The effect of layer spacing on the properties of double-layer Fresnel zone plate lens

The Fresnel zone plate (FZP) antenna has lens-like properties. The FZP lens avoiding the feed blockage in the FZP reflector is studied. There are two types of FZP lens, one where the phase correction is introduced in successive zones, and one where the concentric zones are alternately transparent and opaque; the latter is more interesting because of its printed structure. However, a single-layer lens is not efficient, as about half of the incident energy is reflected backward, so a double-layer lens was proposed to enhance the aperture efficiency. An improved full-wave analysis based on the vector Hankel transform and spectral domain immittance matrix approach were used to analyse the electrically large double-layer FZP lens, and measurements were carried out for a single-layer FZP antenna with a 6-ring and 15-cm focal length at a wavelength of 3.2 cm. In the case of a double-layer lens filled with dielectric substrate, the reflection from the interface of the substrate and possible emergence of the spurious surface waves degrades the focusing properties, therefore, only the case without a dielectric substrate is under consideration in this paper. Some practical design criteria for a double-layer FZP lens can be obtained from various results of field distributions in the focal region of the FZP lens under plane wave incidence. Furthermore, the requirements for feed design can be determined by means of the field distributions along or across the focal axis.