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A full coarse-grid based finite-difference time-domain (FDTD) model is proposed for an efficient analysis of resistively loaded cylindrical antennas driven by coaxial feed lines. In the case of the electrically thin resistive antenna, the thin-wire approximation is applied to the near fields around the antenna. The resistive antenna is equivalently represented by a series connection of piecewisely lumped resistors along the antenna axis. And the coaxial line is replaced by an equivalent source over the feed aperture of the line. Then the corresponding FDTD update equations make it possible to implement the full coarse-grid model without additional grid refinements for the antenna and the feed line. The transient reflected feed voltage and the input impedance of resistive antennas are calculated numerically and compared with those of a full fine-grid. Index Terms--Coaxial transmission lines, cylindrical antennas, finite-difference time-domain (FDTD) methods, loaded antennas.

For the finite-difference time-domain (FDTD) analysis of antennas excited by coaxial lines, an equivalent model of the antenna feed is presented here. Under the quasi-static approximation, the coaxial aperture is simply represented as the equivalent magnetic-frill current and the equivalent load circuit including the effects of the coaxial line. It leads us to a simple modification of the standard FDTD update equation at the aperture without any additional cell modeling of the line. The validity of the proposed model is confirmed by comparing of the FDTD results and the measured data. Index Terms--Antenna feed, equivalent feed model, finite-difference time-domain (FDTD), magnetic-frill current.

The diffraction problem is treated for the incidence of an E-polarized plane wave on both interfaces of an obtuse-angled dielectric wedge. Based on the dual integral equation, the total field is obtained by sum of the physical optics solution and the edge-diffracted correction term. Calculated diffraction coefficients and field patterns are plotted in figures for wedge angle 120 degrees, incident angles 60 degrees and 70 degrees, and relative dielectric constants 2 and 10. It is shown that the Neumann expansion to the nonuniform currents provides a more accurate correction to the physical optics currents than the multipole expansion as the angle of dielectric wedge increases.