Brandon L. Good

Broadband Antireflective Properties of Inverse Motheye Surfaces

A new method for synthesizing broadband antireflective (AR) surfaces at microwave and millimeter wave frequencies is demonstrated. The AR surface, we call an inverse motheye, was formed by machining a multi-layer grating of subwavelength circular holes into a non-absorptive dielectric. This created low reflected energies (<; - 30 dB) over reasonably large bandwidths and incidence angles. An optimization algorithm, based on a direct pattern search, integrated with a rigorous electromagnetic model was used to design the grating geometries. Experimental results are provided at Ka-band demonstrating the validity of the method.

94-GHz Imager With Extended Depth of Field

We describe a computational imaging technique to extend the depth-of-field of a 94-GHz imaging system. The technique uses a cubic phase element in the pupil plane of the system to render system operation relatively insensitive to object distance. However, the cubic phase element also introduces aberrations but, since these are fixed and known, we remove them using post-detection signal processing. We present experimental results that validate system performance and indicate a greater than four-fold increase in depth-of-field from 17rdquo to greater than 68rdquo.

Design and fabrication of microwave flat lenses using a novel dry powder dot deposition system

We describe a new methodology for creating flat lenses operating in the microwave spectrum using a custom designed additive manufacturing system. This method utilizes a novel dry powder 3D printing system to achieve graded index lenses integrated within a structural composite. We also describe a new iterative dot patterning algorithm to achieve a desired graded dielectric distribution, and we compare the iterative dot patterning algorithm to other dot patterning techniques. Computational and experimental results are provided validating the design and fabrication process.

Rapid prototyping of frequency selective surfaces by laser direct write

In this work we describe the use of laser direct-write for the rapid prototyping of frequency selective surfaces. Frequency selective surfaces are generally described by a periodic array of conducting or dielectric features (i.e. crosses, loops, grids, etc.) that when properly designed can pass or reject specific frequency bands of incoming electromagnetic radiation. While simple frequency selective surfaces are relatively straight forward to design and fabricate, operational demands, particularly military, have motivated the design and fabrication of much more complicated patterns. These new designs combine features of significantly different length scales, randomly dithered patterns and combinations of passive and active elements. We will demonstrate how laser direct-write is an ideal tool for the rapid prototyping of these new more complicated frequency selective surface designs. We will present experimental results for devices fabricated using several different laser direct-write processes.

Design of Antireflection Grading Using Magneto-Dielectric Materials

A number of practical application areas would benefit from the development of thin wideband antireflective (AR) surfaces. Here, we incorporate magnetic materials into AR gradings using two approaches. 1) We use the analytic approach demonstrated in a recent paper that details an optimal wideband AR design using a continuous one-dimensional (1-D) grading of dispersive dielectrics [1]. We extend the dispersive nonmagnetic equations in [1] to design perfect AR magneto-dielectric gradings. 2) We derive a spatial-coordinate transformation (SCT) approach that transforms the nonmagnetics solution to achieve a perfect AR magneto-dielectric grading. We find, in general, that the SCT method enables more realizable and flexible solutions. We detail a practical approach for realizing the magneto-dielectric gradings using subwavelength texturing (i.e., motheye method). Several numerical examples demonstrate the utility of this approach in realizing very thin, yet broadband, and AR designs.

Fabrication of flat Luneburg lens using functional additive manufacturing

Summary form only given. The Luneburg Lens offers an exciting and elegant solution to beam scanning and multiple beam forming that can be used to implement low resolution radar systems and high directivity wireless communications. Traditionally, these lenses have been created by aligning discrete layers of materials with varying effective permittivity to emulate the Luneburg dielectric distribution given by: er = es(2 - (r/R)2). To realize this graded permittivity distribution experimentally can be quite challenging. Traditional methods include subtractive manufacturing, which utilizes a computer numerically controlled (CNC) drill or milling machine to produce an array of voids within a homogenous dielectric material (Sato K. and Hiroshi U., Electronics and Communications in Japan, 85, 1-12). Spatially varying the local volume fraction of the voids results in an effective permittivity distribution. While subtractive methods achieve reasonably good electromagnetic performance, the resulting part is often too fragile for many practical applications. Other designs have employed metamaterials and transformational electromagnetics to reproduce the properties of the Luneburg lens. This approach, while quite interesting, often results in a narrow frequency band of operation. In this presentation we will describe an alternative fabrication methodology based on functional additive manufacturing. Commercial additive manufacturing systems or 3D printers create three dimensional (3D) structures by patterning consecutive layers of base material. Unfortunately, these current additive manufacturing techniques are limited in their base materials and not well suited for creating electromagnetically functionalized structures with graded EM properties. Recently our group has built a custom printer for realizing rigid substrates with embedded three dimensionally varying graded dielectrics (Roper D. et. al, Smart Materials and Structures, accepted for publication). Our system employs an ultrasonic powder deposition system (S. Yang and J. Evans, Powder Technology, 129, 55-60, 2004) designed to pattern high dielectric powders onto a low loss dielectric substrate. The powder dispenser acts like an ink jet printer head for dry powders. The powders - which could be high dielectric powders, magnetic powders, conductive powders or polymer powders - are injected under computer control onto a structurally reinforced substrate. To produce a 3D distribution of dielectric properties, multiple layers are patterned, aligned, stacked and post-processed (i.e. cured in an autoclave). The final sample is a solid structure with integrated 3D graded dielectric properties. With this system, we designed, fabricated and experimentally characterized a Luneburg Lens within a structurally rigid composite. In this presentation we will provide the design and characterization of our graded dielectric printer as well as the construction and analysis of the Luneburg Lens.

Effective properties of cylinders in a square lattice

Periodic subwavelength structures on a boundary can enhance the transmission through the boundary. While rigorous methods are accurate for designing these structures, recent methods could benefit from an improved effective medium approximation. In this work, a modified Rayleigh mixture fomula is used to approximate the effective properties of periodic cylinders in a square lattice for transverse electromagnetic plane waves. The modified Rayleigh mixture formula outperforms the Maxwell Garnett formula where the height is greater than the diameter. A comparison is shown for completeness.

Luneburg lens created from distributed water cylinders

In this work, a Luneburg lens operating at 15 GHz is created from water cylinders. The approach uses effective media of water and an iterative dielectric distribution method to achieve the Luneburg lens electromagnetic profile. From the approach, the focusing of energy using the proposed Luneburg lens is shown numerically. While the approach is successfully creates a luneburg lens, the losses due to the water significant.

Effective media theory of dry powder dot printing

Fiberglass composites with three dimensionally varying dielectric properties have been created using a novel dry powder dot deposition system. The out of plane and in plane effective properties were previously determined empirically with separate methods. This work establishes an analytic effective media approach to characterizing the dielectric constant for both cases.

Validation of C-Band Permittivity of Fresh Water using Periodic Jets

This letter presents the results of a free space experiment that validated the Meissner and Wentz model of fresh water permittivity at 13°C in C-Band. The experiment consisted of creating a periodic water jet array inside a focused beam measurement system. The focused beam system measures the radio frequency (RF) response to a finite array of free flowing water jets. The results of the experiment are compared to a one-dimensional Rigorous Coupled-Wave Analysis (RCWA) prediction for infinitely periodic cylinders of water. The consistency between the measured and predicted values appears to validate the permittivity of fresh water at 13°C in C-Band as predicted by the Meissner and Wentz model. This method for measuring and predicting the one-dimensional RCWA solver could be used to characterize properties of other liquids.