Chapter 3 - The Effects of Ground

The ground around and under an antenna is part of the environment in which any actual antenna must operate. Chapter 2 dealt mainly with theoretical antennas in free space, completely removed from the influence of the ground. This chapter is devoted to exploring the interactions between antennas and the ground. The interactions can be analyzed depending on where they occur relative to two areas surrounding the antenna: the reactive near field and the radiating far field. You will recall that the reactive near field only exists very close to the antenna itself. In this region the antenna acts as though it were a large lumped-constant inductor or capacitor, where energy is stored but very little is actually radiated. The interaction with the ground in this area creates mutual impedances between the antenna and its environment and these interactions not only modify the feed-point impedance of an antenna, but often increase losses. In the radiating far field, the presence of ground profoundly influences the radiation pattern of a real antenna. The interaction is different, depending on the antenna’s polarization with respect to the ground. For horizontally polarized antennas, the shape of the radiated pattern in the elevation plane depends primarily on the antenna’s height above ground. For vertically polarized antennas, both the shape and the strength of the radiated pattern in the elevation plane strongly depend on the nature of the ground itself (its dielectric constant and conductivity at RF), as well as on the height of the antenna above ground.