Showing papers by "Yahya Rahmat-Samii published in 2013"

Design and Implementation of Electro-Textile Ground Planes for Wearable UHF RFID Patch Tag Antennas

Abstract: Localization, wireless monitoring, and emerging body-centric wireless systems demand low-cost and low-power devices that are efficient, maintenance-free, and comfortable to wear. Patch-type UHF radio frequency identification (RFID) tag antennas realized using electro-textiles are promising candidates for this purpose. In this letter, we design a patch-type tag antenna operating at 900 MHz and investigate how different types of ground planes using electro-textiles affect the antenna performance. Various conductive fabrics and embroidery structures are considered. We demonstrate that depending on the ground plane structure and density, it is possible to influence the tag impedance behavior and radiation characteristics. Furthermore, wireless reflectometry measurements are conducted to characterize the sheet resistance for the investigated electro-textiles. We then use the sheet resistance in modeling the conductive fabric ground planes in a full-wave electromagnetic solver. Our results contribute to deeper understanding of the complex electro-textiles structures and guidelines for future practical wearable antenna designs.

Impact of Moisture and Washing on the Performance of Embroidered UHF RFID Tags

Abstract: Antennas can be seamlessly integrated into clothing by means of embroidery with conductive thread. In the real-life applications, wearable antennas are exposed to various environmental conditions. We studied the impact of moisture and washing on the performance of fully functional RFID tags based on embroidered antennas. The moisture absorbed in the tag structure can cause a temporary reduction in the tag read range. Repeated washing of the tag resulted in permanent reduction, which accumulated over the wash repetitions. Overall, the results indicated that protective coating is needed for sustained operation. Flexible, durable, and hydrophopic polymers, such as polydimethylsiloxane (PDMS), and polymer-ceramic composites are prominent candidates in terms of mechanical and electromagnetic properties.

Generalised correction to the friis formula: quick determination of the coupling in the fresnel region

Abstract: A simple yet effective correction term to the Friis formula is presented to improve its accuracy in the Fresnel region. Based on the concept of gain reduction factor, the proposed asymptotic term with an empirical coefficient can be widely applied to a variety of antennas without lengthy calculations. The information required in the computation is only the antenna gain, separation and wavelength for the boresight scenarios. Case studies of several representative antennas using full-wave simulations show that the accuracy of the estimated power transmission levels between two antennas in the Fresnel region can be improved by up to 2-5 dB, for both boresight and off-axis cases. Measurement results using two horn antennas also confirm the validity of the proposed correction term.

GTD, UTD, UAT, and STD: A Historical Revisit and Personal Observations

Abstract: In his landmark paper, dated February 1962, Prof. Joseph Keller detailed the notion of Keller's cone. He stated, “Geometrical optics, the oldest and most widely used theory of light propagation, fails to account for certain optical phenomena called diffraction....” I had a personal encounter with Keller's cone at a hotel in Florida! In the early morning on October 14, 2007, I witnessed Keller's cone on the door of my hotel room, resulting from edge diffraction from a TV stand due to the sun's rays coming through an opening of a window curtain. By now we know how to construct the local plane-wave behavior of the diffracted field along the diffracted rays, by invoking the fact that an edge forms one of the caustics of the diffracted wavefront. We also know that the diffracted field is of the order of k-1/2, in comparison to the geometrical-optics field, which is of the order of k0. In many antenna and scattering problems, this added diffracted term immensely enhances the accuracy of the total field. Due to the boundary-layer properties of the diffracted ray field along the incident and reflected shadow boundaries and also caustics, the original form of Keller's construction fails. These shadow-boundary shortcomings have been overcome through construction of the Uniform Theory of Diffraction (UTD, by Kouyoumjian and Pathak), the Uniform Asymptotic Theory (UAT, by Ahluwalia, Boersma, Lewis, Lee, and Deschamps), and the Spectral Theory of Diffraction (STD, by Rahmat-Samii, Ko, and Mittra). An overview and the salient features of these theories are revisited in a novel and unified manner, and a representative reflector-antenna example is highlighted.

Embroidered RFID tags in body-centric communication

Abstract: Wearable Radio Frequency Identification (RFID) tags for body-centric communication can be used in security, healthcare and biomedical applications. Embroidered tags using conductive threads are strong candidates for the implementation of wearable antennas. They provide several important features, such as flexibility, integrability, and light-weight structures. In this paper, the on-body performance of embroidered dipole-type ultra-high frequency (UHF) RFID tags is studied. First, modeling techniques for the embroidered tag antennas and for the human body are presented. The simulation models are then used to design and optimize an embroidered dipole tag to provide a read range of 2.5 m when separated from the human arm with a 1-mm thick layer of cotton fabric.

High-Frequency Techniques in Diffraction Theory: 50 Years of Achievements in GTD, PTD, and Related Approaches [Special section introduction]

Abstract: As stated in the title, this special section of the IEEE Antennas and Propagation Magazine was conceived to celebrate 50 years since the publications of two fundamental scientific works: the paper by J. B. Keller, “Geometrical Theory of Diffraction,” Journal of the Optical Society of America, 52, 1962, pp. 116–130; and the book by P. Ya. Ufimtsev, Method of Edge Waves in the Physical Theory of Diffraction, Moscow, Soviet Radio, 1962 (in Russian).

Durability of embroidered antennas in wireless body-centric healthcare applications

Abstract: Embroidered tags using conductive sewing thread are believed to be strong candidates for the implementation of wearable antennas in wireless body-centric healthcare applications. These applications demand wearable antennas that are durable and mechanically robust. Here, the durability of embroidered wearable tags is investigated using various washing procedures. The results provide important knowledge of how different water temperatures, detergents, and mechanical strain produced by the washing machine affect the embroidered tag performance. Using the attained results, we propose guidelines for increasing the embroidered tag durability and demonstrate that polydimethylsiloxane (PDMS) polymer is a potential candidate for its outstanding properties of being a soft, hydrophobic, heat resistant, low loss, and flexible material.

Experimental Demonstration of Reflectarrays Acting as Conic Section Subreflectors in a Dual Reflector System

Abstract: This paper experimentally demonstrates the use of a microstrip reflectarray as a low-profile planar substitute to a conic section subreflector (hyperboloidal type) in a symmetric dual reflector system at Ku-band. At first, a brief discussion on the simulation and measurement techniques utilized in the paper is provided. A nominal dual reflector Cassegrain system is synthesized through simulations where a feed horn is used to illuminate the hyperboloidal subreflector. Next, a flat metallic subreflector is placed at the subreflector location. This is a critical task as it shows the importance of phase compensation. Due to the flat subreflector, the feed is defocused from the image of the focus and creates phase aberration, leading to beam bifurcation, pattern degradation, and performance deterioration of the dual reflector system. A planar microstrip patch-type subreflectarray is then designed to mimic a hyperboloidal subreflector. Ray tracing is applied to the subreflector-feed system to calculate the phase needed to compensate for the axial defocusing of the feed. A prototype subreflectarray based on the ray-optics approach is fabricated. Radiation pattern measurements and back-projection holographic diagnostics demonstrate that the subreflectarray acts as a hyperboloidal subreflector and restores the antenna performance with a well-defined main beam and low side lobes.

Characterization of Electro-textiles Using Wireless Re∞ectometry for Optimization of Wearable UHF RFID Tags

Abstract: Flexible and maintenance-free wearable ultra-high frequency (UHF) radio fre- quency identiflcation (RFID) tag antennas are desired in wireless sensor systems in security, healthcare, and biomedical applications. Optimization of wearable antennas demands compre- hensive knowledge of the electrical properties of the conductive electro-textiles used in the antenna structure. In this paper, we flrst propose a wireless re∞ectometry technique to accurately extract the sheet resistance of various electro-textiles. The technique relies on the measurement of the resonant peak of a scatterer. Its bandwidth and magnitude are strongly correlated with the con- ductive losses of the strips so that a relationship can be extracted. We then demonstrate that the electro-textiles, including conductive fabrics and embroidery textiles, can be accurately modeled using the sheet resistance as the only conductive material parameter for wearable antenna design. 1. INTRODUCTION Wearable body-centric radio frequency identiflcation (RFID) supports the wellness of people and play a key role in the development toward independent living. Wireless monitoring of people by means of low power and low cost technology is nowadays considered as one of the most promising features of wearable body-centric RFID systems (1,2). These systems include electronic devices, and sensing and signal processing abilities to enable functions such as monitoring of human vital signs (3), human movement and tracking (4), and detection of parameters of the environment, e.g., temperature or toxic gases. Wearable antennas play a key role in establishing a reliable and an e-cient wireless communication link between body-worn electronics and the surrounding environment. These inexpensive antennas should be comfortable to wear and totally maintenance- free. Typically the wearable antenna is using conductive ∞exible materials that are easy to integrate with clothing (2). These conductive materials are known as electro-textiles and include embroidery structures created from conductive threads and commercially available conductive fabrics Wearable passive UHF (860{960MHz) RFID tag antennas are of great interest thanks to their low cost, easy integrability, and large communication ranges. They contribute toward an intelligent environment, where tag antennas will be seamlessly integrated with daily garments to enable wireless body-centric communication everywhere and at anytime (5). As new unconventional materials for wearable antennas are emerging, it is of paramount impor- tance to characterize their applicability in body-centric systems. Compared to conductive fabrics, embroidered structures are inhomogeneous. The conductivity is dependent on the direction of the current ∞ow in the structure and the structure stitching density (2). The thickness of the structure is not unambiguously deflned. This creates challenges in modeling the embroidered structures for wearable antenna design. In this paper we eliminate the uncertainty related to the embroidered structure thickness by using the sheet resistance as the modeling parameter. First, we apply wireless re∞ectometry to characterize the sheet resistance for two difierent embroidered patterns and for three difierent conductive fabrics. We then design and simulate dipole tag antennas using the measured sheet resistance as the only electro-textile modeling parameter. Finally, to verify the validity of the electro-textile model, we fabricate and measure dipole tag antennas in terms of their theoretical read ranges and we compare the measured read ranges with corresponding simulated values.

Spatial Diversity Performance of Multiport Antennas in the Presence of a Butler Network

Abstract: The main concern of this paper is to study the presence of a cascaded Butler network in diversity antenna systems. The advantages of a cascaded Butler network in 2-D multipath environments have been already shown. These advantages are limited to either correlated or nonuniform multipath environments with a line-of-sight component. In the frame of this paper, for the first time, we show a further advantage of a cascaded Butler network in 3-D multipath environments, including the effects of coupling. This advantage is associated with an improved convergence in received signals' covariance matrix in a multipath environment wherein the number of independent incoming waves is limited, i.e., nonrich multipath environment. For this purpose, we use a simulation tool in which a multipath scenario with certain number of incoming waves is emulated and some selected multiport antennas are exposed to it. Afterwards, the simulated random received signals are used to yield different performance metrics like spatial correlation. This advantage achieves considerable practical interest since a typical actual multipath scenario is most likely a nonrich multipath environment.

High-Frequency Techniques in Diffraction Theory: 50 Years of Achievements in GTD, PTD, and Related Approaches (Second Part)

Abstract: This is the second part of a special section of the IEEE Antennas and Propagation Magazine that - as stated in the title - was conceived to celebrate 50 years since the publications of two fundamental scientific works: the paper by J. B. Keller, ?Geometrical Theory of Diffraction,? Journal of the Optical Society of America, 52, 1962, pp. 116?130; and the book by P. Ya. Ufimtsev, Method of Edge Waves in the Physical Theory of Diffraction, Moscow, Soviet Radio, 1962 (in Russian).

Beam-tilted dipole-EBG array antenna for future base station applications

Abstract: A dipole-EBG array that utilizes the beam-tilting single element is presented. This enables down-tilted beam with a high directivity toward a mobile device. The proposed antenna design is fabricated and measured. The directivity of 11.2 dB is achieved at the angle of 25° at 3.5 GHz. This is higher than 9.8 dB of the directivity for the reference case that utilizes the broadside EBG-dipole single element. The array antennas exhibit a reasonable impedance matching across the operational frequency.

Electromagnetic Characterization of High Absorption Sub-Wavelength Optical Nanostructure Photovoltaics for Solar Energy Harvesting

Abstract: This paper presents a detailed numerical electromagnetic characterization of GaAs photovoltaic (PV) nanopillar array solar cells recently developed for solar energy harvesting. Through electromagnetic theory, full-wave simulations, and an optical measurement, a deeper understanding of the electromagnetic operation of these nanostructure arrays is achieved by revealing the mechanisms that allow for its inherent improvement of optical absorption over conventional PV solar cells. Initial investigations include incorporating and verifying material optical properties through measurements of bulk GaAs samples, simulating the effects of nanopillar geometry and configuration, and an analysis of the optical absorption mechanism of the nanopillar arrays through the graphical visualization of the electric fields in the vicinity of the nanopillars. These investigations will offer critical insights into the effects of pillar dimensions and configuration that can significantly increase optical solar energy absorption approximately 1.5 times that of conventional solar cells spanning the entire visible spectrum and for angles of incidence up to 60 $^{\circ}$ . Furthermore, comparisons between nanopillar arrays with and without substrates will demonstrate the mechanism that drives the efficient optical absorption. In addition, the importance of the nanopillar structure (i.e., dimensions) and reflecting substrate in providing energy coupling and improving the air-to-array interface will be discussed.

Sigmoid profiled septum: Evaluation of the parabolic reflector with the septum feed horn

Abstract: A sigmoid function is used to define a smooth profile for possible use in septum designs. In addition, Particle Swarm Optimization (PSO) is used to optimize the sigmoid-profiled septum polarizer. PSO is applied to a sigmoid septum as well as a stepped septum, and the performance of both designs is further compared. Both septum designs are selected to be optimized for a 25% bandwidth, and the impedance matching and axial ratio is analyzed. Furthermore, the reflector antenna system is investigated utilizing the optimized septum horns as a feed for a 15λ parabolic reflector antenna. A directivity of approximately 32 dB is attained for both parabolic reflector antenna systems.

Washing durability of embroidered polymer coated RFID tags

Abstract: Embroidered RFID tags can be used in fabric integrated on-body applications such as tracking and access control. Moreover, embroidered antennas are important also in other wearable, body-centric wireless systems. The durability of the embroidered tag antenna is essential for garment-integrated applications due to the high probability to encounter different environmental conditions. In this paper, the effect of washing on an embroidered RFID tag with a polydimethylsiloxane (PDMS) polymer coating is investigated by measuring the realized gain of the antenna before and after each washing procedure. It was found that coating the embroidered tag antenna with a polymer increased its washing durability.

Ensuring robust antenna designs using multiple diverse optimization techniques

Abstract: The use of multiple optimization algorithms to solve a design problem in electromagnetics can offer more reliable runs and further ensure robustness in finding the global optimum design. Utilizing multiple techniques can more thoroughly search the solution space, and the advantages of each algorithm can be exploited. The PSO and CMA-ES techniques are selected to demonstrate this due to the substantial differences in their exploration mechanisms in multi-dimensional spaces. These techniques are applied to mathematical benchmark functions as well as a stacked patch design for use in weather radar systems.

A Novel MEMS reconfigurable wideband E-shaped patch element for advanced cognitive radio base stations

Abstract: Cognitive radio offers new possible enhancements through its use of dynamic spectrum access, exploiting unused spectrum bands to expand bandwidth and overall system capacity. Development of cognitive radio systems has been underway, but there have only been a few antenna designs fitting the required functionality for these dynamic systems, which could benefit from dynamic antennas as well. The functionality required in a system also depends on the network architecture being utilized, and this research focuses on cognitive radio networks using a pre-existing infrastructure such as base stations and access points. For base station applications, directional patch antenna arrays can be implemented in order to provide sectoral access for nearby users.

Three-Parameter Elliptical Aperture Distributions for Sum and Difference Antenna Patterns Using Particle Swarm Optimization (Invited Paper)

Abstract: This paper presents a unifled analysis of the three- parameter aperture distributions for both sum and difierence antenna patterns, suitable for communications or telemetry applications with either a stationary or tracking antenna, and with the parameters automatically determined by Particle-Swarm Optimization (PSO). These distributions can be created, for example, by re∞ector, phased array, or other antenna systems. The optimizations involve multiple objectives, for which Pareto e-ciency concepts apply, and are accelerated by compact, analytical closed-form equations for key metrics of the distributions, including the far-fleld radiation pattern and detection slope of the difierence pattern. The limiting cases of the three-parameter distributions are discussed and shown to generalize other distributions in the literature. A derivation of the generalized vector far flelds provides the background for the distribution study and helps clarify the deflnition of cross-polarization in the far- fleld. Examples are given to show that the three-parameter (3P) distributions meet a range of system-level constraints for various applications, including a sidelobe mask for satellite ground stations and maximizing pointing error detection sensitivity while minimizing clutter from sidelobes for tracking applications. The equations for the relative angle sensitivity for the difierence pattern are derived. A study of the sensitivity of the 3P parameter values is presented.

EBG-dipole array antenna creating beam-tilt for base-station applications

Abstract: Summary form only given. For a typical base-station, tilted beam towards the ground has been employed in order to increase the signal reception from the mobile device. This can be realized with either an electronically controlled phased array antenna or a mechanically tilted array antenna. These techniques have been used; however, there exists inherent shortcomings with respect to performance at a tilted angle and practical implementation. For a common phased array, an array pattern is tilted toward a desired direction while element pattern stays toward the broadside. This mainly results in antenna gain reduction at the tilted angle. For the mechanically adjusted array, careful and stable installation is often required, which sometimes make it difficult to implement. Therefore, an enhancement in the conventional basestation array antenna is needed in order to achieve improved antenna performance with robust implementation. In this paper, a novel base-station array element design that consists of a single dipole mounted on an EBG ground plane is presented. Each dipole on an EBG structure provides tilted radiation pattern. This is achieved by employing 6×6 cell miniaturized EBG structure that is modified by connecting four of its unit cells. This EBG structure allows a low profile, high directivity, and beam-tilting single element design. With this dipole-EBG element, the radiation pattern can be steered from the broadside to the desired tilted angle which is suitable for the base-station applications. Based on the element design, an array antenna with four dipole-EBG elements is designed in order to verify the base-station like antenna performance. A down-tilt array pattern incorporated with a progressive phase is utilized in the array along with the tilted element pattern. To validate the proposed array, similar array that consists of non-modified standard EBG unit cell (a broadside element pattern) is employed as a reference. It is observed that the proposed array, compared to the reference case, achieves better performance with respect to the gain and sidelobe characteristics. The proposed EBG-dipole element incorporated into an array antenna is simulated using full-wave simulator. The proposed EBG antenna operates at 3.5 GHz, which covers part of WiMAX service bands. The directivity of more than 11.5 dB is obtained at the down-tilted angle of 25°. An impedance matching better than -10 dB S-parameter is attained across a reasonable bandwidth. The proposed dipole-EBG element array antenna should allow for performance efficient realization of future base-station antennas.

Wearable and implantable antennas for wireless body-centric sensing systems

Abstract: In this poster abstract, we present recent advances on implantable and wearable antennas for wireless body-centric sensing systems. Prominent examples of such systems are the wireless brain-machine interface (BMI) system and wearable embroidered antennas. In the poster, we present more detailed simulation and measurement results.

A comparative study between CMA evolution strategies and Particle Swarm Optimization for antenna applications

Abstract: Nature-inspired optimization techniques have been at the forefront of research within electromagnetics due to their unique properties as global optimization algorithms. These algorithms are stochastic techniques which direct the optimizer towards the most likely position based on previously tested points. The biggest question for current researchers in this area is which algorithm performs the fastest, provides the best solution, and offers robust convergence for a variety of different function topologies. Within the domain of nature-inspired optimization techniques, the Covariance Matrix Adaptation (CMA) Evolution Strategies (ES) and the Particle Swarm Optimization (PSO) techniques have transpired due to their rapid convergence for many electromagnetics optimization problems.

Accelerating the Spectral Domain Moment Method for reflectarray's by two-orders of magnitude

Abstract: The Spectral Domain Moment Method (SDMoM) as applied to reflectarray antennas is unique over that of the Space Domain Formulation in that the impedance matrix elements do not contain highly oscillatory exponential terms as they become absorbed in the transform definitions for the basis/testing functions when constructing the EFIE. Since a typical reflectarray contains thousands of elements and the design requires computation of a unique reflection phase curve for each, the computational demand is heavy. Understanding where these demands stem from allow development of methods to help alleviate, this paper both describes the root causes and provides remedies for each. When all remediation's are implemented, efficiency grows over two orders of magnitude as compared to that of the classical SDMoM.

Recent advances in evolutionary optimization techniques in applied electromagnetics

Abstract: This presentation will focus on: (a) a tutorial introduction to GA, PSO and CMA-ES by describing in a novel fashion the underlying concepts and recent advances for those who have used these techniques and for those who have not had any experiences in these areas, (b) a unique approach in performing fundamental comparative studies among these algorithms, (c) demonstration of the potential applications of these algorithms to a variety of electromagnetic and antenna designs, and (d) assessment of the advantages and the limitations of these techniques.

Letter to the editor: Comment on special sections on diffraction: Guest editors' reply

Abstract: In two recent issues of the Antennas and Propagation Magazine (specifically, Volume 55, No. 3, June 2013, and Volume 55, No. 4, August 2013), the history of diffraction and the associated chief contributors/scholars to same were impeccably presented.