M. G. Bray

A broadband open-sleeve dipole antenna mounted above a tunable EBG AMC ground plane

A broadband open-sleeve dipole antenna is mounted above a tunable electromagnetic bandgap (EBG) surface resulting in a low-profile narrowband antenna system that is tunable over an octave bandwidth. The EBG surface is composed of an array of square metallic patches connected via tunable capacitors over a thin PEC backed dielectric substrate. The capacitors tune the resonance of the surface down in frequency resulting in a low-profile antenna system that can be tuned from 1.2 GHz to 2.3 GHz with a relative thickness of only /spl lambda//30 to /spl lambda//15 at the lower and upper frequency limits respectively.

Low Cost and Broadband Dual-Polarization Metamaterial Lens for Directivity Enhancement

Metamaterials have been used in many different configurations to enhance the radiation properties of antennas. However, the vast majority of these metamaterial applications only consider linearly polarized antennas. This paper discusses the theory, design, implementation, and measurements of a far-field collimating lens for use with a circularly-polarized crossed-dipole antenna constructed from a 3D-volumetric metamaterial slab. Zero-index materials (ZIM) and low-index materials (LIM) cause the magnitude and phase of the radiated field across the face of the lens to be distributed uniformly, increasing the broadside gain over the feed antenna alone. Full-wave simulations were used in design of the lens, and a prototype metamaterial lens (meta-lens) was constructed and measured to verify the theoretical predictions. The meta-lens was found to increase the measured directivity of a crossed-dipole feed antenna by more than 6 dB, in good agreement with numerical simulations.

Thinned aperiodic linear phased array optimization for reduced grating lobes during scanning with input impedance bounds

The scan volume of a thinned periodic linear phased array is proportional to the spacing between array elements. As the spacing between elements increases beyond a half wavelength, the scan range of the array will be significantly reduced due to the appearance of grating lobes. This paper investigates a method of creating thinned aperiodic, linear phased arrays through genetic algorithms that suppress the grating lobes with increased steering angles. In addition the genetic algorithm places restrictions on the driving point impedance of each element so that they are well behaved during scanning.

A novel design approach for an independently tunable dual-band EBG AMC surface

This work investigates a novel design approach for creating an independently tunable dual-band electromagnetic bandgap (EBG) surface for a use as an artificial magnetic conductor (AMC). The EBG geometry consists of two reactively loaded concentric square loops. The outer loop is used to tune for the first resonance, while the inner loop is used to tune the second resonance. The resonance frequency of each loop is independent of the other and can be tuned down in frequency with capacitors and up in frequency with inductors.

Fast simulation of lumped-element loaded AMC antenna systems using embedded element theory

In this paper we present a new efficient method for the simulation of low-profile antenna systems with Artificial Magnetic Conducting (AMC) ground planes that incorporate tunable lumped-elements. This technique works by first extracting an impedance matrix consisting of lumped elements and feed elements from a full-wave method of moments simulation. The embedded element theory is then employed to estimate the feed impedance of the AMC/antenna system with any set of arbitrary loading.

A simple dispersive chiral FDTD formulation implemented on a Yee grid

A new simple dispersive chiral FDTD formulation based on the traditional Yee grid is introduced. This methodology uses an explicit form of the dispersive chiral constitutive equations in conjunction with the Z-transform technique to incorporate dispersion. The main advantage of this technique is its simplicity and straightforward incorporation with traditional FDTD methods. This method was also extended to an implicit method introduced by A. Grande et al. (see IEEE Trans. on Microwave Theory and Tech., vol.52, no.2, p.773-84, 2004) that used two transverse FDTD grids to align the E-fields and H-fields. These methodologies were used to simulate the scattering from a chiral slab and both compared favorably with analytic results. The advantage of the explicit method is that, in a full 3D formulation, it only requires a single traditional Yee grid.

Anisotropic metamaterial realization of a flat gain-enhancing lens for antenna applications

A metamaterial design is presented that implements a far-field collimating lens for microwave antenna gain enhancement applications. Using a combination of dual-split ring resonators and end-loaded dipole meta-molecules, the bulk properties are shown to be a good approximation of a low-index uniaxial material with a z-oriented optical axis for both permittivity and permeability between 8 and 9 GHz. Full-wave simulations demonstrate that a large-area lens comprised of the designed unit cell and fed by a crossed-dipole successfully acts as a collimating lens with 6dB gain improvement over the same antenna without the meta-lens.

Matching network design using genetic algorithms for impedance constrained thinned arrays

This paper investigates a technique for optimizing matching networks for thinned aperiodic dipole arrays to achieve a 2:1 or better VSWR for each antenna element over the entire scan range of the array. The thinned array is optimized via a genetic algorithm to have a suppressed grating lobe over the scan range of the array. In addition, the genetic algorithm places restrictions on the driving point impedance of each element so that they are well behaved during scanning. The impedance constraint allows a three element reactive matching network to be optimized for each element of the array using a separate genetic algorithm.

Reconfigurable dipole chaff elements for passive standoff detection of chemical agents

Reconfigurable dipole chaff elements for the remote passive detection of chemical agents are presented in this paper. Each chaff element consists of a metallic reference dipole, and a detection dipole with a switch that changes conductivity in the presence of a chemical analyte. In the absence of an analyte, both dipoles are resonant at the same frequency; however, when the analyte is present, the resonance frequency of the detection dipole shifts. This shift in frequency is remotely detected via a RADAR system.

GA optimized reconfigurable frequency selective surfaces for passive standoff detection of chemical agents

In this paper we present a new class of reconfigurable frequency selective surfaces (FSS) for the remote passive detection of chemical agents. The frequency selective surface is composed of a periodic tiled pixelized pattern on a thin dielectric substrate. The pixelized pattern consists of squares of either metallic patches or chemoresistive patches. In the absence of an analyte, the chemoresistive patches have a high impedance and in the presence of an analyte they have a low impedance. The FSS is designed to provide a sharp narrowband reflection resonance that shifts down in frequency with exposure to the analyte. The geometry of the FSS is selected via a Genetic Algorithm (GA) [1].