Numerical Analysis of the Detection Performance(2)

S11(dB)

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Fig. 1. Our GPR environment of interest, simulated with a time-domain electromagnetic CAD tool. The typical scenario for target detection is represented by an upper half-space (for instance, air) and a lower region describing a ground medium (represented by appropriate permittivity, permeability, and conductivity parameters). Buried scatterers having arbitrary geometry (shape, dimension, and location) and electromagnetic parameters can be considered, detectable with proper GPR antenna systems conveniently located close to the interface y = 0 (a simple Tx/Rx dipole configuration is sketched).

(b)

Fig. 2. Return loss (|s11|, dB) vs. frequency f (GHz) of various designed radiating elements for GPRs: (a) λ/2 dipole, resistively-loaded λ/2 dipole, printed folded loaded λ/2 dipole, printed monopole; in these cases a free-space external environment is considered. (b) Vivaldi antenna, radiating either in free space or coupled to a half-space dielectric ground having εr = 3.2. See labels for the colors associated to the various curves.

Among the various radiating elements that can be considered in the applications, we have designed and tested some fundamental configurations of GPR Tx/Rx systems often commonly employed in practice: e.g., i) Hertzian dipole probes; ii) resonant half-wavelength dipoles; iii) resistively-loaded λ/2 dipoles; iv) printed folded loaded λ/2 dipoles; v) printed monopoles; vi) printed Vivaldi antennas. In order to make consistent comparisons, in the following we refer to the design of antennas operating in a range centered around the reference frequency of 1 GHz. It should be emphasized that in such GPR applications it is extremely important to test the antenna features in the ‘effective’ operation conditions, that is taking into account various ‘realistic’ issues affecting the overall performance of the system, such as the nonhomogeneous external environment, the possible coupling effects between radiators, etc..

Examples concerning the matching features of the various simulated antennas are derived by calculating the return loss as a function of frequency, as shown in Fig. 2 for a number of topologies: various types of dipole and monopole configurations are in Fig. 2(a), while Vivaldi elements are in Fig. 2(b). The capability of reaching significant wideband behaviors is particularly noticeable in the latter case. In order to assess the influence of the external environment in ground-coupled configurations, in Fig. 2(b) the matching features are evaluated by considering both the radiating antenna in free space (red curve) and the antenna placed at the interface between air and a specific ground medium (blue curve).

The spatial field distribution radiated by the antennas has been evaluated considering both ‘ideal’ far-field radiation patterns and ‘real’ field configurations in typical near-field operating conditions, which take into account also the presence of the inhomogeneous environment. Fig. 3(a) shows the far field radiated in free space (Fraunhofer region) of different types of antennas (folded loaded dipole, monopole, Vivaldi radiator, compared also to an ideal current line). Fig. 3(b) shows a near-field distribution radiated at a fixed distance by different antennas (monopole, Vivaldi, and ideal current line as a reference), located at the air/ground interface (see in the relevant caption the geometrical and physical parameters).

Far Field (V/m)

E (V/m)

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Fig. 3. Simulated field distributions for different GPR antenna systems, as labeled by the different colors of the curves. (a) Far-field radiation patterns (Fraunhofer region) for different antennas located in free space (polar form in a cross section xy of the scenario as in Fig. 1). (b) Near-field distribution at a fixed distance (15 cm) for different antennas located at the interface y = 0 between air (upper space) and a dielectric ground medium with εr = 3.2 (lower space).

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

This basic analysis shows to what extent the antenna performance can be affected by the practical GPR operative conditions. These aspects, in conjunction with the specific features of the GPR transmitted signal, heavily influence the detection performance of the system, as discussed further.

III. WAVEFORMS AND DETECTION FEATURES The potential of our numerical implementation has been tested to obtain results for various GPR scenarios of practical interest. We refer here for instance to a ‘sandy’ dielectric ground (εr = 3.2) in which targets having different location, shape, dimensions, and contrast can be buried.

As said, among the other parameters, the choice of the signal waveform significantly influences the detection capability for scatterers in particular critical conditions [3-5]. Some test cases of typical traces considered here are: i) Gaussian-modulated pulse; ii) Ricker waveform; iii) compressed chirp signal.

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