The selection of a near-field or far-field ground-penetrating radar (GPR) model is an important question for an accurate but computationally effective characterization of medium electrical properties using full-wave inverse modeling. In this study, we determined an antenna height threshold for the near-field and far-field full-wave GPR models by analyzing the variation of the spatial derivatives of the Green's function over the antenna aperture. The obtained results show that the ratio of this threshold to the maximum dimension of the antenna aperture is approximately equal to 1.2. Subsequently, we validated the finding threshold through numerical and laboratory experiments using a homemade 1-3 GHz Vivaldi antenna with an aperture of 24 cm. For the numerical experiments, we compared the synthetic GPR data generated from several scenarios of layered medium using both near-field and far-field antenna models. The results showed that above the antenna height threshold, the near-field and far-field GPR data perfectly agree. For the laboratory experiments, we conducted GPR measurements at different antenna heights above a water layer. The near-field model performed better for antenna heights smaller than the threshold value (≈29 cm), while both models provided similar results for larger heights. The results obtained by this study provides valuable insights to specify the antenna height threshold above which the far-field model can be used for a given antenna.
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