The zero phase-front in microwave optics

Many microwave optical devices have been designed and studied, at least in part, by the methods of geometrical optics. Common practice in the microwave field has been to focus attention on the family of rays rather than on the surfaces normal to them, the so-called phase-fronts. When phase-fronts are examined, they are usually obtained from the rays by passing along each ray a constant electrical distance from a reference phase-front. The clumsy analytical expression for a phase-front thus obtained renders even a routine investigation of it a burdensome exercise in mathematical manipulation.