Electromagnetic Studies

Thin-wire Reciprocal Multiradius Implementation of the Electromagnetic Moment Method

by Mark A. Tilston, PhD Dissertation, Dept. Elec. Eng., Univ. Toronto, 1989


The original version of the uniradius thin-wire frequency-domain moment-method program developed by J.H. Richmond has been re-formulated to suppress the computation of non-physical asymmetric fields. Two alternative new versions are described, termed the point-charge version and the bridge-current version. The original program and the two new versions are compared with each other and with simplified theory, where applicable, on the following symmetrical structures: a rectangular wire loop, a two-wire transmission line, a coaxial-cage transmission line, and a log-periodic dipole antenna. The bridge-current version is shown to be the most accurate, and the most invariant with respect to variations in segmentation for the above structures. It is noted that this version is particularly advantageous for structures that include close-spaced parallel wires connected by short wire segments.

An implementation of the moment method for electromagnetic analysis of multiradius thin-wire structures, including multiwire, multiradius junctions is presented. It is an extension of the uniradius bridge-current reformulation of J.H. Richmond's uniradius thin-wire theory described above. The method features an exactly symmetric mutual impedance matrix ensuring reciprocity between sources, it is unconstrained with respect to both the length ratio and the radius ratio of adjoining segments, and it permits the self-consistent inclusion of coaxial-cable sections in the configuration under analysis. The method is validated (a) through comparison with simplified theory and an alternative moment-method analysis on both an electrically small rectangular loop and a short coaxial cage transmission line, (b) through comparison with transmission-line theory for a two-wire line and a coaxial cable, (c) through comparison with measurements on a folded monopole antenna, a sleeve monopole antenna, and a log-periodic dipole antenna, and (d) through comparison with the uniradius bridge-current moment method and transmission-line theory for a bazooka-balun-fed dipole antenna.

Comparison is made between the presented moment method, the NEC (version #1) moment method, and the MININEC moment method, on the analysis of a detuning stub used to minimize the reradiation of AM broadcast signals from a sounded tower, and on the analysis of a bent two-wire transmission line whose dimensions are typical of what one would find on a printed circuit board.  In these two specific problems, the presented moment method is shown to be a significant improvement over the NEC and MININEC moment methods.

Download: Mark's PhD Thesis (2.2 MB) - This is scanned as only hard copy is available.