First the assumptions and limitations of the k-space gain formulation are enumerated. These include the assumption of observing the gain in the far field of the array, the demarcation of which is explained further in Appendix 1. Next are descriptions of antenna performance metrics used to evaluate performance: peak gain, maximum sidelobe level, beamwidths, and integrated sidelobe level (ISL). Then it is shown that plane waves are solutions of Maxwell’s equations (or more specifically, of the Helmholtz wave equation). This is a key result since the k-space gain formulation is based on two-dimensional (2-D) expansions of the ESA spatial weighting function into the summation of plane waves, each with its own amplitude and phase, through continuous or discrete 2-D Fourier transforms.