Surface-Wave Coupling and Antenna Properties in Two Dimensions

Abstract

Antennas are characterized by their gain and effective aperture area, and the coupling between two antennas in 3-D free space is governed by the Friis transmission equation. In this paper, we derive the properties of antennas in 2-D space, and the equivalent coupling equation. This is useful for evaluating surface-wave coupling between antennas that share the same ground plane or substrate. We propose a quantity which is the effective width for surface-wave coupling, and derive its value for an isotropic surface-wave radiator in two dimensions. We also determine the surface-wave directivity for dipole-like modes, which is relevant to many small planar antennas. The total coupling between two coplanar antennas is a combination of surface waves and space waves, and these two components are distinguished in simulations by calculating antenna coupling as a function of distance. Several simple examples are illustrated including patch and monopole antennas on various substrates. Quantifying the effective surface wave width can serve as a useful tool for optimizing the coupling between coplanar antennas.