Resistively loaded antennas for ground-penetrating radar: A modeling approach

Abstract

The design of surface ground-penetrating radar (GPR) antennas is inherently difficult, primarily because the presence of the air-soil interface greatly complicates both analytic and laboratory-based approaches aimed at characterizing the antennas. Versatile numerical simulation techniques capable of describing the key physical principles governing GPR antenna radiation offer new solutions to this problem. We use a finite-difference time-domain (FDTD) solution of Maxwell's equations in three dimensions to explore the radiation characteristics of various bow-tie antennas (including quasi-linear antennas) operating in different environments. The antenna panels are either modeled as having an infinite conductivity [i.e., a perfect electrical conductor (PEC)], a constant finite conductivity, or a Wu-King finite-conductivity profile. Finite conductivities are accommodated through a subcell extension of the classical FDTD approach, with the model space surrounded by highly efficient generalized perfectly matched layer (GPML) absorbing boundary conditions. Our results show that input impedances, radiated waveforms, and radiation patterns of bow-tie antennas with Wu-King conductivity profiles are largely invariant when placed in free space or above diverse half-space earth models. By comparison, antennas with PEC or constant finite-conductivity panels have variable characteristics that depend somewhat on their operating environment. Quasi-linear antenna designs tend to be less sensitive in this respect, and hence may be suitable for a somewhat larger variety of soil conditions than planar bow-tie antennas characterized by large flare angles. Antennas with constant finite-conductivity panels are considerably more robust (i.e., less sensitive to their environment) than their PEC analogs because the loss resistance is increased, and the range over which a significant amount of current flow occurs is decreased when the antenna panels are resistively loaded. For the extreme case of Wu-King conductivity profiles, the current in the antenna panels approaches that of a quasi-infinitesimal electric dipole. This is shown by the surface-charge distributions on the various antennas and by the corresponding energy radiation patterns. Unfortunately, the favorable characteristics of the latter antennas are counterbalanced by markedly lower radiation efficiency. For the antenna designs considered in this study, we found that the peak energy radiated into earth models from bow-tie antennas with Wu-King conductivity profiles is about one order of magnitude lower than for antennas with PEC terminals.