Any piece of metal will radiate. The challenge for Ground Penetrating Radar (GPR) antenna designers is to specify a piece of metal (plus dielectric) that will take an ultra-wideband input pulse of 2 octaves or more, and radiate it cleanly into the ground with minimal feed point reflection and energy storage in the antenna. Radiation into the air should be minimal, and any radiation which does occur should also involve minimal energy storage. All these conditions should be satisfied subject to the presence of earth materials of variable dielectric properties in the near-field of the antenna.
As a primary tool for this design task, a rigorous electromagnetic model has been developed. The model has the flexibility to handle complex, arbitrarily shaped elements including the shield. The ground is idealised as a homogeneous lossy dielectric half-space. The influence of the air-ground interface upon the antenna is thus rigorously included. The approach taken treats other inhomogeneities, such as layers, as second order for the purpose of characterising the antenna. The Complex Image Method (CIM) is used to approximate the Green's functions for matrix filling. The fact that many ground materials have an approximately constant complex permittivity over the frequency band of interest to GPR allows the use of a single set of complex images over the whole band. Triangular Rao-Wilton-Glisson basis functions were used to provide the flexibility to handle curved surfaces and arbitrary shapes.
The model was tested using monopole antennas mounted on a vertical PEC ground plane in a water tank. Close agreement was found for impedance and radiation patterns of both shielded and bare bowtie antennas above water over the band 50MHz-3GHz.
Typical lossy ground materials were also modelled. Gain and coupling efficiency were plotted against frequency and antenna height. It was shown that bowtie antennas give rise to a broadband beam at the critical angle if they are elevated above the interface a few percent of their length. The effect of the shield was seen in cavity resonance, reduction of low frequency gain and the generation of very narrow beams at the critical angle from the vertical currents supported, particularly near resonance.
Another valuable tool in design is the set of heuristic principles that are based on qualitative understanding of antenna operation. Work in this area is represented by a collection of GPR antenna design principles. The sensitivity of a wire dipole to height above the interface is dependent on the wire diameter, for example, and dipole antennas have the lowest cross-coupling when in each other's broadside direction. A summary of theoretical work on the issue of surface waves on the half-space surface is also included.
The modelling approach taken of constant Q, CIM with triangular basis functions is thus shown to be a useful tool for GPR antenna design. Other antenna parameters may be readily investigated with the existing model including alternative shapes for the radiating elements and resonance damping methods. A similar approach would also be possible for the investigation of the scattering properties of targets.