Douglas J. Kern

The design synthesis of multiband artificial magnetic conductors using high impedance frequency selective surfaces

This paper introduces several different design methodologies for multiband artificial magnetic conducting (AMC) surfaces. The paper begins by investigating the multiband properties exhibited by a conventional electromagnetic bandgap (EBG) AMC that consists of a frequency selective surface (FSS) on top of a thin dielectric substrate with a PEC back plane. The higher-order resonances associated with these surfaces have not been discussed in detail to date, as previous research has been concerned only with exploiting the primary resonant frequency. However, it will be shown that by understanding and making appropriate use of these higher order resonances, it is possible to design multiband AMC surfaces that work for nearly any desired combination of operating frequencies. The first multiband AMC design approach that will be considered is based on the introduction of FSS screens that have fractal or nearly fractal unit cell geometries. This is followed by a more general and robust genetic algorithm (GA) technique for the synthesis of optimal multiband AMC surfaces. In this case, a GA is used to evolve multiband AMC surface designs by simultaneously optimizing the geometry and size of the FSS unit cell as well as the thickness and dielectric constant of the substrate material. Finally, several examples of multiband AMC surfaces are presented, including some practical dual-band and tri-band designs genetically evolved for operation at GPS and cellular frequencies, as well as an example illustrating the success in creating a multiband AMC surface with angular stability.

The synthesis of metamaterial ferrites for RF applications using electromagnetic bandgap structures

It has been well known for many years that the desirable properties of conventional ferrite materials are seriously degraded for frequencies above 1 GHz. The paper demonstrates that electromagnetic bandgap (EBG) structures may be interpreted as an equivalent PEC-backed slab of magnetic material with a frequency dependent permeability. A design methodology is presented for realizing a metaferrite material using a GA optimization procedure. A standard HZ-FSS (high-impedance frequency selective surface) structure is shown to be equivalent to a thin PEC-backed slab of magnetic material with a frequency dependent permeability. By optimizing the surface impedance of the HZ-FSS, it is possible to synthesize a metaferrite with nearly any desired real and imaginary values of permeability. Finally, this design procedure allows for a low-loss negative permeability metaferrite to be realized, with potential application to the design of left-handed or double negative media.

Metaferrites: using electromagnetic bandgap structures to synthesize metamaterial ferrites

A methodology is presented for the design synthesis of metamaterial ferrites, or metaferrites, that retain their desirable magnetic properties at frequencies above 1 GHz. The design synthesis is accomplished by optimizing a high impedance frequency selective surface (HZ-FSS) structure via a genetic algorithm (GA) for the desired effective permeability of an equivalent magnetic substrate backed by a perfect electric conductor ground plane. The ability to optimize the design parameters of these HZ-FSS structures allows for the possibility of synthesizing low-loss dispersive metaferrites with either a positive or a negative real part of the effective permeability at the desired operating frequency band. The results presented in this paper demonstrate five possible metaferrite designs: two with the associated real and imaginary permeabilities for use as low-loss magnetic materials, and three designs for use as absorbing materials.

Active negative impedance loaded EBG structures for the realization of ultra-wideband Artificial Magnetic Conductors

A new design methodology is introduced for an ultra-wideband Artificial Magnetic Conductor (AMC) that is based on loading the elements of Electromagnetic Bandgap (EBG) structures with active devices. The types of active loads used for this application belong to the class of devices known as Negative Impedance Converters (NICs). NICs are active two-port networks for which the impedance looking into one port is the negation of the impedance connected to the other port scaled by the impedance conversion coefficient of the device. Several design examples will be presented that demonstrate the considerable enhancement in bandwidth that can be achieved, compared to conventional passive AMC surfaces, by using EBG structures actively loaded with NICs.

Novel BI-FDTD approach for the analysis of chiral cylinders and spheres

A versatile time-domain technique, known as bi-isotropic finite difference time domain (BI-FDTD), has recently been introduced for the numerical analysis of electromagnetic wave interactions with complex bi-isotropic media. However, to date only one-dimensional BI-FDTD schemes have been successfully implemented. This paper presents novel two-dimensional (2-D) and three-dimensional (3-D) dispersive BI-FDTD formulations for the first time. The update equations for these new 2-D and 3-D BI-FDTD approaches are developed and applied to the analysis of electromagnetic wave scattering by chiral cylinders and spheres in free space. The distinctive feature of this technique is the use of two independent sets of wavefields representing the left- and right-polarized waves in the chiral medium. This wavefield decomposition approach allows dispersive models for the chirality parameter as well as the permittivity and permeability of the medium to be readily incorporated into an FDTD scheme. The 2-D and 3-D BI-FDTD simulation results are compared with available analytical solutions for the scattering from a circular chiral cylinder and a chiral sphere respectively

Ultra-thin electromagnetic bandgap absorbers synthesized via genetic algorithms

A design methodology is presented for utilizing electromagnetic bandgap meta-materials, also known as artificial magnetic conductors, to realize ultra-thin absorbers. One approach that has recently been proposed is to place a resistive sheet in close proximity to a frequency selective surface acting as an artificial magnetic conductor. However, we demonstrate that incorporating the loss directly into the frequency selective surface can eliminate the additional resistive sheet, thereby further reducing the overall thickness of the absorber. A genetic algorithm is used to optimize the geometrical structure and corresponding resistance of the lossy frequency selective surface in order to achieve the thinnest possible design. Two examples of genetically engineered electromagnetic bandgap meta-material absorbers are presented and discussed.

A robust GA-FSS technique for the synthesis of optimal multiband AMCs with angular stability

A Genetic Algorithm (GA) design methodology is presented for synthesizing multiband Artificial Magnetic Conductors (AMCs) with angular stability. The GA is used to optimize the Frequency Selective Surface (FSS) screen geometry and other design parameters for both multiband operation and stability with respect to the angle of illumination at each operating frequency. The design example presented demonstrates the multiband angular stability for a dual-band AMC at a GPS frequency of 1.575 GHz and a cell phone frequency of 1.96 GHz.

Design of reconfigurable electromagnetic bandgap surfaces as artificial magnetic conducting ground planes and absorbers

The research presented in this paper adapts the reconfigurable FSS technology to EBG AMC surface design, which utilizes a periodic array of fixed metallic cross-dipole elements connected by ideal switches. This structure can be reconfigured to achieve the AMC condition for various resonant frequencies. The introduction of loss into the FSS screen also allows the design of reconfigurable EBG absorbers

Optimization of multi-band AMC surfaces with magnetic loading

This work presents single-bband and multi-band artificia magnetic conducting (AMC) surface designs that are shown to have significant bandwidth enhancement achieved by loading the substrate with a magnetic material. In both cases, a conventional AMC structure consisting of a high impedance frequency selective surface (HZ-FSS) is optimized using a genetic algorithm (GA) for maximum bandwidth. The single-band design operates at 2 GHz, while the multiband design has targeted resonant frequencies of 860 MHz, 1.575 GHz, and 1.88 GHz. These design examples serve to demonstrate that the inclusion of a modest amount of magnetic material within the substrate allows for improved bandwidth at all resonant frequencies.

Matched impedance thin composite magneto-dielectric metasurfaces

The advantages and disadvantages of dielectric loading applied to electromagnetic devices such as antennas using high permittivity materials is well known. Sometimes overlooked, however, is the same effect using a material with magnetic properties. This is mainly due to the fact that most natural magnetic materials exhibit large losses that make them virtually unusable at high frequencies. If materials that exhibit magnetic and dielectric properties with reasonable losses were available then more advanced RF devices and antenna systems could be created. For instance, the use of these materials in conjunction with antennas would facilitate the development of designs with much smaller physical footprints than are typically possible, with few performance compromises [1]. Recently, composite magneto-dielectric substitutes, called metaferrites [2], have been engineered as a possible way to address this need for magnetic materials that are usable beyond 1 GHz. In [2], Kern et al. demonstrated that the properties of a PEC backed slab of magnetic material with frequency dependent permeability could effectively be achieved using a high impedance electromagnetic bandgap (EBG) structure. It was also shown that the real and imaginary parts of the effective permeability of an equivalent magnetic material slab could be related to the values of the surface impedance for the EBG structure. In this paper a new design technique for creating matched magnetodielectric metamaterial slabs is introduced. The technique is based on using a genetic algorithm (GA) to optimize [3,4] thin metallo-dielectric metasurfaces comprised of a periodic array of electrically small unit cells and backed by a perfectly conducting ground plane. Examples will be presented to demonstrate the effectiveness of this technique.