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A printed bowtie antenna for a Post Reception Synthetic Focussing Surface Penetrating Radar (PRSF-SPR) has been developed with the aid of FDTD analysis. Antenna free space characteristics were compared against practical measurements and its performance was analysed when soil is present. Introduction: The Post Reception Synthetic Focussing Surface Penetrating Radar (PRSF-SPR) (1, 2) system requires the use of a planar array of antenna elements with each element having wide band matching and wide beam radiation patterns over this frequency band. For an operating frequency of 1 GHz, the system requires approximately 400MHz bandwidth for transmission of a three cycle pulse. The first comprehensive investigation of the input impedance and radiation from bowtie antennas was performed by Brown and Woodword (3). Where they investigated flat top Unipolar Bowties fed through an image plane. In this paper the Bowtie antenna analysis is performed using the Finite Difference Time Domain (FDTD) method. The FDTD analysis includes modelling of antenna element in free space for comparison with practical measurements and investigation of the antenna properties when ground is present. Accurate practical measurements of the radiation pattern in subsurface is difficult due to the complexities involved in such measurement. FDTD Modelling in free space and measurements: The printed bowtie dipole antenna was represented by the FDTD model shown in Fig 1. The antenna element 210mm in length, and the feed were produced on a substrate of dielectric constant 2.2. The antenna element was fed by a balanced co-planar strip line, which passed through a rectangular hole in the ground plane. The co-planar strip line was fed from a coaxial cable via a λ/4 balun with the strip line being tapered to obtain suitable impedance matching. The model space was limited using absorbing boundary conditions (4). For wideband excitation, a single Gaussian pulse of 250 ps width was employed in the simulation process. Better antenna-feed matching was obtained by varying the model parameters. Fig 2 shows the comparison of the final FDTD model and measured responses of the practical antenna. It can be seen that the antenna has a -10dB antenna-feed match from 0.8 to 1.8 GHz. The far-field radiation pattern of the dipole antenna was found by post processing the FDTD frequency domain data at specific frequencies and these are compared with measurements performed in an anachoic chamber. The calculated (shown only for 1.2 GHz) and the measured co-polar radiation patterns for the principal planes are shown in Fig 3. The measured cross-polar levels (not shown) were 20dB lower than the co-polar levels. The far field patterns were obtained for 0.8 and 1.2 GHz, which corresponds to the lower and upper operating frequencies of the antenna. Although better than -10dB antenna feed match was obtained from 0.8 to 1.8 GHz, due to the formation of a null at 00 in the H plane pattern, the antenna could only usefully be operated from 0.8 to 1.2 GHz. The null formation is due to bowtie-ground plane spacing, which corresponds to λ/4 at 1 GHz. It should be noted that in the FDTD model, the tapered strip line and the bowtie are staircase approximated to fit the actual dimensions. The connector, the dielectric substrate losses and the finite size of the antenna ground plane were also not incorporated in the FDTD model. These effects were the prime cause of the small difference seen in FDTD model and practical measurements. Analysis with soil: Since the antenna element is to be employed in a SPR system, the input and radiation characteristics have been analysed incorporating soil in the FDTD model. Homogenous soil was

Wideband printed bowtie antenna element development for post reception synthetic focusing surface penetrating radar