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Finite Element Analysis (FEA) is used to determine the sensitivity of feed placement on a Luneburg lens (LL) having large scale cubic discretization of its permittivity distribution. This is of practical importance for lenses fabricated using additive manufacturing, allowing accurate prediction of performance, and potentially reducing overall print time. It is shown that the far-field relative side lobe level (RSLL) is most sensitive to this form of discretization, and the impact to multi-feed and single-feed applications is considered. It is shown that for single-feed applications, large cubic macro cells are beneficial and provide a RSLL above that achieved with the continuous and non-uniform shelled counterparts.

This research applies Effective Medium Theory and 3D Finite Element Analysis to model the transmissive loss through a waveguide fed additively manufactured Luneburg lens. New results are presented that provide rational function approximations for accurately modeling the aperture, beam, and radiation loss factors of the antenna. It introduces a normalized loss tangent and shows that the loss factors are dependent on the product of this parameter and the lens radius. Applying the constraint that the main beam of the radiation pattern contains 50% of accepted power, a maximum useful radius is tabulated for common polymers used in additive manufacturing.

Carbon–carbon composites (CCCs) are a unique form of carbon fiber-reinforced materials that exhibit excellent thermomechanical properties under extreme environmental conditions. Due to the need for the retention of mechanical properties at temperatures exceeding 2000 °C, CCCs have been utilized in heat shields, rocket nozzles, aircraft brakes, and leading edge material in hypersonic vehicles. In order to expand the applicability of CCCs, the fabrication process must be modified such that there is a reduction in cost or processing time. It is hypothesized that maximizing the permeability of the composite, during processing, will grant the largest reduction in the fabrication time as it leads to a larger volume of pores filled during re-densification. This review attempts to capture the various parameters that have led to increased permeability, as well as outlining process modifications that have demonstrated influence over the carbonized microstructure. In addition, this review seeks to differentiate itself by systematically outlining research advances that have been made in each step of the fabrication process. In doing so, scientific gaps that exist can be expounded upon while simultaneously summarizing what is necessary to advance the field.

The gradient-index (GRIN) Luneburg lens antenna offers significant benefits, e.g. high aperture efficiency, low-power, minimal cost, wide beam scanning angle and broad bandwidth, over phased array antennas and reflector antennas. However, the spherical shape of the Luneburg lens geometry complicates the integration of standard planar feed sources and poses significant implementation challenge. To eliminate the feed mismatch problem, the quasi-conformal transformation optics (QCTO) method can be adopted to modify the lens’ spherical feed surface into a planar one. However, Luneburg lenses designed with QCTO method are limited to poor performance due to the presence of the reflections and beam broadening arising from the quasi-conformal mapping. In this paper, we present a new method of implementing QCTO-enabled modified Luneburg lens antenna by designing a broadband anti-reflective layer along with the modified lens’s planar excitation surface. The proposed anti-reflector layer is inherently broadband in nature, has a continuously tapered inhomogeneous dielectric permittivity profile along its thickness, and ensures broadband impedance matching. To show the new QCTO modified Luneburg lens antenna, an example lens antenna was designed at Ka-band (26–40 GHz) and fabricated using fused deposition modeling (FDM) based additive manufacturing technique. Electromagnetic performance of the lens antenna was experimentally demonstrated.

Additive manufacturing (AM) is a rapidly evolving technology set that has been proven to be a significant value add to a wide range of industries. In the space of RF design and engineering AM has been used for a wide range of applications. However, a lack of printable materials with properties suitable for use in RF applications has limited the performance and frequency range where printed devices can be utilized. At DeLUX Advanced Manufacturing we bring together a unique set of skills from RF engineers, to Mechanical Engineers, to Materials Scientists and Physicists to develop printable materials and deposition techniques specifically for RF applications. In this presentation we will show materials that were developed at DeLUX, in partnership with the University of Delaware, for use specifically in RF applications. We will present characterization data of our line of high dielectric low loss thermoplastic filaments for use in standard fused filament fabrication (FFF) systems. These filaments can have custom tailored dielectric constants ranging from 1.6-8 with loss tangents ranging from 0.0001-0.004. We will also present the use of these filaments in various applications such as, wide steering angle RF graded index (GRIN) lenses, as well as printed antennas on fully printed substrates with discrete spatial dielectric properties. These antennas will also showcase DeLUX’s ability to print integrated RF connectors with performance comparable to commercial of the shelf (COTS) connectors. The conductors for these antennas are also printed using DeLUX developed conductive pastes. These conductive pastes are specially formulated for high conductivity at frequencies up to 30GHz, and for filling long, high aspect ratio, channels and enclosed cavities. Many currently available conductive inks and pastes have greatly reduced conductivities at frequencies greater than a few gigahertz and many cannot be used within tight channels or enclosed spaces due to outgassing during the curing process. High frequency characterization data of these inks will be presented as well as performance data in various printed antenna structures like the stacked dual band patch antenna shown in Figure 1 .

This research explores the feasibility to fabricate and characterize novel conformal metasurface antennas and radiofrequency (RF) filters developed using hybrid multi-material 6-axis additive manufacturing (AM). The interest to embed RF communication systems within structural components has been increasing for many defense and warfighter readiness applications such as integrating them within various platforms from aircrafts to naval ships. To accomplish this type of mission a blend of desired mechanical properties and electrical properties are needed. Conformal printing allows for optimal aerodynamic or hydrodynamic performance while also being able to functionalize this structural component as an active electronic device. This pushes the boundaries of AM and RF device manufacturing capabilities. AM is a rapidly growing technology that is more often being implemented because of its unique ability to create complex designs that conventional manufacturing methods are not capable of producing. AM traditionally has been used for prototyping and developing structures, however, to advance this technology, more functional and applicable AM prints need to be performed. Conformal 6-axis and hybrid multi-material AM would allow for growth in RF electronics and is a different type of fabrication method that is investigated here.

Designing low cost, broadband and wide scan angle beamforming systems is critical for a diverse range of commercial and military applications. Many current beamforming approaches either involve complex phased arrays or large mechanically steered antennas. The phased arrays are often prohibitively expensive while the mechanically steered systems are often too slow, heavy, and susceptible to mechanical failures. An alternative approach involves the use of graded index (GRIN) RF lenses as passive beamformers. Recently the reliable fabrication of RF GRIN lenses is made possible with the use of advanced additive manufacturing (AM) systems and materials. Through AM, GRIN based RF lenses have recently been reported that possess high radiation efficiencies, high antenna gains, and ultra-wideband operations [1] . Current GRIN devices, while greatly improved, still often lack the wide-angle steering and efficient conformal feed structures needed for some of the more challenging applications. In this presentation, we describe the design and fabrication, via additive manufacturing, of an array of conformal feeds. When integrated with a graded index beamforming lens, such as the Luneburg lens or modified versions, results in an ultra-wide scan angle beam steering device. Specifically, we will describe the design and fabrication, via AM, of an integrated beam steering device that combines: (1) printed conformal RF connectors, (2) printed broadband baluns, (3) printed broadband feed antennas, and (4) a novel GRIN lens.

A novel radar retroreflector is created using fused deposition modeling techniques. The three-dimensionally printed reflector is comprised of a variable dielectric structure generated using a set of space-filling curves, capped with a thin hemisphere of copper tape. The retroreflective performance is modeled using commercial software and radar cross section measurements are taken in an anechoic chamber. The modeled and measured results are shown to be in good agreement and demonstrate good retroreflective properties up to ±70° elevation and azimuth acceptance angles.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

In this paper, the design concept, optimization methods, simulated radiation properties, and additive manufacturing methods for 3D wire grid antennas in symmetric and non-symmetric arbitrary volumes are presented.

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) region from 8 to 12 μm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) substrate with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. The tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one-dimensional gratings are polarization dependent and those based on two-dimensional gratings are polarization independent at normal incidence of light. Both 1-D and 2-D gratings were designed and characterized. Anti-reflection coatings were applied on substrate interfaces to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using an automated tunable room temperature quantum cascade laser (QCL) system operating from 7.3 to 12.9 μm, and a modified Fourier Transform Spectrometer with a custom designed chamber for normal incidence of light on the sample. We present detailed characterization experiments and compare the theoretical and experimental results.

We demonstrate a new method for realizing Luneburg lens antennas with nearly continuously graded permittivity profiles in three dimensions. The method combines fused filament fabrication, a nearly ubiquitous additive manufacturing technique, with an effective media approach that employs space-filling curves. We experimentally validate our design methodology by fabricating and characterizing two Luneburg lenses designed to operate at separate frequency bands (26–40 GHz and 70–110 GHz) with antenna gains of 20 and 24 dBi, respectively. The results demonstrate good agreement with rigorous electromagnetic simulations. We also demonstrate the ability to passively beam steer over a wideband of operation (26–40 GHz and 70–110 GHz) with less than 0.5% power loss due to dielectric material losses within the Luneburg lens.

Among the processing parameters of additive manufacturing, printing direction is of critical importance. While studies on effects of printing direction have so far mainly focused on mechanical properties of solid specimens, the present research is intended to demonstrate printing direction dependence of mechanical behavior of additively manufactured 3D preforms and their composites. Compressive behavior of additively manufactured 3D braid preforms and composites was investigated for three distinct printing directions (0°, 45° and Z-direction). Fused filament fabrication (FFF) of acrylonitrile-butadiene-styrene (ABS) filament and short carbon fiber/ABS (CF/ABS) filament was adopted. First, solid cube specimens were fabricated; the parts printed along 0° and 45° directions showed more fabrication-induced pores. Then, 3D braid preforms were fabricated and infused with silicone matrix. For preforms printed along 45° direction, inter-yarn adhesion was observed, which enhanced specimen initial modulus. On the other hand, Z-direction specimens showed higher structural ductility, due to inter-yarn slippage.

In this paper we present the development of feedstock materials, specifically tailored permittivity materials used in fused filament fabrication (FFF), for microwave applications. One of the major bottlenecks in utilizing additive manufacturing for useful radiofrequency (RF) applications is the lack of suitable materials. Flexible, high-dielectric constant and low dielectric loss tangent composite materials, consisting of a mixture of polymer and ceramic nanoparticles in controlled volume fractions, are manufactured. Effective medium approximations (EMA) to predict composite properties, such as Bruggeman approximations, are validated with measurements between 18 – 40 GHz and are presented.

In this paper the authors present a novel design tool for realizing dielectric structures with spatially varying electromagnetic properties via additive manufacturing (AM). To create tool paths ideal for AM processes, space-filling curves were utilized. Using fused deposition modeling (FDM), spatially varying structures were printed that produced a spatially varying relative permittivity. A wide range of varying fill fractions were printed and evaluated, demonstrating good agreement between the simulated and measured results. Furthermore, the authors verified that this design tool can be applied to practical structures by designing, printing and testing a gradient index flat lens.

The long-wave IR spectral region spanning ∼3 to 13 μm contains spectral bands useful for many scientific and industrial applications. We discuss design, fabrication, and testing of example devices based on guided-mode resonant metamaterials.

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron based on the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect incident broadband light at one (or more) narrow spectral band while fully transmitting the other wavelengths. Tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1-D) gratings are polarization dependent and those based on two dimensional (2-D) gratings are less sensitive to incident polarization of light. Both filters were fabricated using standard dielectric deposition and photolithographic techniques and characterized. Simple 2-layer anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a modified commercial Fourier Transform Infrared Spectrometer (FTIR) system with a separate chamber for a collimated incident beam of light, focusing optics, a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector, a wire grid polarizer and a micro-engineered filter. We will present detailed characterization experiment and compare the theoretical and experimental results.

We present metamaterial tunable notch filters operating from 8 to 12 µm based on the guided-mode resonance phenomenon using high-refractive index dielectric materials and their characterization using a customized infrared spectrometer.

Additive manufacturing features “direct” and “layer-by-layer” fabrication and has significantly facilitated the microstructural design and fabrication of a wide range of highly complex parts. To enable the application of additive manufacturing in major industries and composites, it is necessary to evaluate the microstructural features of additively manufactured parts. Among the various advanced characterization techniques, X-ray micro-computed tomography (μ-CT) showed unique advantages as a high resolution, nondestructive and 3D visualization and measurement technique for material characterization. In the research reported in this article, we have fabricated an array of multi-directional preforms and composites and have characterized their microstructural features via X-ray μ-CT. First, a solid specimen as well as 3D orthogonal and 3D braided preforms have been fabricated using fused filament fabrication (FFF) and inspected with X-ray μ-CT. Then, the fabricated preforms have been infused with silicone matrix and the multi-directional preforms and composites have been tested in compression at different strain levels, to reveal their damage evolution under compressive loading. The preliminary effort made in this research demonstrates the feasibility of characterizing microstructure of additively manufactured parts via X-ray μ-CT technique and enables an investigation of the microstructural features and damage evolution of multi-directional preforms and composites.

In contrast to conventional preforming techniques, additive manufacturing features direct and layer-by-layer fabrication, which provides viable new capabilities for the fabrication of reinforced composites. In this article, we explore the microstructural design as well as additive manufacturing and characterization of 3D orthogonal, short carbon fiber/acrylonitrile-butadiene-styrene (ABS) preforms and composite. First, an array of 3D orthogonal preforms is designed based on topological consideration and validated by fused filament fabrication of pure ABS wire; high fidelity between models and preforms is accomplished. Then, short carbon fibers are introduced into the designed 3D orthogonal preforms as reinforcement, using a short carbon fiber/ABS wire. Lastly, the compressive behavior of a 3D orthogonal, short carbon fiber/ABS preform and that of its silicone infused composite are characterized. The preform design methodology developed in this research as well as the preliminary effort made in composite fabrication and characterization demonstrates the feasibility of additive manufacturing of 3D orthogonal preform based fiber composites.

Previous electromagnetic computations of multilayered dielectric/metallic spheres identified the ideal dimensions and composition for achieving optimized mass extinction coefficients ( m 2 / g ). A hollow metallic sphere, with a thin metallic shell, is one such example of a spherical structure that can theoretically achieve high mass extinction coefficients in the long wave infrared (LWIR) region (8–12 µm). To this end, we endeavored to demonstrate a cost-effective and scalable manufacturing approach for synthesizing and experimentally validating the mass extinction coefficients of hollow metallic spheres. Specifically, we detail a novel approach for fabricating hollow aluminum spheres using radio frequency (RF) magnetron sputter deposition. Sacrificial high-density polyethylene polymer microspheres were used as substrates for the deposition of thin layers of aluminum. The core shell structures were subsequently thermally processed to form the hollow micron sized aluminum shells. The mass extinction coefficients of the hollow aluminum spheres were subsequently measured and compared to computational results. A strong agreement between experimental and theoretical predictions was observed. Finally, the LWIR mass extinction coefficients of the hollow spheres were compared to high aspect ratio brass flakes, a common pigment used for LWIR attenuation, and other materials and geometries that are used for LWIR filtering applications. This comparison of both performance and availability revealed that the fabricated hollow aluminum spheres exhibited competitive LWIR properties using a more scalable and cost-effective manufacturing approach.

—In this paper we present the design and fabrication of modified Luneburg lens antennas. The lens was designed using a quasi-conformal transformation optics approach and fabricated using additive manufacturing. We experimentally validated our methods by designing and fabricating a modified Luneburg lens operating at 24–40 GHz with a beamforming capability of -50° to 50°.

This paper presents progress made towards the design and fabrication of complex RF components and systems using multi material additive manufacturing (AM).

Recent improvements in microparticle synthesis and handling have prompted new research into the engineering and fabrication of single and multilayered microspheres through traditional physical and chemical vapor depositions. At the University of Delaware, we have developed a custom batch coating process utilizing a vibro-fluidized mixing vessel to deposit thin-films onto the surface of microparticle substrates through R.F. magnetron sputtering. This process opens up a number of design possibilities for single and multilayered microsphere technologies that can be used to improve the optical performance of several optical filtering applications. Through the use of custom design and simulation software, we have optimized a number of filter designs and validated these findings through commercial software. Specifically, we have aimed to improve upon the mass extinction performance seen by traditional materials in the long wave infrared spectrum (LWIR, λ=8-12μm). In order to do this, we have run a series of experiments aimed at creating ultra-lightweight metallic hollow-spheres. Aluminum thin-films have been successfully deposited onto a number of substrates including hollow glass microspheres, high density polyethylene microspheres, and polystyrene foam spheres. By depositing the thin-films onto polymer substrates we have been able to remove the solid core after deposition through a thermal decomposition or chemical dissolution process, in an effort to reduce particle mass and improve mass extinction performance of the filter. A quantum cascade laser measurement system has been used to characterize the optical response of these fabricated aluminum hollow-spheres and have largely agreed with the expected simulated results.

We describe performance characterization of spectrally tunable nano-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on using the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. The tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1D) gratings are polarization dependent and those based on two dimensional (2D) gratings are close to polarization independent. To design the filter with a strong narrow band reflectance, we used the rigorous coupled wave (RCW) algorithm to simulate the filter. Here we will describe design and characterization of prototype filters with 1D grating. Anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a QCL system operating at room temperature, nanoengineered filter and an uncooled broadband sensor. We will present the filter design, detailed characterization experiment and compare the theoretical and experimental results.

We describe the design of metamaterial tunable notch filter operating from 8 to 12 µm based on the guided-mode resonance phenomenon using dielectric materials germanium and zinc selenide and their characterization using quantum cascade lasers.

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.

A method to fabricate continuously tapered antireflective (AR) coatings using fused deposition modelling (FDM) is presented. The method leverages FDM's ability to fabricate spatially varying 3D structures with variable polymer fill volumes, thus producing a spatially varying effective permittivity profile. The method is validated experimentally within the Ka-band. Experimental results closely match predictions, and AR treated samples demonstrate much less transmission loss than non-AR treated samples.

We present the development of a long-wave infrared regime multiband absorption filter with simultaneous wavelength and intensity selectivity. The approach employs a technique we call “spectral dithering” to place single-band absorbing pixels across a unit cell such that their weighted sum describes a multiband absorption spectrum. The number of absorption bands is proportional to the number of unique pixels present. Pixel patterning controls wavelength selectivity, whereas pixel distribution controls intensity selectivity. Using the rigorous coupled-wave method for modelling, a filter with three absorption bands between 6 and 14 μm is designed using the spectral dithering technique. The device is fabricated and experimentally verified using Fourier-transform infrared spectroscopy.

Current micro-bolometers are broadband detectors and tend to absorb a broad window of the IR spectrum for thermal imaging. Such systems are limited due to their lack of sensitivity to blackbody radiation, as well as the inability to spectrally discern multiple wavelengths in the field of view for hyperspectral imaging (HSI). As a result, many important applications such as low concentration chemical detection cannot be performed. One solution to this problem is to employ a system with thermoelectrically cooled or liquid nitrogen cooled sensors, which can lead to higher sensitivity in detection. However, one major drawback of these systems is the size, weight and power (SWaP) issue as they tend to be rather bulky and cumbersome, which largely challenges their use in unmanned aerial vehicles. Further, spectral filtering is commonly performed with large hardware and moving gratings, greatly increasing the SWaP of the system. To this point, Lumilant’s effort is to develop wavelength selective uncooled IR filters that can be integrated onto a microbolometer, to exceed the sensitivity imposed by the blackbody radiation limit. We have demonstrated narrowband absorbers and electrically tunable filters addressing the need for low-SWaP platforms.

As infrared (IR) imaging technologies improve for the commercial market, optical filters complementing this technology are critical to aid in the insertion and benefit of thermal imaging across markets of industry and manufacturing. Thermal imaging, specific to shortwave infrared (SWIR) through longwave infrared (LWIR) provides the means for an observer to collect thermal information from a scene, whether being temperature gradients or spectral signatures of materials. This is beneficial to applications such as chem/bio sensing, where the identification of a chemical species being present or emitted could compromise personnel or the environment. Due to the abundant amount of information within an environment, the difficulty lies within the observer’s ability to extract the information. The use of optical filters paired with thermal imaging provides the means to interrogate a scene by looking at unique infrared signatures. The more efficient the optical filter can either transmit the wavelengths of interest, or suppress other wavelengths increases the finesse of the imaging system. Such optical filters can be fabricated in the form of micro-spheres, which can be dispersed into a scene, where the optical filter’s intimate interaction with the scene can supply information to the observer, specific to material properties and temperature. To this extent, Lumilant has made great progress in the design and fabrication of such micro-sphere optical filters. By engineering the optical filter’s structure, different optical responses can be tuned to their individual application.

High-impedance surfaces like electromagnetic band gap (EBG) ground planes have been used to realize low profile antennas. In this paper, an alternative ground plane design is presented for antennas with circular current distributions, which consists of an annular slot backed by a thin grounded substrate. The slot ring generates a high-impedance condition at a resonant frequency in the vicinity of the slot. This annular slot loaded ground plane results in efficient radiation of the low profile antenna in the broadside direction. Since the slot loaded ground plane consists of a single slot ring rather than periodic structures, it is much simpler to construct than the standard EBG ground plane. Numerical simulations and experimental measurements are shown to validate the design concept and are compared with a mushroom-type EBG ground plane. A parametric analysis was also conducted by varying the slot ring's dimensions and tuning varactors are shown to vary the ground plane's resonant frequency.

In this paper we describe a method for creating flexible composite filaments with dielectric constants below 2.0 over a wide frequency band (i.e. 18 - 40 GHz). We demonstrate that a low dielectric constant composite filament, useful for FFF printing, can be manufactured by combining a base thermoplastic polymer with hollow microspheres and a plasticizer. Experimental results are provided for filaments made from two different base polymers (i.e. ABS and HDPE) and varying volume fractions of hollow microspheres. We also describe an effective media model to predict the dielectric properties of the composite filaments as a function of the properties of the constituent materials (e.g. base polymer, hollow microspheres) and their relative volume fractions within the composite filament. Experimental test samples were printed using the new low-K filaments and experimental characterization results are provided that validate this approach.

An analytical method for determining the optimal reflection phase of an electromagnetic band gap (EBG) ground plane to match a low profile dipole antenna is introduced. Image theory is used to incorporate the near field coupling between a dipole antenna and the ground plane. The main contribution of this paper is to show that the optimal EBG reflection phase can be determined at discrete frequencies where a theoretically perfect return loss occurs. The optimal reflection phase is then obtained over a wider frequency band of interest and is related to the antenna's return loss for a given feed impedance and antenna height above the EBG. The resulting reflection phase can be used as a reference for designing an EBG ground plane that is well matched to the antenna without time consuming iterative full wave numerical simulations. Numerical modeling results are compared to the optimal return loss derived from the analytical method to validate the design process. It is also shown that, for certain antennas, vias are not always necessary in the construction of the EBG, which eases the manufacturing process. Finally, a dipole and EBG are constructed using the optimal design method and measurements are compared to the simulations.

This letter presents an experiment using a fully polarimetric hybrid-polarization (CL-pol) radar that employs separate spiral antennas for transmit of the orthogonal circular polarizations and operates from 8 to 16 GHz. We propose a model for the antenna cross-polarization isolation (XPI) and calibration of this radar. The transmit XPI is characterized by leveraging the Doppler shift caused by a continually rotating dihedral target illuminated by a circularly polarized wave. Experimental results show the validity of the radar model and antenna measurement technique .

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 seperate methods. This work establishes an analytic effective media approach to characterizing the dielectric constant for both cases.

In this paper we present a novel design tool for the generation of graded dielectric structures via additive manufacturing. Space filling curves are utilized to create tool paths ideal for the additive manufacturing (AM) processes. Fused deposition modeling (FDM) was utilized to print spatially varying structures resulting in a spatially varying relative permittivity. A wide range of varying fill fractions were printed and evaluated; the simulated and measured results displayed good agreement. To verify that the design tool can be applied to practical structures a gradient index flat lens was designed, printed, and tested.