Ground penetrating radar antennas: theoretical and experimental directivity functions

The prediction of the directivity function of a GPR antenna still remains a partially unsolved problem because of the subject complexity. First, the far-field conditions are often not satisfied and second, the antenna design has little in common with the Hertzian dipole for which an analytical approach can be used. The authors' contribution is both theoretical and experimental. On one side, they solve the (electromagnetic) EM integral equations numerically to derive the wavefield components from near to far-field distances. On the other side, they experiment with two novel techniques for measuring the directivity functions in the near to far-field range on dry and saturated sand. Theoretical and experimental results show that neither the analytic approximation of far-field directivity for the numerical integration of near-field directivity can perfectly match the measured functions, although near-field solutions are generally more consistent. The mismatch should be attributed to the present-day GPR antenna design that includes absorbers and shields. Although the effects of these elements are not included in the present numerical near-field solutions, they believe the approach to be of practical value to predict an average directivity function. A smoothed version of the analytic far-field solution can also be used in the range of the near to far-field transition but near-field solutions are really recommended when TX-RX distances are shorter than five wavelengths.