Numerical analysis of antenna fields using multipole expansions

A new systematic approach is described which can be used to efficiently obtain the frequency- and time-domain multipole amplitudes of the electromagnetic field due to an arbitrary linear antenna. In particular, the method is perfectly fitted for the finite-difference time-domain (FDTD) method, which is first used to obtain the time-domain multipole amplitudes of the antenna driven by a wide-band signal. Though the time-domain representation is valid in the far-field only, a single numerically performed Fourier transform leads to the frequency-domain multipole amplitudes valid for the entire spectrum of the input impulse and at any point outside a minimum sphere which contains all radiating elements.