@article{oh_kordsmeier_askari_adams_2023, title={Low Profile GRIN Lenses With Integrated Matching Using 3-D Printed Ceramic}, volume={4}, ISSN={["2637-6431"]}, DOI={10.1109/OJAP.2022.3227747}, abstractNote={In this paper, we investigate a shortened horn antenna with high gain that is enabled by a 3D-printed gradient index (GRIN) lens composed of high permittivity zirconia <inline-formula> <tex-math notation="LaTeX">$(ZrO_{2})$ </tex-math></inline-formula>. The baseline H-plane sectoral horn antenna is designed with length that is 1/3 of the optimal horn antenna and exhibits a low gain due to the high flaring rate of the horn. Increased gain is achieved by adding a flat GRIN lens at the horn aperture. High permittivity <inline-formula> <tex-math notation="LaTeX">$ZrO_{2}\,\,(\varepsilon _{r} = 23)$ </tex-math></inline-formula> enables lens miniaturization; however, when interfaced with air, reflections at the air interface increase the impedance mismatch. Two different methods for mitigating the reflections are studied. One is a simple <inline-formula> <tex-math notation="LaTeX">$\lambda /4$ </tex-math></inline-formula> matching layer that matches the bulk permittivity of <inline-formula> <tex-math notation="LaTeX">$ZrO_{2}$ </tex-math></inline-formula> to air. As expected, the <inline-formula> <tex-math notation="LaTeX">$\lambda /4$ </tex-math></inline-formula> layers reduce the reflections in part of the 13–18 GHz band but produce high reflection in other parts. The second approach is a GRIN lens with integrated tapered matching layer to match phase and impedance simultaneously. Three tapering methods are studied (exponential, Klopfenstein, linear) for impedance matching. Analytical expressions of the minimum thickness and permittivity distribution are derived. The lens is discretized for print and three types of unit cells are proposed to create a wide range of permittivities ranging from bulk ceramic to air. A <inline-formula> <tex-math notation="LaTeX">$ZrO_{2}$ </tex-math></inline-formula> lens prototype printed with an XJet Carmel 1400 is measured and results show good agreement with simulations, including gain performance equivalent to a horn of 2.4x longer length. The measured gain and beamwidth of the lens are 5.4dB higher and 52° narrower than those of the shortened horn alone at 15 GHz, respectively.}, journal={IEEE OPEN JOURNAL OF ANTENNAS AND PROPAGATION}, author={Oh, Yongduk and Kordsmeier, Neal and Askari, Hussain and Adams, Jacob J.}, year={2023}, pages={12–22} }
 @article{yang_oh_hu_adams_2021, title={Implementation of A Flat-Bottom Luneburg Lens Based on Conformal Transformation Optics}, ISSN={["2576-7216"]}, DOI={10.1109/IMS19712.2021.9574844}, abstractNote={Using transformation optics based on numerical conformal mapping, a 2D flat-bottom Luneburg lens is designed and realized using isotropic materials. The gradient-index material profile is discretized and realized through unit cells. Full wave simulation is conducted to verify the performance. Maximum 2D directivity of 14.9 dBi is achieved for a line source when the 2D lens has a free-space electrical size of 10λ. A wide field of view (~ 120 degree) is obtained for a set of planar offset feed points spanning 3.3λ.}, journal={2021 IEEE MTT-S INTERNATIONAL MICROWAVE SYMPOSIUM (IMS)}, author={Yang, Binbin and Oh, Yongduk and Hu, Xinchen and Adams, Jacob J.}, year={2021}, pages={556–558} }
 @article{burden_oh_mummareddy_negro_cortes_du plessis_macdonald_adams_li_rojas_2021, title={Unit cell estimation of volumetrically-varying permittivity in additively-manufactured ceramic lattices with X-ray computed tomography}, volume={210}, ISSN={["1873-4197"]}, DOI={10.1016/j.matdes.2021.110032}, abstractNote={Additive manufacturing of ceramics is transforming electromagnetics by providing density-varying lattices and stochastic foams within arbitrary envelopes. Periodic structures can now be fabricated with zirconia which offers the highest permittivity of any 3D printable material possible to be printed with nearly-full-density. By arranging a lattice with variation in the strut and node sizings as well as unit cell dimensions, the effective density of a structure can be spatially-modulated gracefully and with unprecedented freedom. These variations in density directly translate into variations in the effective permittivity of the bulk lattice (estimated locally and globally with a combination of mixing formulas, curve fits, and capacitance models). A lattice had previously been fabricated with a rectangular envelope for evaluation of effective global permittivity of the overall structure using a network analyzer. For this work, the structure was scanned with X-ray computed tomography (CT) to capture the three dimensional density of the structure including both the solid ceramic elements as well as the interstitial space. Software was developed that reads CT scan data and provides a 3D data structure with a pointwise unit cell estimation of the effective permittivity throughout the volume - a model well suited for electromagnetic simulations to optimize advanced microwave devices. The proposed technique serves as a foundation for the non-destructive estimation of space-varying permittivity within 3D printed lattices and foams.}, journal={MATERIALS & DESIGN}, author={Burden, Edward and Oh, Yongduk and Mummareddy, Bhargavi and Negro, Dylan and Cortes, Pedro and Du Plessis, Anton and MacDonald, Eric and Adams, Jacob and Li, Frank and Rojas, Roberto}, year={2021}, month={Nov} }
 @article{bharambe_oh_adams_negro_macdonald_2020, title={3D Printed Zirconia for UWB Stacked Conical Ring DRA}, ISSN={["1522-3965"]}, DOI={10.1109/IEEECONF35879.2020.9330248}, abstractNote={In this paper, we present a 3D printed ultrawideband (2.95 GHz-20 GHz) dielectric resonator antenna (DRA) fabricated using Nano-Particle Jetting (NPJ) of high permittivity, low loss, mechanically tough ceramic, zirconia. 3D printing enables fabrication of geometrically complex DRA design without machining or high pressure shaping. The wideband impedance response was achieved with a hybrid monopole/DRA design that uses the fundamental and higher order modes of the monopole and the parasitic DRA rings for a wide impedance bandwidth (VSWR lt; \mathbf{3}$). In the future, the 3D printing technique can enable building DRAs with more complex shapes or ceramic lattices to build graded index lenses.}, journal={2020 IEEE INTERNATIONAL SYMPOSIUM ON ANTENNAS AND PROPAGATION AND NORTH AMERICAN RADIO SCIENCE MEETING}, author={Bharambe, Vivek T. and Oh, Yongduk and Adams, Jacob J. and Negro, Dylan and MacDonald, Eric}, year={2020}, pages={41–42} }
 @article{oh_bharambe_adams_negro_macdonald_2020, title={Design of a 3D Printed Gradient Index Lens Using High Permittivity Ceramic}, ISSN={["1522-3965"]}, DOI={10.1109/IEEECONF35879.2020.9330193}, abstractNote={In this paper, we investigate the design of a gradient index (GRIN) horn-integrated lens using a zircona lattice printed using a Nanoparticle Jetting process. The electrical properties of the ZrO2 lattice for a range of geometric parameters are simulated to realize a range of effective dielectric constants from 3 - 23. The properties of a 3D printed lattice are found to be consistent with the simulations. A shortened horn antenna combined with a flat GRIN lens is designed to collimate the beam and enhance directivity. The simulated directivity is 14.6 dBi at 15 GHz, which is 6.2 dB higher than the same horn without the lens.}, journal={2020 IEEE INTERNATIONAL SYMPOSIUM ON ANTENNAS AND PROPAGATION AND NORTH AMERICAN RADIO SCIENCE MEETING}, author={Oh, Yongduk and Bharambe, Vivek T. and Adams, Jacob J. and Negro, Dylan and MacDonald, Eric}, year={2020}, pages={1431–1432} }