This paper treats the old problem of defining and understanding stored
electromagnetic energy. Three different concepts for evaluating the stored
energy and the radiation Q factor of an antenna are introduced. The first two
use time harmonic quantities and are based on the work of Vandenbosch and
Yaghjian. The third concept is a time-domain method which aims to deliver the
true stored energy. The concepts are discussed and compared on the basis of
examples of varying complexity, including evaluation of Q factors for
non-radiating lumped RLC circuits and a canonical dipole radiator. It is shown
that all three concepts unite for special cases of parallel and series RLC
circuits. For other (even very simple) circuits, the approaches yield
significantly different results.
The scattering from a thin conducting wire is computed by representing the induced current as a sum of driven and resonant terms, the latter with complex propagation constant mk perturbed from its free space value k. Using Galerkin’s method, the central problem of determining m reduces to a minimization problem. For the limiting cases of highly conducting or highly absorbing wires simplifications are found. For short wires the Rayleigh cross sections are obtained; for longer wires with high absorption, accurate cross section formulas are constructed based on the unperturbed infinite wire currents. For general wire lengths and conductivities the method is computationally very simple and results are in excellent agreement with independent computations of both current and far field quantities, as well as experimental measurements.