Decay of groundplane currents of small antenna elements

Expressions are derived for the surface current induced by vertical and horizontal dipoles on a nearby infinite groundplane. The expressions are obtained by means of the exact magnetic field. It is found that for a vertical dipole the surface current density decays roughly as the inverse of the distance from the source, while for a horizontal dipole it decays as the inverse distance squared. The result furnishes an explication for the empirical observation that antennas carrying mainly horizontal currents tend to be less sensitive to the dimensions of a finite groundplane than antennas carrying vertical currents. A formula is given by means of which the groundplane current of a compound source can be evaluated and controlled.