Browsing by Subject "Electric impedance"

Now showing 1 - 11 of 11

Open Access
Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)
(Institute of Electrical and Electronics Engineers Inc., 2014) Hafalir, F. S.; Oran, O. F.; Gurler, N.; Ider, Y. Z.
Images of electrical conductivity and permittivity of tissues may be used for diagnostic purposes as well as for estimating local specific absorption rate distributions. Magnetic resonance electrical properties tomography (MREPT) aims at noninvasively obtaining conductivity and permittivity images at radio-frequency frequencies of magnetic resonance imaging systems. MREPT algorithms are based on measuring the B1 field which is perturbed by the electrical properties of the imaged object. In this study, the relation between the electrical properties and the measured B1 field is formulated for the first time as a well-known convection-reaction equation. The suggested novel algorithm, called 'cr-MREPT,' is based on the solution of this equation on a triangular mesh, and in contrast to previously proposed algorithms, it is applicable in practice not only for regions where electrical properties are relatively constant but also for regions where they vary. The convective field of the convection-reaction equation depends on the spatial derivatives of the B1 field, and in the regions where its magnitude is low, a spot-like artifact is observed in the reconstructed electrical properties images. For eliminating this artifact, two different methods are developed, namely 'constrained cr-MREPT' and 'double-excitation cr-MREPT.' Successful reconstructions are obtained using noisy and noise-free simulated data, and experimental data from phantoms.
Open Access
Design of dual-frequency probe-fed microstrip antennas with genetic optimization algorithm
(IEEE, 2003) Ozgun, O.; Mutlu, S.; Aksun, M. I.; Alatan, L.
Dual-frequency operation of antennas has become a necessity for many applications in recent wireless communication systems, such as GPS, GSM services operating at two different frequency bands, and services of PCS and IMT-2000 applications. Although there are various techniques to achieve dual-band operation from various types of microstrip antennas, there is no efficient design tool that has been incorporated with a suitable optimization algorithm. In this paper, the cavity-model based simulation tool along with the genetic optimization algorithm is presented for the design of dual-band microstrip antennas, using multiple slots in the patch or multiple shorting strips between the patch and the ground plane. Since this approach is based on the cavity model, the multiport approach is efficiently employed to analyze the effects of the slots and shorting strips on the input impedance. Then, the optimization of the positions of slots and shorting strips is performed via a genetic optimization algorithm, to achieve an acceptable antenna operation over the desired frequency bands. The antennas designed by this efficient design procedure were realized experimentally, and the results are compared. In addition, these results are also compared to the results obtained by the commercial electromagnetic simulation tool, the FEM-based software HFSS by ANSOFT.
Open Access
Fast direct (noniterative) solvers for integral-equation formulations of scattering problems
(IEEE, 1998) Gürel, Levent; Chew, W. C.
A family of direct (noniterative) solvers with reduced computational complexity is proposed for solving problems involving resonant or near-resonant structures. Based on the recursive interaction matrix algorithm, the solvers exploit the aggregation concept of the recursive aggregate T-matrix algorithm to accelerate the solution. Direct algorithms are developed to compute the scattered field and the current coefficient, and invert the impedance matrix. Computational complexities of these algorithms are expressed in terms of the number of harmonics P required to express the scattered field of a larger scatterer made up of N scatterers. The exact P-N relation is determined by the geometry.
Open Access
Hybrid model for probe-fed rectangular microstrip antennas with shorting pins
(IEEE, 2000) Mutlu, Selma; Aksun, M. İrşadi
For a probe-fed microstrip antenna, it is quite common to employ the cavity model to find the field distribution under the patch and other electrical properties. Therefore, a multiport analysis technique based on the cavity model is usually employed to predict the input impedance of a probe-fed microstrip antenna with shorting pins. However, this approach does not provide any information about the field distribution under the patch with the shorting pins, which is usually used to calculate the radiation properties of the patch antenna. In this study, shorting pins are considered as current sources with unknown amplitudes, and the field distribution under the patch is obtained as a linear superposition of the contributions from each source via cavity model. Then, the unknown current densities over the shorting pins are determined by implementing the boundary condition of the tangential electric field on the pins. This is a hybrid approach because the field distribution is calculated from the cavity model, and the current densities over the shorting pins are obtained from the point matching of the resulting field distributions over the shorting conductors. The input impedance results found from this approach agree extremely well with those obtained from the multiport analysis, which shows that the proposed approach predicts both the input impedance and the field distribution under the patch. In addition, since the feeding probe is also made of PEC, the electric field under the patch should satisfy the boundary condition on this conductor as well. In the application of the cavity model, this is always ignored, with the assumption that the source probe is too thin to affect the field distribution under the patch significantly. In this study, the boundary condition of the electric field is implemented over the source, and its effect on the field distribution, in turn on the resonant frequency, is demonstrated.
Open Access
Investigation of the inaccuracy of the MFIE discretized with the RWG basis functions
(IEEE, 2004-06) Ergül, Özgür; Gürel, Levent
The inaccuracy of the magnetic-field integral equation (MFIE) discretized with the Rao-Wilton-Glisson (RWG) basis functions is discussed. The electric-field integral equation (EFIE) gives accurate results with the usage of RWG basis functions for the conducting surfaces with arbitrary planar triangulations. Two reasons i.e., logarithmic singularity in the field integration and solid angle expression in the MFIE, that lead to inaccuracy of the MFIE, are also discussed. It is found that the current distribution and the radar cross section (RCS) obtained by the MFIE does not perfectly match their counterparts obtained by the EFIE.
Open Access
Magnetic resonance electrical impedance tomography (MREIT) based on the solution of the convection equation using FEM with stabilization
(Institute of Physics Publishing, 2012-07-27) Oran, O. F.; Ider, Y. Z.
Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on reconstructing the internal conductivity distribution of a conductive object from the Laplacian of only one component of the magnetic flux density (∇ 2B z) generated by the internal current distribution. In this study, a new algorithm is proposed to solve this ∇ 2B z-based MREIT problem which is mathematically formulated as the steady-state scalar pure convection equation. Numerical methods developed for the solution of the more general convectiondiffusion equation are utilized. It is known that the solution of the pure convection equation is numerically unstable if sharp variations of the field variable (in this case conductivity) exist or if there are inconsistent boundary conditions. Various stabilization techniques, based on introducing artificial diffusion, are developed to handle such cases and in this study the streamline upwind Petrov-Galerkin (SUPG) stabilization method is incorporated into the Galerkin weighted residual finite element method (FEM) to numerically solve the MREIT problem. The proposed algorithm is tested with simulated and also experimental data from phantoms. Successful conductivity reconstructions are obtained by solving the related convection equation using the Galerkin weighted residual FEM when there are no sharp variations in the actual conductivity distribution. However, when there is noise in the magnetic flux density data or when there are sharp variations in conductivity, it is found that SUPG stabilization is beneficial.
Open Access
On the evaluation of spatial domain MoM matrix entries containing closed form Green's functions
(IEEE, 1997-07) Kinayman, Noyan,; Mittra, R.; Aksun, M. I.
The method of moments (MoM) is widely-used for the solution of mixed potential integral equations (MPIE) arising in the analysis of planar stratified geometries. However, the application of this technique in the spatial domain poses some difficulties since the associated spatial-domain Green's functions for these geometries are improper oscillatory integrals, known as Sommerfeld integrals, that are very computationally-intensive to evaluate. It is possible to eliminate the time-consuming task of computing these integrals by using closed form versions of the spatial domain Green's functions and the time required to evaluate the reaction integrals in the MoM matrix can be reduced considerably. Furthermore, the reaction integrals resulting from the application of the MoM can also be evaluated analytically by using piecewise linear basis and testing functions (Alatan et al., 1996). Hence, an efficient EM simulation algorithm can be developed by using the closed form Green's functions in the MoM formulation that involves no numerical integration. However, despite the time-saving realized from the analytical evaluation of the reaction integrals with the closed-form Green's functions, the need for further reducing the matrix fill-time is not obviated for many problems. Thus the objective of this paper is to present a hybrid technique for the evaluation of the MoM reaction integrals in a numerically-efficient manner that further reduces the time needed for their computation. A microstrip patch antenna is used as an example.
Open Access
Paraxial space-domain formulation for surface fields on dielectric coated circular cylinder
(IEEE, 2002-11) Ertürk, V. B.; Rojas, R. G.
A new method to evaluate the surface fields excited within the paraxial (nearly axial) region of an electrically large dielectric coated circular cylinder is presented. This representation is obtained by performing the Watson’s transformation in the standard eigenfunction solution and using the fact that the circumferentially propagating series representation of the appropriate Green’s function is periodic in one of its two variables. Therefore, it can be approximated by a Fourier series where the two leading terms of the expansion yield engineering accuracy in most cases. This work can be used in conjunction with a method of moments solution for the design/analysis of conformal microstrip antennas and arrays. Numerical results are presented and compared with a standard eigenfunction expansion.
Open Access
Proof-of-concept energy-efficient and real-time hemodynamic feature extraction from bioimpedance signals using a mixed-signal field programmable analog array
(IEEE, 2017) Töreyin, Hakan; Shah, S.; Hersek, S.; İnan, O. T.; Hasler, J.
We present a mixed-signal system for extracting hemodynamic parameters in real-time from noisy electrical bioimpedance (EBI) measurements in an energy-efficient manner. The proof-of-concept system consists of floating-gate-based analog signal processing (ASP) electronics implemented on a field programmable analog array (FPAA) chip interfaced with an on-chip low-power microcontroller. Physiological features important for calculating hemodynamic parameters (e.g., heart rate, blood volume, and blood flow) are extracted using the custom signal processing circuitry, which consumes a total power of 209 nW. Testing of the signal processing circuitry has been performed using ∼580 sec of an impedance plethysmography dataset collected from the knee of a subject using a custom analog EBI front-end. Results show the similarities of variations in heart rate, blood volume, and blood flow calculated using features extracted by the ASP circuitry implemented on an FPAA and a MATLAB digital signal processing algorithm.
Open Access
Spectrally accelerated biconjugate gradient stabilized method for scattering from and propagation over electrically large inhomogeneous geometries
(John Wiley & Sons, 2005) Babaoglu, B.; Altintas, A.; Ertürk, V. B.
Scattering from and propagation over rough-terrain profiles, as well as reentrant surfaces are investigated using an integral equation (IE)-based spectrally accelerated biconjugate gradient stabilized (SA-BiCGSTAB) method, with a storage requirement and a computational cost of O(N) per iteration, where N is the surface unknowns in the discretized IE. Numerical results in the form of current and path loss are presented and compared with previously published as well as measured results in order to assess the accuracy and efficiency of this method.
Open Access
A triple-band antenna array for next-generation wireless and satellite-based applications
(Cambridge University Press, 2016) Razzaqi, A. A.; Khawaja, B. A.; Ramzan M.; Zafar, M. J.; Nasir, S. A.; Mustaqim, M.; Tarar, M. A.; Tauqeer, T.
In this paper, a triple-band 1 × 2 and 1 × 4 microstrip patch antenna array for next-generation wireless and satellite-based applications are presented. The targeted frequency bands are 3.6, 5.2 and 6.7 GHz, respectively. Simple design procedures and optimization techniques are discussed to achieve better antenna performance. The antenna is designed and simulated using Agilent ADS Momentum using FR4 substrate (r = 4.2 and h = 1.66 mm). The main patch of the antenna is designed for 3.6 GHz operation. A hybrid feed technique is used for antenna arrays with quarter-wave transformer-based network to match the impedance from the feed-point to the antenna to 50. The antenna is optimized to resonate at triple-bands by using two symmetrical slits. The single-element triple-band antenna is fabricated and characterized, and a comparison between the simulated and measured antenna is presented. The achieved simulated impedance bandwidths/gains for the 1 × 2 array are 1.67%/7.75, 1.06%/7.7, and 1.65%/9.4 dBi and for 1 × 4 array are 1.67%/10.2, 1.45%/8.2, and 1.05%/10 dBi for 3.6, 5.2, and 6.7 GHz bands, respectively, which are very practical. These antenna arrays can also be used for advanced antenna beam-steering systems. Copyright © Cambridge University Press and the European Microwave Association 2014.