Dielectric fresnel zone-plate lenses and antennas

Abstract

The Fresnel zone lens witb transparentlreflecting (absorbing) rings has inadequate focusing properties.The radiation efficiency of the corresponding lens antenna is less than 15 % [l]. To increase the lens focusing quality Wiltse (1976) replaced the teflecting (absorbing) rings by phase-reversing dielectric Ms. On the base ofthis dielectric zoneplate lens antennas have been developed and examiued for which the radiation efficiency is of about 25-30 % [2-4]. Recently, a new variety of quarter-wave Fresnel zone-plate lens with increased focusing quality was proposed [5]. Each full-wave zone of the Fresnel plate lens was divided into four quarter-wave subzones which were covered by dielectric rings having equal thickness but different pennittivities. To accomplish a quarter-wave stepwise phaseandon the relative permittivities has to obey the following sequence: E 1’1, E2=6.25, €3‘4, and Eq”2.25. This is evident from the computed “infinite” transmission phase-shift characteristics of the dielectric plates having the above sequence of permittivity values, and thickness of the ideal dielectric phase-shifter, i.e. l/2 for & 4. This lens configuration was used to design a tmsmissive-type Fresnel zone-plate antennas for DBS TV and millimeter-wave bands with a computed radiation efficiency higher than 50% [5-71. They were examined theoretically using the multiple reflection/transmission coefficients for low-loss dielectric plates and vectorial Kirchhoff diffraction antenna theory. With dielectric rings made in sandwich-type manner the Fresnel zone-plate antenna reaches about 60% theoretical aperture efficiency. References