On the Number of Modes in Spherical Wave Expansions

Since the early days of spherical near-field far-field transformations a recommendation for the necessary number of polar modes has been given by N = kro + 10, being the wavenumber and r the radius of the minimum sphere. The almost explosive development in computer speed and storage capacity witnessed during the last two decades has made transformations of fields from antennas exceeding thousands of wavelengths feasible, and a closer investigation of the above expression seems to be appropriate.

An improved expression for the number of modes, N, related to the antenna size and the required accuracy will be developed. The impact of truncation of the modal expansion at a given level will be illustrated. This is especially important for measurements where noise is present, or where there is undesirable scattering from objects.

Publication: AMTA 26th Annual Symposium (AMTA2004)
Place: Stone Mountain Park, GA, USA, 17-22 October, 2004