Derek A. McNamara

Research and Development on Phase-Shifting Surfaces (PSSs)

Phase-shifting surfaces (PSSs) developed in recent years are reported. Research and development on these phase-shifting surfaces are set in the historical context of prior-art free-standing lensing devices, as well as in the context of recent studies performed by other research groups. In addition, initial work on a phase- and amplitude-shifting surface (PASS), an extension of the phase-shifting surface, is demonstrated in a beam-shaping application.

Electronically Frequency-Reconfigurable Rectangular Dielectric Resonator Antennas

The full implementation of a slot-fed frequency-reconfigurable rectangular dielectric resonator antenna (DRA) is described. It uses two conducting walls, on opposite vertical faces of the DRA, which are switched via conducting tabs to be in contact or not with the groundplane. Theoretical and experimental performance results for such DRAs, using either PIN or varactor diode switches, are described, where prototypes were designed to operate between 3-8 GHz. The PIN diode-loaded and varactor-loaded DRAs achieved a tuning range of 91% and 55%, respectively, with other performance metrics detailed in the communication.

Antenna Shape Synthesis Without Prior Specification of the Feedpoint Locations

An antenna shape synthesis method is proposed that allows shaping of the antenna geometry prior to specification of the feed location and type. This reduces the constraints placed on the optimization process and can lead to potentially new designs due to the increased degree of freedom afforded. An appropriate feedpoint is easily chosen after shape optimization by selecting a location on the resulting structure for best impedance matching. The procedure is made possible through the use of characteristic mode concepts. Examples show that the antenna-Q values of the resulting shaped radiators closely approach the fundamental bounds.

Printed Hybrid Lens Antenna

A novel printed hybrid lens antenna made of a 2-layer phase shifting surface (PSS) has been developed. It is a hybrid lens because the required operation is achieved by a combination of true phase correction (similar to a conventional lens, and previously reported PSS phase-correcting lenses) and amplitude-only correction (similar to a Fresnel zone plate). A prototype is fabricated and measured, revealing that the hybrid lens antenna has roughly twice the aperture efficiency of a Fresnel zone plate, and half that of a conventional lens. It provides an attractive compromise between performance, fabrication complexity and cost.

Periodic plasmonic nanoantennas in a piecewise homogeneous background

Periodic rectangular gold nanomonopoles and nanodipoles in a piecewise inhomogeneous background, consisting of a silicon substrate and a dielectric (aqueous) cover, have been investigated extensively via 3D finite-difference time-domain simulations. The transmittance, reflectance and absorptance response of the nanoantennas were studied as a function of their geometry (length, width, thickness, gap) and found to vary very strongly. The nanoantennas were found to resonate in a single surface plasmon mode supported by the corresponding rectangular cross-section nanowire waveguide, identified as the sa(b)(0) mode [Phys. Rev. B 63, 125417 (2001)]. We determine the propagation characteristics of this mode as a function of nanowire cross-section and wavelength, and we relate the modal results to the performance of the nanoantennas. An approximate expression resting on modal results is proposed for the resonant length of nanomonopoles, and a simple equivalent circuit, also resting on modal results, but involving transmission lines and a capacitor (modelling the gap) is proposed to determine the resonant wavelength of nanodipoles. The expression and the circuit yield results that are in good agreement with the full computations, and thus will prove useful in the design of nanoantennas.

Thin phase-correcting lens antennas made using a three-layer phase-shifting surface (PSS) at Ka band

This paper presents the design, fabrication, and measurement of three phase-correcting lens antennas made using an electrically thin three-layer phase-shifting surface (PSS). The three phase-correction schemes realised using the PSS are the 90 degree, 45 degree and quasi-continuous cases. These lenses, which present identical cost, size and weight, are compared in terms of their far-field performance. It is shown that decreasing the size of the phase steps used in the phase-correction scheme improves the performance of the PSS lenses. As a reference to conventional designs, the performance of a dielectric plano-hyperbolic lens antenna is also presented. The performance of the PSS lenses are as good as, or better, than this conventional dielectric lens antenna within a specific operating band, over which they also offer a significant reduction in weight, thickness and cost.

Observations on computational outcomes for the characteristic modes of dielectric objects

Some computational aspects for the characteristic modes (CM) of dielectric objects are examined. Volume integral equation (VIE) formulations are reliable but computationally burdensome. The use of surface integral equation (SIE) formulations results in a lighter computational load but, we show here, obtrudes certain non-physical modes in addition to the physical ones. We show that the non-physical ones are easily identified using a radiated power check, and can be discarded, so allowing use of the SIE for finding the true CMs of dielectric objects. The SIE can be used with fine meshing with relatively modest computational resources; this is of practical importance.

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.

Comparison between conventional lenses and an electrically thin lens made using a phase shifting surface (PSS) at Ka Band

This paper compares the performance of a thin lens based on a new phase shifting surface (PSS) concept to three conventional lenses at Ka-Band. The pros and cons of each lens antenna are highlighted and verified by experimental results. It is shown that a 90° phase-correcting PSS lens antenna can achieve almost the same gain as the corresponding dielectric plano-hyperbolic lens antenna within a specific operating band along with significant improvements in weight, thickness and cost.

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.