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A method for determining the complex permittivity for material specimens with isotropic or biaxially anisotropic dielectric properties is described and representative measured results are presented. The method extracts the material permittivity through the utilization of computational electromagnetic simulations of specimen transmission measurements made with a broadband (50 MHz to over 500 MHz) coaxial airline with a material specimen which partially fills the waveguide cross-section. A number of independent transmission measurements equal to the number of unknown permittivity terms is required for unique material permittivity determination. For isotropic specimens, a single specimen orientation is required, whereas for anisotropic specimens, different specimen orientations in the airline are required to probe the unknown permittivity axes and thus ensure unique material permittivity determination. Measurements using a 3.5-inch outer diameter coaxial airline were made for known isotropic specimens as well as an anisotropic lossy material specimen. This technique provides the material measurement community with a mechanism to characterize a single specimen of complex shape and biaxial dielectric anisotropy at low frequencies covering multiple rectangular waveguide bands simultaneously and represents a significant advance in the current measurement state-of-the-art particularly at frequencies below 100 MHz. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1864–1868, 2015

Metasurfaces enable a new avenue to engineered responses of the electromagnetic scattering characteristics through the control and manipulation of the wavefronts. The wavefronts are tuned by altering deeply sub-wavelength structural features on the metasurface. In this work, a heuristic technique is used where the conductive topology of the metasurfaces for a fixed form factor is optimized by a multi-objective genetic algorithm mimicking evolutionary processes. Analysis is performed for the engineered metasurface responses for frequency-selectivity, absorption, rasorber characteristics, and photonic nanojet generation. Computational electromagnetic simulations for these engineered metasurfaces are presented and discussed. The concepts presented in this work can be applied to design metasurfaces and metamaterials from the microwave to the optical regimes.

In this work, we present a new X-band waveguide (WR90) measurement method that permits the broadband characterization of the complex permittivity for low dielectric loss tangent material specimens with improved accuracy. An electrically-long polypropylene specimen that partially fills the cross-section is inserted into the waveguide and the transmitted scattering parameter (S21) is measured. The extraction method relies on computational electromagnetic simulations, coupled with a genetic algorithm, to match the experimental S21 measurement. The sensitivity of the technique to sample length was explored by simulating specimen lengths from 2.54 to 15.24 cm, in 2.54 cm increments. Analysis of our simulated data predicts the technique will have the sensitivity to measure loss tangent values on the order of 10e-3 for materials such as polymers with relatively low real permittivity values. The ability to accurately characterize low-loss dielectric material specimens of polypropylene is demonstrated experimentally. The method was validated by excellent agreement with a free-space focused-beam system measurement of a polypropylene sheet. This technique provides the material measurement community with the ability to accurately extract material properties of low-loss material specimen over the entire X-band range. This technique could easily be extended to other frequency bands., 6 pages, 7 figures, Review of Scientific Instruments

The optimization of Fresnel zone plates using a genetic algorithm and a the body-of-revolution, finite-difference time-domain simulator is studied. This technique is applied to zone plates for use in a free-space, focused-beam system to measure the electrical properties of materials. The results are compared with a traditionally designed zone plate and a doubly hyperbolic lens. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2223–2227, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24520

A Fresnel zone plate is designed to perform the function of a doubly-hyperbolic lens. The focusing characteristics of the zone plate and the lens are compared using both measured data and body-of-revolution finite-difference time-domain simulations. The results from this comparison are used to evaluate the use of zone plates as focusing elements in a free-space, focused-beam measurement system. A set of two zone plates is used to perform measurements of the material properties of a dielectric sheet, and the results are compared to the results from a standard system which uses two doubly-hyperbolic lenses.

Grooved-dielectric, phase-correcting, Fresnel zone plate antennas are analyzed using the body-of-revolution finite-difference time-domain (BOR-FDTD) method. Parametric studies of the focusing ability of these antennas are performed to examine the effects of the focal length, diameter, number of zones, and the thickness of the lens, as well as the number of phase corrections per zone. The results of these studies are presented as design graphs and are used to lend insight into the focusing mechanism of phase-correcting zone plates. The BOR-FDTD analysis is validated by comparison with previous measurements.

Antennas based on the Soret and folded zone plates are analyzed using the body-of-revolution finite-difference time-domain method. Parametric studies of the focusing performance of these antennas are performed by varying the number of zones, the focal length, and the diameter of the zone plate. The results of the studies provide insight into the factors that influence the performance of these zone plate antennas and are used to generate accurate design graphs. The full analysis is validated by comparison with previously measured data for an integrated-circuit zone plate antenna

This paper describes leading-edge research to explore and demonstrate new and unique nanoscale magnetic composites for high-frequency RF applications. Passivated cobalt nanoparticles were chemically synthesized and dispersed in epoxy to fabricate nanocomposite thick films. The high permeability comes from enhanced coupling between the metal nanoparticles while the insulating polymer matrix prevents eddy current loss and improves stability with frequency. Test vehicles were fabricated to demonstrate integration of these composites in organic substrates and to characterize the high-frequency properties. The frequency-dependent magnetic properties in 100–500 MHz range were extracted by impedance spectroscopy. Magnetic toroids were mechanically pressed with the metal-insulator powder. By refining the processing, permeability of 2.7 was demonstrated at VHF frequencies. The loss tangent was less than 0.04 at these frequencies. The GHz frequency-dependent material characteristics of the magneto-dielectric films were extracted from corner-probing of parallel-plate resonators and strip inductors. By engineering the composite structures at nanoscale, a combination of stable permeability of ∼2 at 1–5 GHz and permittivity of 7, not previously reported, was achieved with polymer composites for antenna miniaturization. The magnetic nanomaterials with low loss, described in this paper, can benefit several other RF and power components, leading to their miniaturization and performance enhancement in emerging RF sub-systems. The metal composite structures also lead to high permittivity in the GHz frequencies which can benefit such RF components as antennas, by allowing closer impedance matching with air.

Fresnel zone plates have been used as replacements for doubly-hyperbolic lenses in free-space, focused-beam systems to measure the electrical properties of materials [1]. These zone plates were designed using the traditional method for determining the profile of a zone plate, which is based on optical, ray-tracing principles [2]. In this work, a non-traditional, optimization-based design methodology featuring the genetic algorithm will be explored to determine if the performance of a zone plate can be improved for the focused-beam application. Results will be presented that compare the optimized design to the traditional zone plate and to a doubly-hyperbolic lens.

In this paper, the body-of-revolution finite-difference time-domain (BOR-FDTD) method is used to study the performance of a Fresnel zone plate in a focusing system. Results from a comparison of a zone plate with a doubly-hyperbolic lens will be presented to illustrate the benefits and limitations of the zone plate.

At optical frequencies, the features of a zone plate are electrically large, so geometrical and scalar theories are adequate for their analysis. At microwave and millimeter-wave frequencies, however, this is often no longer true, and a full electromagnetic analysis is required to obtain accurate results. In this paper, the body-of-revolution finite-difference time-domain (BOR-FDTD) method is used to study the performance of the grooved-dielectric, phase-correcting Fresnel zone plate antenna. Graphs are presented that examine the effect of various parameters on the focusing ability of these antennas, such as the focal length F, the diameter D, the number of full-wave zones N, and the number of subzones per full-wave zone P. This information should be useful to the designers of these antennas.