A power wave theory of antennas and coupling into complex systems

We introduce here a simple theory of antenna radiation and scattering that fully extends into the time domain a number of standard antenna terms, including gain, realized gain, effective length, antenna pattern, beamwidth, scattering cross section, and radar cross section. Power wave theory applies to linear reciprocal antennas of all feed impedances and feed types, including waveguide feeds. It also applies to antennas embedded in any lossless medium. The theory also leads to a natural definition of mutual coupling coefficient in antenna arrays. The approach is analogous to that used to describe circuits with generalized scattering parameters, with different reference impedances at each port. We identify receiving and transmitting impulse responses, and prove that they always have a simple relationship to each other, provided that the antenna has no nonlinear or nonreciprocal components. We also identify a scattering impulse response that can be applied to either an antenna or an arbitrary scatterer. From these functions, we build a Generalized Antenna Scattering Matrix (GASM), which provides a complete description of antenna response in the far field. This establishes a formalism that allows one to calculate antenna response under a variety of conditions, including, for example, a source or load of arbitrary impedance. The approach simplifies and clarifies terminology for characterizing antenna performance in both the time and frequency domains. Since a complex system may be considered a poor antenna, the approach provides a uniquely useful method of characterizing radiation from and coupling into a complex system, using the same parameters in transmission and reception.