Flat and conformal zone plate antennas with new capabilities

We call attention to the existence of a free parameter in the design of Fresnel zone plates. Historically, zone plates have been designed with a specific choice for this parameter, which can be taken as a type of phase reference. We present two methods of interpreting the parameter, either in terms of a reference radius or equivalently a reference phase. Importantly, the parameter can be chosen to have non-standard values which are shown to improve important aspects of antenna performance and to add new functionality to zone plate antennas