Investigation of the directivity of a commercial Ground-Penetrating Radar antenna using a Finite-Difference Time-Domain antenna model

There have been a number of studies using theoretical predictions, numerical solutions, and measured data for analysing the radiation patterns of Ground-Penetrating Radar (GPR) antennas in free-space and over lossless and low-loss half-spaces. These studies demonstrated that theoretical far-field patterns do not apply in the near-field, and that results from simplistic models do not well match measured data from real antennas. In this paper a Finite-Difference Time-Domain (FDTD) model based on a high-frequency commercial GPR antenna was used to conduct an initial study into the near- and far-field radiation patterns in different environments. The antenna model replicates all the detailed geometry and main components of the real antenna. Taguchi´s optimisation method was used when developing the antenna model to determine values for any unknown parameters (due to commercial sensitivity). E-and H-plane patterns from the antenna model were analysed and compared with theoretical far-field patterns over a range of different lossless half-spaces (ε_r = 3, 6, 12), and at a range of distances (0.1-0.6 m). It was found that the modelled E- and H-plane patterns converged towards a consistent shape between 0.1-0.2 m indicating a transition between field zones occurring at this range. Significant differences in the shape and amplitude of the modelled and theoretical patterns were observed. This indicates that the complexity of GPR antennas means that theoretical far-field patterns cannot be assumed for the environments typically encountered by high-frequency GPR antennas.