Recent improvements to the model for wire antennas in the code NEC

The author describes modifications of the numerical electromagnetics code (NEC) algorithm to improve the accuracy for wires with discontinuous radius and tightly coupled junctions and to avoid loss of precision in modeling electrically small antennas. To model wires with discontinuous radius, the current is now located on the wire surface with match points on the axis, reversing the previous convention. Openings in the surface at wire ends, voltage source gaps, and steps in radius are closed with radial caps so that the location of match points on the axis is justified under the extended boundary condition. A condition on the charge density at junctions is established by a small moment-method solution for the continuity of the scalar potential at each junction or change in radius. Steps to improve precision at low frequencies include changes in the evaluation of basis functions and field due to a segment, and a provision to use a constant basis and weighting function around selected small loops.<>