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A thin wire-based finite-difference time-domain (FDTD) model for a simple analysis of insulated and resistively loaded cylindrical antennas fed by coaxial lines is proposed. The resistive loading and the coaxial feed line are approximated to equivalent resistors along the antenna axis and the equivalent source over the infinitesimal feed gap, respectively. The effects of the insulation are corrected by employing thin-wire approximation and the boundary condition. A full coarse-grid FDTD algorithm is then implemented without additional grid refinements for the insulation, the resistive loading and the feed line. As a function of insulation properties and resistive loading profiles, the transient reflected feed voltage and the input impedance of antennas are calculated numerically. The validity of the proposed model is proved by comparing it with the results of the full fine-grid FDTD.