N.K. Uzunoglu

Focusing Properties of Dipole Arrays Placed Near a Multilayer Lossy Sphere

Focusing properties of dipole arrays placed near a three-layer lossy sphere are investigated analytically. An exact solution is obtained employing the spherical wave functions. The multilayer sphere is used to model the human head to investigate the feasibility of employing external dipole arrays in hyperthermia applications. Several array geometries and current excitations are considered operating at 432 and 915 MHz.

Measurement of Three-dimensional Temperature Distribution Inside Dielectric Objects Using Near-field Radiometry

The performance of near-field microwave radiometry systems to measure non-invasively the temperature distribution inside dielectric objects is analyzed. A rigorous solution based on the Greens function theory and the fluctuation-dissipation theorem is pursued to determine the power received by an aperture antenna located at the near field of the object to be measured. Resolution properties of this measurement technique are considered to estimate its imaging capabilities. The performance of antennas located at different distances from the object to be measured are examined. It is shown that contacting type antennas are superior as far as imaging properties are considered. Numerical results are presented for several cases.

Analysis of a Dielectric Resonator Antenna by Applying a Combined Semi-Analytical Method and Simulation

The radiation properties of a hemispherical dielectric resonator, consisting of a monopole antenna placed in a dielectric hemisphere is theoretically examined. A method based on the combination of Greens function theory, in conjunction with the Method of Auxiliary Sources (MAS), is used to solve analytically and calculate the electromagnetic field at any arbitrary point of the structure. The Greens function theory is used to calculate the response of the hemisphere to an excitation consisting of an elementary dipole placed inside its volume. The Method of Auxiliary Sources (MAS) is applied by distributing a set of elementary dipoles along the axis of the antenna probe. A number of matching points, lying on the surface of the cylindrical antenna, are used to calculate the dipole moments of the auxiliary sources, by imposing appropriate boundary conditions. Additionally, the input impedance and the current distribution on the antenna are also derived through application of the above-mentioned methodology. The numerical results are finally validated by comparing them with the simulation results of the relevant configuration elicited using a commercial simulation FEM tool.

Inverse scattering using a variational principle

A variational principle for the scattering amplitude is applied to obtain a relationship, in the Fourier domain, between the scattering potential and the scattered field recorded over a measurement line outside the potential. This relationship leads to a novel iterative reconstruction algorithm for recovering the distribution of the index of refraction from measurements of the scattered field. The proposed technique uses and builds upon the well-known filtered backpropagation algorithm of diffraction tomography. Computer simulations are presented that compare the reconstructions obtained with the proposed algorithm to the ones obtained with Borns approximation.

Propagation of electromagnetic waves on the lateral surface of a ferrite/semiconductor superlattice at quantum hall-effect conditions

The principal result of this work is the existence of coupled magnon-plasmon surface polaritons, i.e., nonradiative spin-electromagnetic surface waves on the lateral surface of a superlattice that consists of the alternating layers of GaAs-AlGaAs quantum well system and ferromagnetic insulator. Dispersion relations for the waves are considered at quantum Hall-effect conditions when a static quantizing magnetic field is perpendicular to the quantum well plane.

Vacuum Conditions and Microwave Output Characteristics of a Vertical Extraction Vircator

The Virtual Cathode Oscillator (Vircator) is a high power microwave tube with simple design principles, ease in operation, low maintenance and capability of operating in a wide range of vacuum conditions. The effect of residual gas pressure on the output of a low-voltage, vertical extraction Vircator is examined. It can be experimentally observed that a rough vacuum, in the order of 10−3 Torr, can still be adequate for normal Vircator operation. Higher pressures result in significant reduction of the power and duration of the microwave pulses. Deterioration on the output is rapid and microwaves cease for pressures over 10−2 Torr.

A generalised diffraction tomography technique based on non-linear optimization and Gaussian basis expansion of the scatterer

A novel method for the nonperturbative solution of the inverse scattering problem is presented. The method is based on the description of the unknown scatterer in terms of Gaussian basis functions and the discretization of the scattering integral equation (SIE) by applying a Gauss quadrature integration procedure. The inverse scattering problem is solved by minimizing the squared error between measured and predicted values for the scattering amplitude, subject to non-linear equality constraints imposed by the SIE for the field in the interior of the scattering object.

Analysis of the interaction between a layered spherical human head model and a helical dipole antenna

The interaction between a three-layer spherical head model and a helical dipole antenna is described by applying the hybrid method of Greens functions and auxiliary sources (Green/MAS). The main purpose of the paper is to provide a reliable tool for the solution of certain canonical problems pertaining to the dosimetric evaluation of potential health hazards in mobile telecommunications. As the analysis presented has a semi-analytical character, it can be used for benchmarking of purely numerical codes applied to the solution of more realistic geometries. Furthermore, it can give rough estimates of all the electromagnetic parameters of interest. The head is modeled by a three-layer lossy dielectric sphere whose dyadic Greens function is calculated. The antenna is replaced by a distribution of elementary dipole sources lying along the curved axis of the helical wire and the feeding gap. Appropriate boundary conditions are applied on the surface of the antenna leading to a linear system of equations. By solving the system, the unknown dipole moments are determined. Subsequently, the electric field at any point in space (including the interior of the head and the far field) as well as other parameters of interest such as the current on the antenna surface, the input impedance etc. can be easily calculated.

Pulse waveform design using time reversal for cardiac ultrasound imaging

The time‐reversal method is being used to design pulse waveform signals for cardiac ultrasound imaging. The objective is to remove the impact of the dominant scatterers of the human body located between the sensors and the heart’s near region. The proposed pulse waveform design method has been evaluated with numerical experiments that simulate the acoustical scattering effects of generic cardiac ultrasound imaging operations. Moreover, it is shown that the time‐reversal method, when it is being applied on a portion of the received signal, provides a direct real‐time assessment of the dominant scatterers’ relevant transfer function.

Acoustic source estimation based on physical modeling and optimization algorithm

The propagation of acoustic waves in the sea is analyzed by employing a horizontally stratified model for the sea medium with an arbitrary number of layers. Through this physical model, the acoustic field distribution, originating from an elementary source at an arbitrary location point inside the sea medium, is calculated by implementing a Green’s function approach. The proposed optimization algorithm is based on the minimization of the quadratic error function between estimated source signal values and signal values corresponding to known acoustic sources for the frequency content of the measured signal. Initial source estimation is based on the measured signal and the transfer function obtained by the above described propagation model. The minimization of the error function with respect to the model parameters described is carried out independently for each signal, corresponding to a known source, by employing the downhill‐simplex minimization method. The measured signal is positively recognized as one corresponding to a known acoustic source if the minimization procedure results in an error value less than a predefined limit.