Hung Loui

A dual-band dual-polarized nested Vivaldi slot array with multilevel ground plane

In this paper, we present a systematic approach to the design, optimization and characterization of a broadband 5:1 bandwidth (0.8 to 4.0 GHz) antenna subarray. The array element is an optimized-taper antipodal Vivaldi slot with a bandwidth of 2.5:1. Two such elements of different sizes and with 0.4 GHz (10% of the highest frequency) overlapping bandwidths are arrayed in a nested lattice above a multilevel ground plane that shields the feeds and electronics. Return loss, radiation patterns, cross-polarization and mutual coupling are measured from 0.5-5.0 GHz. This array demonstrates E plane patterns with 50/spl deg/ and 45/spl deg/ 3-dB beamwidths in the lower and upper frequency bands, respectively. The coupling between the elements is characterized for different relative antenna positions in all three dimensions.

Bandwidth control of forbidden transmission gaps in compound structures with subwavelength slits

Phase resonances in transmission compound structures with subwavelength slits produce sharp dips in the transmission response. For all equal slits, the wavelengths of these sharp transmission minima can be varied by changing the width or the length of all the slits. In this paper we show that the width of the dip, i.e., the frequency range of minimum transmittance, can be controlled by making at least one slit different from the rest within a compound unit cell. In particular, we investigate the effect that a change in the dielectric filling, or in the length of a single slit, produces in the transmission response. We also analyze the scan angle behavior of these structures by means of band diagrams and compare them with previous results for all-equal slit structures.

A Negative Refractive Index Metamaterial Based on a Cubic Array of Layered Nonmagnetic Spherical Particles

A low-loss passive metamaterial exhibiting negative refractive index or “double negative” electromagnetic properties at microwave frequencies is proposed. The metamaterial is a lattice of spherical particles made up of multiple dielectric materials in concentric layers. Because no magnetic constituents (that tend to have higher losses) are involved, the negative-index behavior is possible with very low values of attenuation. A negative-index metamaterial based on dielectric-coated metal spheres is also proposed, and is predicted to have lower attenuation than other structures based on metallic scatterers. Numerical results and design principles are given.

Radar-cross-section reduction of wind turbines. part 1.

In recent years, increasing deployment of large wind-turbine farms has become an issue of growing concern for the radar community. The large radar cross section (RCS) presented by wind turbines interferes with radar operation, and the Doppler shift caused by blade rotation causes problems identifying and tracking moving targets. Each new wind-turbine farm installation must be carefully evaluated for potential disruption of radar operation for air defense, air traffic control, weather sensing, and other applications. Several approaches currently exist to minimize conflict between wind-turbine farms and radar installations, including procedural adjustments, radar upgrades, and proper choice of low-impact wind-farm sites, but each has problems with limited effectiveness or prohibitive cost. An alternative approach, heretofore not technically feasible, is to reduce the RCS of wind turbines to the extent that they can be installed near existing radar installations. This report summarizes efforts to reduce wind-turbine RCS, with a particular emphasis on the blades. The report begins with a survey of the wind-turbine RCS-reduction literature to establish a baseline for comparison. The following topics are then addressed: electromagnetic model development and validation, novel material development, integration into wind-turbine fabrication processes, integrated-absorber design, and wind-turbine RCS modeling. Related topics of interest, including alternative mitigation techniques (procedural, at-the-radar, etc.), an introduction to RCS and electromagnetic scattering, and RCS-reduction modeling techniques, can be found in a previous report.

Thick FSSs for large scan angle applications

Frequency selective surfaces (FSSs) have applications in radomes, dichroic plates for antenna feed systems and quasi-optical filters for spectroscopy. Most FSSs have been printed on thin substrates, and possibly cascaded for larger scan angles. There are a limited number of theoretical discussions of thick metal FSSs. The paper presents the comparison of measurements to a mode-matching finite element numerical method for the analysis of arbitrarily thick FSS. A simple thick FSS was chosen for validation. As expected, since there are no matching dielectric layers on either side of the FSS, the TE transmission response is poor for high incidence angles. Current work is focused on the design of flat and curved radomes for large scan angle applications, and THz frequency low-loss quasi-optical filters. Our analysis tool is validated and allows us to design for frequency limits, sharpness of stop or pass bands, and impedance matching at the input and output surfaces while taking fabrication tolerances into account. The shape of the aperture can be chosen to tailor the cutoff frequency. It should also be noted that the geometry in the z-direction affects the sharpness of the transmission response while dielectric layers improve the matching at different incidence angles.

Dual-polarization large scan angle broadband thick-metal FSS

This paper describes the design, fabrication, and characterization of a large scan angle FSS comprised of a periodic array of dielectric-filled tapered holes as shown in Figure 1(a). The performances of three FSSs with different taper profiles and equal hexagonal lattices are compared. FSSs with cylindrical, linearly tapered, and cosine tapered holes (Figure 1(b)) show dramatically different TE and TM wide scan angle transmission responses between 18 and 27 GHz. Measurements are compared to simulation for a cosine tapered K-band thick-metal FSS that shows the widest bandwidth and largest scan angle for TE and TM transmission responses. Significant reduction in the pass-band ripple is also observed for this design.

Plasmonic antireflection surfaces for the mid-infrared

In a similar manner to the frequency selective surfaces commonly used in the microwave regime, we have designed antireflective surfaces in the mid-infrared (2-5 μm). Translation of microwave designs to the infrared is not trivial for several reasons. Properties of applicable IR materials are significantly different than their microwave counterparts. Additionally, the required feature sizes need a completely different fabrication methodology. Our surfaces are metallic, yet have a high-transmission angular and frequency passband. We take advantage of photon-plasmon interaction to maximize transmission through holes in the metal surface. Simulations have been completed using both rigorous coupled wave analysis and method of moments codes. The design process has followed a path that insures that we are able to fabricate the designed structures considering cases of normal and off-angle incidence. We designed our surfaces to be compatible with shapes that we will etch in silicon and then coat in gold: this process allows the greatest flexibility in etching shapes for vias while maintaining a metallic layer for plasmon propagation on the surface. We anticipate over 90% transmission in the infrared passband. Our design methodology would also be applicable to the 8-12 μm band.

Plasmonic antireflection coatings in the infrared

Antireflective (AR) coatings are used in many optical applications such as flat-panel displays, solar cells, lasers and other optoelectronic devices. Until recently, efforts for realizing low surface reflectivity have been concentrated on thin dielectric films, where the structural profile of the film is used to create a gradual change in the refractive index between the air and substrate. As an alternative to this approach, there has been much interest in pushing frequency-selective-surface (FSS) technology, a technology which is well known in the literature for its filtering characteristics at microwave and millimeter frequencies, to infrared frequencies. With FSSs (examples include periodic conducting elements placed on a dielectric substrate or apertures perforating a conducting sheet), the filtering response is realized due to the photon-plasmon coupling associated with the structured periodic surface rather than through an impedance-matching concept. Following along the path of the microwave community, attention in this work is confined to the transmission response of single-layer aperture-array screens, with the goal of ultimately realizing an AR surface with a high angular and frequency bandwidth in the mid-infrared frequency range.

A new approach for in-situ scan impedance characterization of scanned antenna arrays

Scanned phased array antennas require active scan impedance determination and mitigation. This paper addresses the former by introducing a novel in-situ measurement architecture and associated mathematics for efficiently determining the real-time active scan impedance of arbitrary sized scanned arrays in the field. The in-situ nature of the proposed architecture reduces the need for large numerical simulation and/or estimation of scan impedance variations due to possible diverse antenna array placement in the field. Direct experimental characterization also enables direct validation of numerical simulation. The mathematics developed are for an M by N antenna array utilizing direct in-situ mutual coupling characterization. The mathematical model was implemented in MATLAB and verified through simulation using CST Microwave Studio (MWS) for a 2×2 monopole planar antenna array. The models robustness is tested by varying the inter-element spacing.

Transmissive infrared frequency selective surfaces and infrared antennas : final report for LDRD 105749.

Plasmonic structures open up new opportunities in photonic devices, sometimes offering an alternate method to perform a function and sometimes offering capabilities not possible with standard optics. In this LDRD we successfully demonstrated metal coatings on optical surfaces that do not adversely affect the transmission of those surfaces at the design frequency. This technology could be applied as an RF noise blocking layer across an optical aperture or as a method to apply an electric field to an active electro-optic device without affecting optical performance. We also demonstrated thin optical absorbers using similar patterned surfaces. These infrared optical antennas show promise as a method to improve performance in mercury cadmium telluride detectors. Furthermore, these structures could be coupled with other components to lead to direct rectification of infrared radiation. This possibility leads to a new method for infrared detection and energy harvesting of infrared radiation.