E. Almajali

On Beam Squint in Offset-Fed Reflectarrays

A full-wave receive-mode analysis is used to show that it is the shift in the location of the focal point with frequency (caused by nonconstant path delays over the surface of the reflectarray) that is principally responsible for the beam squint that occurs in offset-fed reflectarray antennas, and not the element type used or the lattice size. A similar transmit-mode analysis confirms that it is this focal point shift that implies noncoincidence of the focal point and the feed phase center at off-center frequencies, which results in a phase distribution over the reflectarray aperture with a slope other than that required to have the main beam in the desired direction. We further demonstrate this fact (computationally and experimentally) by showing that if the feed is physically moved to the shifted focal point at some off-center frequency, the main beam pointing direction at this frequency is restored.

Derivation and Validation of the Basic Design Equations for Symmetric Sub-Reflectarrays

The basic design equations for symmetrical sub-reflectarrays are derived. These provide the required phase-distribution on the sub-reflectarray surface. Both ellipsoidal-type and hyperboloidal-type sub-reflectarrays are designed and fabricated using single-layer rectangular patch elements. Measured amplitude and phase patterns are shown for both types of sub-reflectarray, and compared to predictions. The behaviour of the ellipsoidal-type sub-reflectarray is furthermore compared to measured patterns of its solid subreflector counterpart. These experimental results show that the sub-reflectarrays indeed emulate the behaviour of their solid subreflector counterparts, and the validity of the basic design equations is thus established.

Observations on the performance of reflectarrays with reduced inter-element spacings

Several authors have described the use of reduced inter-element spacing as a means of improving the performance of reflectarray antennas. This paper discusses some additional aspects that complement their conclusions, and emphasizes some of the reasons for these improvements.

Feed Image Lobes in Offset-Fed Reflectarrays: Diagnosis and Solution

The actual source of the “feed image lobe” in fixed-beam single-layer offset-fed reflectarrays composed of variable-size patch or dipole elements is diagnosed using a novel technique based on determining the individual contributions of the conduction currents on the reflectarray elements and the groundplane, to the reflectarray scattered fields. The behavior of the feed image lobe is studied over a range of frequencies about the center frequency, and this is used to establish the fact that both the elements and the groundplane are responsible for the presence of the feed image lobe. We show how to reduce or eliminate the feed image lobe at off-center frequencies in offset-fed reflectarrays through use of sub-wavelength reflectarrays instead of conventional reflectarrays with half-wavelength inter-element spacing. Measured and full-wave predicted results are used in all discussions.

Beam Squint Suppression in Offset-Fed Reflectarrays

A novel approach is proposed to suppress the frequency-dependent beam squint of offset-fed reflectarray patterns. The design comprises two offset feeds located at opposite ends of the reflectarray, in the plane of offset, simultaneously illuminating the reflectarray. This generates a single beam that does not squint from the desired design direction at off-center frequencies. The concept is validated by comparing the full-wave simulated patterns to measured counterparts.

Experimental validation of design expressions for sub-reflectarrays

The basic design equations for reflectarrays that emulate ellipsoidal and hyperboloidal sub-reflectors (hereafter referred to as sub-reflectarrays) are presented. Measurements performed on an example of the ellipsoidal type sub-reflectarray are described, showing the far-zone amplitude and phase patterns, in so doing validating the design equations.

Angle of Incidence Effects in Reflectarray Antenna Design: Making gain increases possible by including incidence angle effects.

The subject of whether or not it is necessary to include a feed field incidence angle in the design of main reflectarrays and subreflectarrays is discussed quantitatively in this article. The performance of reflectarray designs done with and without such incidence angle effects included are compared. The additional gain is achieved if the incidence angle is indeed included in the design process. The immunity of subwavelength elements to incidence angle effects is confirmed and illustrated using individual element patterns. Subreflectarrays designed with such subwavelength elements are shown to exhibit superior performance compared to those using half-wavelength ones.

Transmitarray Antenna Design Using Forward and Inverse Neural Network Modeling

Transmitarray work begins with the setting up of a database of transmission coefficients versus element parameters. This database has to be inverted as part of the design process. It is shown that inverse neural networks are well-suited to do this, even when the inverse relationships are multivalued.

Improving sub-reflectarray antennas performance using sub-wavelength inter-element spacing

Several authors have described the use of subwavelength inter-element spacing as a means of improving the performance of reflectarray antennas. This paper examines the use of such a sub-wavelength lattice for improving the performance of sub-reflectarray antennas. This does not yet appear to have been done by others. In addition to studying the scattered field patterns of such sub-reflectarrays on their own, we also inspect their performance when they are used to illuminate a main-reflector, as would be the case in practice.

An accessible two-dimensional moment method analysis for composite antenna models

This paper describes the use of a two-dimensional (2D) method of moments analysis for electromagnetic scattering & radiation from objects comprised of both conducting and penetrable material (that is, composite objects). Such analyses are not available as existing commercial codes. Yet two-dimensional analyses are often able to reveal information about a situation whose 3D analysis is not feasible even by present day computational standards. We highlight a formulation that is accessible in the sense that a very complete version is available in such detail that it is relatively easily implementable, add some clarification of certain quantities, and describe some applications.