Reflectarray Antennas: Operating Mechanisms and Remedies for Problem Aspects

Abstract

Reflectarrays that emulate paraboloidal main-reflectors, and hyperboloidal or ellipsoidal sub-reflectors, have undergone a great deal of development over the past two decades. More recently, research on the topic has concentrated on overcoming some remaining disadvantages, re-examining certain design issues, and extending reflectarray functionality. This thesis concerns itself with fixed-beam offset-fed single-layer main-reflectarrays and sub-reflectarrays comprised of square or rectangular variable size conducting elements. Both full-wave analyses and experiment are used in all the deliberations. In order to examine reflectarray operating mechanisms the thesis first describes a component-by-component technique whereby the role of the various reflectarray parts can be assessed by determining their individual and aggregate contributions to the reflectarray near- and far-fields. This technique is used to diagnose the fact that feed-image-lobes that appear at off-centre frequencies are caused not only by the groundplane as first thought, but by an imbalance in the complex currents on the patches and groundplane at such frequencies. The use of sub-wavelength elements is shown to suppress such unwanted lobes. The thesis then uses receive- and transmit-modes analysis to show that beam squint at off-centre frequencies, often not accounted for when stating the gain bandwidth of a reflectarray, is due to the shifting of the true focal points away from the geometrical one at these frequencies. It is demonstrated that a two-feed reflectarray arrangement is capable of eliminating beam squint, and that the use of smaller focal length to aperture size (F/D) ratios removes the grating lobes that can appear in such two-feed reflectarrays due to clustering of the aperture amplitude distribution. Finally, the thesis studies the effect of the reality that the angle of incidence of the feed fields on the various reflectarray elements is not the same for all elements, even though this is most often assumed when using element reflection phase versus element size databases in performing reflectarray designs. Careful full-wave analysis reveals that it is not only the dependence of element reflection phase on incidence angle that is important, but that the individual element pattern beamwidths change and distort as this angle increases. This is important not only from the point of view of the coupling of the feed fields to the elements, but also as far as the angular sector within which the reradiated fields are important. Thus sub-reflectarrays, whose radiation patterns are considerably wider than main-reflectors, are more susceptable to incidence angle effects. It is shown that the use of sub-wavelength elements in a reflectarray largely ensures its immunity to such effects.