Investigation of the frequency dependent antenna transfer functions and phase center position for modeling off-ground GPR

We compared different methods to estimate the phase center of an ultra wideband ground penetrating radar (GPR) horn antenna operating off-ground, namely, (1) extrapolation of peak-to-peak reflection values in the time domain and assuming a fixed phase center, and (2), frequency-domain full-waveform inversion assuming a frequency dependent phase center. For that purpose, we performed radar measurements at different heights above a perfect electrical conductor (PEC). A double ridged horn antenna operating in the frequency range 0.8 - 5.2 GHz was used. In the limits of the antenna geometry, we observed that antenna modeling results were not significantly affected by the position of the phase center, even when considering frequency dependence. This implies that the transfer function model used to model the antenna inherently accounts for the phase center position. This analysis showed that the proposed antenna model avoids the need for the frequency dependent phase center determination and is valid for any applications where the field measured by the antenna can be considered as coming from a distance larger than the antenna aperture itself, such as for off-ground GPR or telecommunications.