Finite-Difference Analysis of EMP Coupling to Thin Struts and Wires

This paper describes the implementation of the thin-strut formalism in the 3-D EMP time-domain finite-difference code THREDE. The thin-strut formalism permits inclusion of arbitrary fine wires in THREDE without imposing any corresponding demand to reduce the cell size to the wire size. The keystone of this technique is the so-called in-cell inductance-the inductance per unit length a thin wire would have with respect to an enclosing conductor half a cell removed. THREDE results using this formalism are compared with analytic EMP solutions for a linear dipole antenna and a loop antenna. Errors are around 10 percent for the loop and 7 percent for the dipole. The 10-percent loop error could probably be improved; the 7-percent dipole error seems to be fundamental to the basic THREDE approximations.