Variable Reference Phase in

u Abstract We review a free parameter in the design of Fresnel-zone-pla te antennas. Historically, zone-plate antennas have been designed with a specific choice for this parameter. Two methods of interpreting the parameter have been identified, either in terms of a reference radius or, equivalently, a reference phase. For simplicity, here we treat this variable parameter as a reference phase. Importantly, the reference phase can be chosen to have non-standard values, which have been shown to improve important aspects of antenna performance and to add a new functionality to zone-plate antennas. In particular, a nearly linear one-to-one relationship between reference phase and the phase of the main-lobe beam exists, which is not present in the phase of sidelobes. This difference in phase response between main lobe and sidelobes is present independently of phase in feed circuitry: a zone-plate phase shift is fundamentally different from a feed phase shift. Frequency modulation of the reference phase thus appears as strong modulation of the main-lobe signal, but not of the signals through the sidelobes. This difference in frequency and phase response allows side lobe signals to be discriminated against, making new applications possible. We review applications that exploit these features, including multipath-fading reduction, radar sidelobe-clutter reduction, and enhanced security in communications.