HF signals into the ionosphere. A system approach is needed to create a scientifically planned station. We pointed out in Chapter 3 that antenna modeling programs do not generally take into account the effects
of irregular terrain, and by “irregular” we mean any sort of ground that is not flat. Most modeling programs based on NEC-2 or MININEC do model reflections, but they do not model diffractions.
On the other hand, while a ray-tracing program like
HFTA (HF Terrain Assessment) does take into account diffraction, it doesn’t explicitly factor in the mutual impedance between an antenna and the ground. Instead, HFTA makes the basic assumption that the antenna is mounted sufficiently high above ground so that the mutual impedance between an antenna and the ground is minimal. In this chapter we’ll look at modeling the antennas themselves on the PC. We’ll evaluate some typical antennas over flat ground and also in free space. |