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