Yakup Bayram

Embroidered Conductive Fibers on Polymer Composite for Conformal Antennas

We provide a novel class of conformal antennas based on embroidered conductive metal-polymer fibers (E-fiber) on polymer-ceramic composites. This new technology offers attractive mechanical and RF performance when compared to traditional flat and rigid circuits and antennas. The proposed E-fiber components are consisted of high strength and flexible polymer fiber cores and conductive metallic coatings. They were fabricated using automatic embroidery process, followed by assembly with polydimethylsiloxane and rare-earth titanate ceramic composites. Such composite substrates were tape-casted, and capable of providing tunable dielectric constant from 3 to 12 with a low tanδ <; 10-2 up to GHz frequencies. Basic RF prototypes, such as transmission lines (TL), patch antennas, and antenna arrays were fabricated for experimental evaluation. Measurement of the prototypes were conducted and compared to their copper counterparts. The RF characteristics of the E-fiber TLs exhibited an insertion loss of only 0.03 dB/cm higher than copper TLs up to 4 GHz . Also, the E-fiber patch antenna and antenna array exhibited 0.3 dB and 0.6 dB lower gains, respectively, than their copper counterparts. When applied onto a cylindrical surface, both the E-fiber patch antenna and antenna array only suffered 1 dB loss in realized gain, which is quite remarkable when compared with traditional antennas.

E-Textile Conductors and Polymer Composites for Conformal Lightweight Antennas

We present a conformal and lightweight antenna technology based on E-textile conductors and polymer-ceramic composites. Unique advantages of the proposed technology are its structural integrity, light weight and conformity to the platform. E-textile conductors are fabricated with single wall carbon nanotube (SWNT) and Ag coated textiles. They demonstrate good structural integrity with polymer composites due to their mechanical compatibility. Similarly, polymer composites demonstrate superior RF performance with permittivity ranging from 3 to 13. Fabrication process for E-textile conductors and integration process with polymer composites is described in detail. We also demonstrated merit of the proposed technology with a simple patch antenna whose radiation performance is measured when it was flat and conformed onto a cylindrical surface. We compared its performance with that of an ideal patch. Experiments suggested that the sample patch antenna based on the proposed technique achieved 6 dB gain, which is 2 dB below a patch which has the same dimensions and made of ideal lossless materials. When it is conformed onto a cylindrical surface, we achieved 2.5 dB less gain than that of antenna realized with a PEC surface. This clearly validates the merit of the proposed conformal antenna technique based on non-traditional materials.

Polymer-Carbon Nanotube Sheets for Conformal Load Bearing Antennas

We propose a conductive carbon nanotube (CNT) sheet to realize conformal antennas on polymer substrates. Polymer-ceramic composites (rubber-like structures) have good RF (high dielectric constant and low loss tangent) and desirable mechanical properties (conformal, flexible and lightweight). However, there is a challenge in printing metallization circuits on polymer substrates due to their hydrophobic nature. Also, they are associated with low metal-polymer adhesion, causing peeling under stain or tensile stresses. To address these issues, in this paper, we consider the approach of embedding high density vertically-aligned carbon nanotubes within the polymer composite to achieve a CNT sheet having high structural compatibility. We present the fabrication process to achieve high conductivity CNT sheets and construct a sample polymer-CNT patch antenna, yielding a 5.6 dB gain. This is only 0.8 dB lower than that of an ideal patch made of perfect electric conductor (PEC). Strain and tensile tests are also carried out to evaluate electrical performance of the polymer-CNT sheet as it is bent and stretched. Our measurements show that the proposed conductive polymer-CNT sheet is highly flexible and preserves good conductivity under small bending and stretching. The CNT sheet retains acceptable performances even after 100° bending and 13% stretching. The proposed polymer-CNT sheets are well suited for load bearing antenna applications.

High-Frequency EM Characterization of Through-Wall Building Imaging

A high-frequency asymptotic technique based on the Uniform Geometric Theory of Diffraction (UTD) is employed for building interior imaging. The analysis is implemented using a ray-tracing technique to account for multiple scattering interactions in a building, along with a set of heuristic diffraction coefficients for dielectric wedges and corners. Imaging of the synthetic aperture radar data is carried out by the conventional fast Fourier transform method to transform to the downrange domain, and combined with a coherent near-zone imaging function for cross-range. Comparisons with experimental data for a scaled-down building model are given to demonstrate the suitability and efficacy of our analysis for through-wall building imaging. The UTD ray mechanisms account for the dominant scattering features observed in the image.

Hybrid

We propose a generalized S-parameter analysis for transmission lines (TLs) with linear/nonlinear load terminations subject to arbitrary plane-wave and port excitations. S-parameters are prevalently used to model TLs such as cable bundles and interconnects on printed circuit boards (PCBs) subject to port excitations. The conventional S-parameter approach is well suited to characterize interactions among ports. However, nontraditional port excitations associated with plane-wave coupling to physical ports at TL terminals lead to forced, as well as propagating, modal waves, necessitating a modification of the standard S-parameter characterization. In this paper, we consider external plane-wave excitations, as well as port (internal) sources, and propose a hybrid S-parameter matrix for characterization of the associated microwave network and systems. A key aspect of the approach is to treat the forced waves at the ports as constant voltage sources and induced propagating modal waves as additional entries (hybrid S-parameters) in the S-parameter matrix. The resulting hybrid S-matrix and voltage sources can be subsequently exported to any circuit solver such as HSPICE and Agilents Advanced Design System for the analysis of combined linear and nonlinear circuit terminations at ports. The proposed method is particularly suited for susceptibility analysis of cable bundles and PCBs for electromagnetic interference evaluations. It also exploits numerical techniques for structural and circuit domain characterization and allows for circuit design optimization without a need to perform any further computational electromagnetic analysis

S

We propose the use of high strength, metal-coated Kevlar yarns to weave flexible, conformal, and load-bearing antennas for an emerging class of applications emphasizing multiple functionality. In particular, here we present a unified, quantitative analysis of multiple properties of conductors as load-bearing materials in stress-, weight-, and shape-critical applications (e.g., in aerial vehicles), suggesting advantageous electrical conductor configurations to be metal-coated, multi-filament, high strength fibers. We then describe the fabrication of highly conductive metal coated Kevlar yarns, their mechanical and electrical properties, and the weaving of a flexible, stretchable, volumetric spiral antenna. The high frequency response of the antenna is found to match that of a traditionally made antenna comprised of electroplated copper on a rigid ceramic (Rogers TMM4) substrate. At low frequencies, the relatively lower conductivity of the metal-coated kevlar yarn leads to higher resistive losses compared to the traditional electroplated copper. We discuss strategies for mitigating such losses, and other means of improvement. More broadly, the results described here suggest a novel direction for multi-functional antenna design and applications, enabled by the superior mechanical characteristics of the composite conducting fibers, and the flexible, conformable, woven antenna architectures they help achieve.

-Parameters for Transmission Line Networks With Linear/Nonlinear Load Terminations Subject to Arbitrary Excitations

This paper presents embroidered conductive fibers (E-fiber) on polymer composites for conformal and light-weight RF circuits and antennas. A polydimethylsiloxane, namely PDMS, is adapted. Our earlier studies showed that it had a remarkable RF performance with a tangent loss of less 0.02 at RF frequencies. In this paper, an embroidered E-fiber microstrip line sample was fabricated with conductive fibers on a polymer substrate, and its RF characteristics were measured. The S-parameters were compared with those of copper microstrip lines. Measurements indicated that the insertion loss of the E-fiber microstrip line was only 0.04dB/cm higher than that of the copper. A multilayer microstrip line using similar fabrication techniques was also printed and measurements were conducted. All results clearly demonstrate the feasibility of the proposed embroidered conductive fiber technique for flexible and conformal RF applications.

High-Strength, Metalized Fibers for Conformal Load Bearing Antenna Applications

We present a hybrid analysis technique based on a decomposition approach to evaluate electromagnetic interference (EMI) effects on RF active and passive circuit components enclosed within multilayer cavities exposed to external electromagnetic waves. The adopted decomposition concept separates the analysis of the passive printed circuits and shielding cavity from the nonlinear components. The method of moments with a layered media Greens function is employed for the passive components and layered printed circuit board. For the nonlinear active devices circuit tools such as HSPICE or Agilent ADS are employed for their analysis. Using this approach, we investigated EMI effects on an enclosed RF power amplifier board with varying slot lengths on the shielding cavities. We found that external EMI could upset active circuits even in the shielded enclosure through a narrow slot aperture for certain frequencies of the interfering source.

Multilayer printing of embroidered RF circuits on polymer composites

A generalized method of moments (MoM)-SPICE iterative technique for field coupling analysis of multiconductor transmission lines (MTLs) in the vicinity of complex structures is presented. Telegraphers coupling equations are modified with additional distributed voltage and current sources for more accurate analysis of the total current induced onto transmission line bundles in the presence of complex structures. These additional voltage and current sources are introduced to enforce the electric field boundary condition and continuity equation on MTLs beyond the quasi-static regime. The surrounding structure is modeled via the MoM and a SPICE-like simulator is used to simulate equivalent circuit model of the MTLs extracted via the partial element equivalent circuit method. The proposed technique is based on perturbation theory with the quasi-static current distributions on the transmission lines still assumed to be dominant. Validation examples for single and MTLs are given in the presence of complex structures.

Hybrid Analysis of Electromagnetic Interference Effects on Microwave Active Circuits Within Cavity Enclosures

We introduce a novel class of flexible Radio Frequency (RF) electronics composed of conductive fibers on polymer or fabric substrates. The proposed fiber conductors and polymer substrates provide excellent RF characteristics, including mechanical flexibility and conformality. Key to the improved conductivity is the increased stitching density of the employed conductive fibers, reaching >;70 stitches per cm2. Prototype flexible antennas and circuits were fabricated and validated for their RF performance. These were realized by embroidering them on organza fabrics or by integrating them on thin polymer substrates. Their RF performance was found comparable to their conventional copper counterparts. Because of their excellent RF performance and high level of flexibility, these embroidered antennas should lead to a new class of devices expected to provide high data rate, low profile, and reliable operation for RF applications.