Clinton P. Scarborough

An octave-bandwidth negligible-loss radiofrequency metamaterial

Metamaterials provide an unprecedented ability to manipulate electromagnetic waves and are an enabling technology for new devices ranging from flat lenses that focus light beyond the diffraction limit to coatings capable of cloaking an object. Nevertheless, narrow bandwidths and high intrinsic losses arising from the resonant properties of metamaterials have raised doubts about their usefulness. New design approaches seek to turn the perceived disadvantages of dispersion into assets that enhance a devices performance. Here we employ dispersion engineering of metamaterial properties to enable specific device performance over usable bandwidths. In particular, we design metamaterials that considerably improve conventional horn antennas over greater than an octave bandwidth with negligible loss and advance the state of the art in the process. Fabrication and measurement of a metahorn confirm its broadband, low-loss performance. This example illustrates the power of clever implementation combined with dispersion engineering to bring metamaterials into their full potential for revolutionizing practical devices.

Experimental demonstration of an isotropic metamaterial super lens with negative unity permeability at 8.5 MHz

We report a low frequency magnetic metamaterial super lens operating at 8.5 MHz. The super lens, composed of a slab of metamaterial with an isotropic effective permeability of negative unity, is capable of resolving two closely spaced magnetic sources that are indistinguishable in the absence of the lens, while simultaneously increasing the magnetic field strength observed in the image plane. Measurements of a prototype demonstrated the increased field strength and a resolution of 1/441 wavelengths, representing a new state-of-the-art in metamaterial-assisted near-field imaging.

Design Synthesis of Metasurfaces for Broadband Hybrid-Mode Horn Antennas With Enhanced Radiation Pattern and Polarization Characteristics

Metamaterial surfaces (metasurfaces) with a low effective index of refraction have been recently proposed for application in the design of hybrid-mode horn antennas, such as soft and hard horns. Here we explore designs of several metasurfaces and their use as liners for coating the interior walls of horn antennas. The design process combines the genetic algorithm optimization technique with a full-wave electromagnetic solver to create dispersion-engineered metamaterials that possess customized surface impedance properties. A metamaterial parameter extraction technique is developed and employed in the optimization process, which is based on the surface impedance expressions for a homogeneous slab backed by a perfectly conducting ground plane illuminated at near grazing incidence. The optimized metasurface is found to be equivalent to a low index metamaterial with a dispersion that can improve the performance of conventional horn antennas over the entire Ku -band while introducing negligible losses. We conclude with a numerical study of a conical horn antenna whose interior is lined with a low index metasurface. The far-field radiation patterns and aperture field distributions confirm hybrid-mode operation over a wide bandwidth, validating the proposed metasurface design methodology.

A

Metamaterials with properly engineered surface properties have been recently proposed for application in the design of broadband hybrid-mode horn antennas, such as soft and hard horns. In this paper, we present the design, fabrication, and measured results of a square dual-polarization horn antenna with thin metasurfaces lining the four walls, demonstrating broadband, negligible-loss hybrid-mode operation. By employing a powerful genetic-algorithm (GA) design optimization technique, we have dispersion-engineered low-index metaliners whose surface impedances satisfy the balanced hybrid condition across the Ku-band. The optimized metaliners were synthesized based on conventional printed-circuit board technology, leading to a lightweight and low-cost construction. To improve the cross-polarization response, a simple dielectric plug was placed in the throat of the horn to perform effective mode conversion. Measurements showed that the fabricated horn antenna prototype provided low sidelobes, low cross-polarization levels, and radiation patterns that are approximately independent of polarization. Excellent agreement was found between measured and simulated results across the entire band of operation. Both the far-field radiation patterns and the aperture field distributions confirm the hybrid-mode operation of the horn, validating the balanced metasurface design. This metamaterial-enabled antenna represents a low-cost alternative to other types of soft feed horns, such as corrugated horns.

K_{u}

Metamaterial and metasurface structures, which satisfy the balanced hybrid conditions, have been recently proposed for designing hybrid-mode soft and hard horn antennas. In this paper, we present the design of broadband soft metasurfaces and their applications in hybrid-mode conical horns. These designs exhibit surface characteristics that can be controlled by adjusting the constituent structural elements. The analysis and design of metasurfaces were performed on both a planar platform and in a cylindrical waveguide structure. Such dispersion-engineered metasurfaces with customized surface properties can support the desired hybrid modes in a cylindrical waveguide, facilitating their usage in horn antennas. By engineering the metasurfaces with regard to their surface impedance as well as tailoring their spatial distributions along the wall of the horn, we demonstrate an inhomogeneous metahorn with superior performance and broader bandwidth compared to those with uniform liners. The symmetric far-field radiation patterns with low sidelobe and cross-polarization levels, together with the corresponding aperture field distributions, signifies the hybrid-mode operation of the horn, validating the efficacy and broadband performance of the metasurfaces. Such metasurface-lined horns can be employed as soft feeds in dual polarization antenna systems.

-Band Dual Polarization Hybrid-Mode Horn Antenna Enabled by Printed-Circuit-Board Metasurfaces

This paper reports on the detailed design and experimental demonstration of a metamaterial-enabled low-sidelobe horn antenna (metahorn) based on principles similar to those of earlier soft horn antennas. The target application is a linearly polarized feed horn in the super-extended C-band (3.4-6.725 GHz) for communication satellite reflector antennas. The paper describes the detailed design and manufacturing of the -plane metamaterial liner (metaliner) based on a freestanding wire grid without the need for a dielectric substrate material. The measured copolarized and cross-polarized antenna patterns from the feed horn demonstrate over an octave pattern bandwidth with negligible loss. The results show similar bandwidth with lower sidelobes and backlobes than those of the trifurcated horn that is currently used as the standard C-band feed for single linear polarization. This demonstration shows promise for lightweight metamaterial horns to replace heavy and expensive C-band corrugated horns.

Inhomogeneous Metasurfaces With Engineered Dispersion for Broadband Hybrid-Mode Horn Antennas

While many studies have been conducted on metamaterials at microwave frequencies, comparatively few have examined their use in high-power applications. Here, we perform a general study of metamaterial geometries to identify configurations that are well-suited for utilization in high-power environments. We further develop a genetic algorithm optimization scheme for synthesizing pixelized geometries with artificial magnetic conducting (AMC) properties and reduced maximum field enhancement factor (MFEF).

Demonstration of an Octave-Bandwidth Negligible-Loss Metamaterial Horn Antenna for Satellite Applications

The design and characterization of a compact, low-profile metasurface-enabled antenna with tunable frequency and polarization characteristics is reported. This antenna is made possible by a tunable artificial magnetic conducting (AMC) substrate combined with tunable crossed end-loaded dipoles. Intertwined miniaturized periodic unit cells comprise the AMC, presenting a homogeneous interface to the dipoles above. Moreover, the AMC structure is predominantly air, making it very lightweight. Tunable capacitors on the dipoles allow the dynamic reconfiguration of their input impedances, which in turn allows for near-arbitrary dynamic adjusting of the antennas polarization for the left- or right-hand circular as well as linear with various orientations. Measurements of a prototype showed comparable performance with commercially available alternatives that were more than three times as thick as the new design reported here.

High-power considerations in metamaterial antennas

Hybrid-mode (HM) horn antennas with internal metamaterial wall liners, here denoted meta-horns and meta-liners, respectively, were first proposed in [1]. A soft meta-horn comprises a low index meta-liner with a given permittivity and thickness (see Figure 1b) that supports a balanced hybrid mode at the design frequency. These soft horns offer considerably larger bandwidth than the widely used corrugated horn due to evanescent propagation of the EM fields normal to the meta-liner. A hard meta-horn has a dielectric liner on top of a low index meta-liner as shown in Figure 2b. These horns also offer considerably larger bandwidth and comparable aperture efficiency when compared with other hard horns, which also is due to evanescent EM propagation normal to the meta-liner. Modeling and analysis of these meta-horns based on a cylindrical model as well as full-wave MoM analysis (WIPL-D) were reported in [1–4]. When compared with dielcore horns (Figures 1a and 2a) [4] both the soft and hard meta-horns exhibit comparable bandwidth performance without the disadvantages of insertion loss in the dielectric core, dielectric-air mismatch and additional weight. Full-wave analysis of both horns have shown that the desired frequency dispersion matches the Drude model, indicating that these low index meta-liners can be implemented with large bandwidth.

Compact Low-Profile Tunable Metasurface-Enabled Antenna With Near-Arbitrary Polarization

Metasurfaces and metamaterials have been explored extensively in recent years for their ability to enable a variety of innovative microwave devices. However, because their exotic properties often arise from resonant structures, the large field enhancements under high-power microwave illumination can lead to dielectric breakdown and damage to the device. In order to develop metasurfaces and metamaterials capable of being utilized in high-power microwave applications, this paper investigates techniques for reducing the maximum field enhancement factor (MFEF) in several types of structures from the literature. Starting with a simple Sievenpiper metasurface, this paper evaluates the dependence of MFEF on the structure design parameters. For more complex metasurface geometries, a genetic algorithm is demonstrated that can evolve structures that have minimal MFEF. In addition, negative-index and low-index metamaterials are evaluated for field enhancement. By optimizing for low loss and by operating in the resonance tails, metamaterials with low MFEF can be realized for high-power applications. To illustrate this, a quad-beam focusing metamaterial lens is presented with an MFEF less than 5 over the entire operating band.