An experimental setup and data reduction method has been developed for noninvasive high frequency electromagnetic impedance measurements of carbon nanotube (CNT)/epoxy nanocomposites. Using time domain reflectometry and parallel plate transmission lines, dielectric properties can be measured with the specimen under tensile loading. Good dispersion and addition of CNTs lead to an increase in high frequency dielectric constant of the nanocomposites. A strong strain dependence of the impedance is observed for the well dispersed nanocomposite while the baseline epoxy showed no strain dependence. A mechanism, based on an increase in CNT-CNT tunneling capacitance with applied tensile strain has been suggested. This research is expected to introduce a noninvasive characterization technique for studying electromagnetic properties of conductive nanocomposites.
Guided and leaky modes for a circular dielectric rod are analyzed in detail in this paper. By considering the field distributions, these modes are well defined and classified. The relations for the mode solutions using different types of special functions and Riemann sheets are figured out. Further, completed forms of characteristic equations used to solve different modes are presented explicitly. Asymptotic expansion method and Lambert W function are employed to derive the initial guesses around cutoff frequency, low frequency limit and high frequency limit for both TM and TE cases. The behaviors of complex transverse attenuation constants for proper and two types of improper modes with different cases are presented with some modes not shown in literatures.
In classical electromagnetics textbooks, the microwave circuits such as circulators, couplers, and filters are solved by non-systematic approaches such as even-odd mode analysis. Hence an electrical engineering student coming from the conventional circuit theory background encounters difficulties in understanding and solving microwave circuits. In this paper, we propose a modified node voltage analysis method in which the circuit branches are represented by their forward transmission matrices so that the electromagnetic wave propagation is taken care of. The Kirchhoff's current rule, tailored for high frequencies, is applied to formulate the simultaneous node voltage equations which are subsequently solved by matrix inversion. The proposed forward transmission matrix (FTM) method is applied to evaluate the S-parameters of some well-known microwave devices including the recently-developed metamaterialbased circuits. The FTM node analysis is a natural extension of the classical node analysis which is taught in the early stages of an Electrical Engineering program. Hence we anticipate that the proposed method will ease up the conceptual transition of electrical engineering students and academicians from the low-frequency alternating current circuits to high frequency RF and microwave circuits.
In this paper, a switch array antenna (SAA) with performance improvement for 77 GHz automotive FMCW radar to detect moving targets and near-field region (NFR) targets is proposed. The SAA generally mitigates hardware complexity, weight and cost, while it has technical difficulties to overcome depending on radar requirements. Firstly, a time-divided transmit by switch array to detect the moving targets causes a phase distortion of echo signals and generates considerably high and periodic side lobes of the SAA beam. In order to suppress side lobes more than 10 dB, a sub-array, a part of the SAA with non-distorted phases, is used to synthesize a compensation beam. Secondly, the SAA to detect the NFR targets, one of the radar requirements, deteriorates a beam performance by a spherical wave. To overcome a partial compensation method is proposed considering beam coverage of the radar operation at each target range. Some of the NFR targets to verify minimum range detection are simulated and the SAA beam gets back main lobe and has side lobes 15~25 dB suppressed with compensation. With the SAA designed, a detection performance of the radar is simulated. Two targets and radar system parameter are used, and signal processing tasks are included in simulation. Also outdoor test is carried out to verify that the proposed compensation methods enhance the detection performance of the radar with real targets. It is confirmed that the moving targets and NFR targets are well detected because multiple false targets are eliminated with the proposed compensation methods.
A worst-case tolerance synthesis problem for low-sidelobe sparse linear arrays is solved by using a novel self-adaptive hybrid differential evolution (SAHDE) algorithm. First, we establish a worst-case tolerance synthesis model for low-sidelobe sparse linear arrays, in which random position errors are considered and assumed to obey the Gaussian distributions. Through the random sampling, the random model is converted to a deterministic optimization problem. Then, a novel SAHDE algorithm is presented for solving the problem. As a modification to the existing hybrid differential evolution algorithm, a simplified quadratic interpolation (SQI) operator is used to tune the control parameters self-adaptively, establishing the connections between control parameters and the fitness values. In order to determine appropriate control parameter values quickly, a selection operation is also used. Detailed implementation procedure for the SAHDE algorithm is presented, and some numerical results show its effectiveness. Finally, for the deterministic optimization problem, we present a fast way for calculating its fitness values. The SAHDE algorithm is used to obtain optimal nominal element positions. Simulated results illustrate that the worst-case peak sidelobe levels for the sparse linear arrays are improved evidently. The SAHDE algorithm is efficient for solving the worst-case tolerance synthesis problem.
The model for penetration of a wire braid is rigorously formulated. Integral formulas are developed from energy principles for both self and transfer immittances in terms of potentials for the fields. The detailed boundary value problem for the wire braid is also set up in a very efficient manner; the braid wires act as sources for the potentials in the form of a sequence of line multipoles with unknown coefficients that are determined by means of conditions arising from the wire surface boundary conditions. Approximations are introduced to relate the local properties of the braid wires to a simplified infinite periodic planar geometry. This is used to treat nonuniform coaxial geometries including eccentric interior coaxial arrangements and an exterior ground plane.
We present a description of the electromagnetic field for the propagation invariant beams using scalar potentials. Fundamental dynamical quantities are obtained: energy density, Poynting vector and Maxwell stress tensor. As an example, all these quantities are explicitly calculated for the Bessel beams, which are invariant beams with circular cylindrical symmetry.
We present in this paper an analytical model for the calculation of the electromagnetic field in a slotless surface mounted permanent magnet machines. This model takes into account the two essential directions of magnetization of the magnets, namely tangential magnetization and radial magnetization. It especially accounts for the parallel form of the magnetization direction. The model uses equivalent currents on the surfaces of the magnets, and in an original way, it uses currents in all volume of the magnet. The model is validated by numerical results obtained with a free software of calculation of the electromagnetic fields (FEMM: Finite Element Method Magnetics) which uses the finite elements method and by an existing experimental structure. The model is extended to the magnets segmented in bars and separated. It is then extended to the shape of magnets close to parallepiped forms which are really used in large machines.
Investigation of a unit cell in terms of reflected wave amplitude and phase, for designing linearly polarized single layer Textile-Reflectarray (TRA) at C-band, is presented. The relative dielectric constant of the material is extracted using resonance method, and a WLAN antenna is designed to verify the accuracy of extracted material parameter. An error of 5% is observed in the extracted dielectric constant, when performance of WLAN antenna is measured at WI-FI Band (2.4 GHz). The extracted dielectric constant is used in the unit cell designing for TRA at the C-Band (5.8 GHz). The radiating element is made using laying technique with conductive thread. A square patch with a ring is selected after analyzing multiple geometries of the patch providing the required reflected phase range and low losses. By varying size of patch and ring of single layer unit cell in CST periodic environment, reflected phase range of 360 degree is achieved, which is required for RA designing. The solid copper ground plane at the bottom of unit cell is replaced with conductive shielded fabric with high level signal attenuation. Four different sizes of textile unit cells are fabricated using conductive thread, and the reflected phase and amplitude are measured using waveguide method. The simulated and measured results are compared when solid copper ground plane at the bottom of unit cell has been replaced with shielded fabric. The proposed method provides the first step towards designing flexible high gain textile reflectarrays.
Filtered backpropagation (FBPP) is a well-known technique used in Diffraction Tomography (DT). For accurate reconstruction of a complex-valued image using FBPP, full 360˚ angular coverage is necessary. However, it has been shown that by exploiting inherent redundancies in the projection data, accurate reconstruction is possible with 270˚ coverage. This is called the minimal-scan angle range. This is done by applying weighting functions (or filters) on projection data of the object to eliminate the redundancies. There could be many general weight functions. These are all equivalent at 270˚ coverage but would perform differently at lower angular coverages and in presence of noise. This paper presents a generalized mathematical framework to generate weight functions for exploiting data redundancy. Further, a comparative analysis of different filters when angular coverage is lower than minimal-scan angle of 270˚ is presented. Simulation studies have been done to find optimum weight filters for sub-minimal angular coverage. The optimum weights generate images comparable to a full 360˚ coverage FBPP reconstruction. Performance of the filters in the presence of noise is also analyzed. These fast and deterministic algorithms are capable of correctly reconstructing complex valued images even at angular coverage of 200˚ while still under the FBPP regime.