Coarse-Grid FDTD Approach for Capacitively Loaded Cylindrical Tube Antennas

A coarse-grid finite-difference time-domain (FDTD) approach for cylindrical tube antennas loaded with capacitive gaps is presented by using equivalent corrections. The loaded section of the antenna is equivalently transformed to the thin wire with the lumped capacitors. The equivalent capacitance of the loaded gap is calculated from the parallel connection of the uniform-field capacitance and the fringing-field capacitance over its gap. The near-field behaviors around the loaded and the unloaded sections of the antenna are approximated to the quasi-static fields with the radial dependency. From contour-path integration, those considerations are represented as the equivalent corrections of the FDTD update equations. This approach is only applied within one cell from the antenna axis. Thus, there are no requirements such as additional grid refinements for the fine geometry of its gap. Numerical results of the proposed approach are verified by comparing with the reference data of the capacitively loaded cylindrical tube monopole antenna driven by a coaxial line.