The theoretical development of planar near-field antenna measurements is usually based on this plane wave spectrum (PWS) representation of electromagnetic (EM) fields. This generalized interpretation can be shown to stem from the free-space solution of the scalar wave equation, which itself follows directly from classical EM theory and Maxwell's equations where the four Maxwell equations are postulated, mathematical generalizations of a great many macroscopic experimental observations of electricity and magnetism. The validity of these phenomenological physical laws is attested to by the extraordinarily good agreement attained between measurement and prediction.