Subcell treatment of 90° metal corners in FDTD

A simple extension to the staircase FDTD algorithm yielding accurate subcell models of flat metal walls and corners is presented. Electric field nodes on the edges of staircase models of perfect electric conductors are replaced by values obtained using interpolation or analytic continuation into the metal. The algorithm is validated by computing the resonant frequencies of cavities     

Highly Accurate Debye Models for Normal and Malignant Breast Tissue Dielectric Properties at Microwave Frequencies

The finite difference time domain (FDTD) method is widely used as a computational tool for development, validation, and optimization of emerging microwave breast cancer detection and treatment techniques. When expressed in terms of Debye parameters, dispersive breast tissue dielectric properties can be efficiently incorporated into FDTD codes. Previously, we experimentally characterized the dielectric properties of a large number of excised normal and malignant breast tissue samples from 0.5 to 20 GHz. We subdivided the large database of normal tissue data into three groups based on the percent adipose tissue present in a particular sample. In addition, we formed a group of all cancer samples that contained at least 30% malignant tissue. We summarized the data using one-pole Cole-Cole models that were rigorously fit to the median dielectric properties of the three normal tissue groups and one malignant tissue group. In this letter, we present computationally simpler one- and two-pole Debye models that retain the high accuracy of the Cole-Cole models. Model parameters are derived for two sets of frequency ranges: the entire measurement frequency range from 0.5 to 20 GHz, and the 3.1-10.6 GHz FCC band allocated for ultrawideband medical applications. The proposed Debye models provide a means for creating computationally efficient FDTD breast models with realistic wideband dielectric properties derived from the largest and most comprehensive experimental study conducted to date on human breast tissue.     

Three-dimensional subgridding algorithm for FDTD

In many computational problems solved using the finite-difference time-domain (FDTD) technique, there is a need to model selected volumes with higher resolution than the whole computational space. An efficient algorithm has been developed for this purpose that provides the mesh refinement by the factor of two in each direction. The algorithm can be used in two-dimensional (2-D) and three-dimensional (3-D) problems and provides for subgridding in both space and time. Performance of the 3-D algorithm was tested in waveguides and resonators. A high accuracy and efficiency were observed in all test cases with insignificant (of an order of -60 dB) reflections from mesh interfaces. Practical applications of the algorithm in the analyses of a resonator with a dielectric rod and of a cellular phone behavior in the vicinity of the operator head are also reported     

A simple recursive formula for calculating the S-parameters offinite periodic structures

A simple and efficient recursive formula to calculate the S-parameters of large, finite periodic structures is introduced. The technique utilizes measured or computed S matrices of structures with a small number of periods. The formula is verified on a simple rectangular waveguide with periodic gratings. Results from the formula are plotted against similar results obtained from finite-difference time-domain (FDTD) simulations, and they show excellent agreement. This formula can be used to compute the S-parameters of large microwave or optical periodic structures, which, by virtue of their finiteness may not be open to the application of periodic boundary conditions     

Extension of FDTD method to nonuniform excitations

A new representation of a line source in the finite difference time domain method has been implemented, using the total/scattered field formulation. Verification for two problems with analytic solutions showed excellent accuracy. One application is the simulation of the exposure of a human body in close proximity to a 60 Hz transmission line     

Precision open-ended coaxial probes for in vivo and ex vivo dielectric spectroscopy of biological tissues at microwave frequencies

Hermetic stainless-steel open-ended coaxial probes have been designed for precision dielectric spectroscopy of biological tissue, such as breast tissue, over the 0.5-20-GHz frequency range. Robust data-processing techniques have also been developed for extracting the unknown permittivity of the tissue under test from the reflection coefficient measured with the precision probe and a vector network analyzer. The first technique, referred to as a reflection-coefficient deembedding method, converts the reflection coefficient measured at the probe's calibration plane to the desired aperture-plane reflection coefficient. The second technique uses a rational function model to solve the inverse problem, i.e., to convert the aperture-plane reflection coefficient to the tissue permittivity. The results of the characterization and validation studies demonstrate that these precision probes, used with the prescribed measurement protocols and data-processing techniques, provide highly accurate and reliable in vivo and ex vivo biological tissue measurements, including breast tissue spectroscopy.     

Realizing an electronically tunable reflectarray using varactor diode-tuned elements

Electronically tunable reflectarrays hold significant promise as cost effective architectures for RF beamforming applications. The successful operation of these arrays depends on the phase agility of the cells used to realize the array. This letter describes the design of an electronically tunable reflectarray based on a novel cell architecture that provides a large degree of phase agility. Experimental results demonstrating the beamforming capabilities of the array at 5.8 GHz are presented.     

Spherical wave sources in FDTD

A method is described which simulates the propagation of electromagnetic waves as spherical wave modes, approximated by an FDTD method. Modal equations in radius and time are discretized for explicit time-stepping. Angular functions are implemented analytically as required. Computed results for two examples are compared with analytic solutions - a resonator and a dipole near a conducting sphere - to demonstrate the validity of the method with very good agreement. This method is intended as a source condition in total/scattered FDTD methods, to allow for modeling of near-field object interactions without explicitly modeling the source     

Effects of mechanical flaws in open-ended coaxial probes for dielectric spectroscopy

A detailed study of dielectric properties of breast tissue in the 0.1 to 20 GHz frequency range currently under way uses open-ended teflon coaxial probes as sensors. This letter quantifies the effects of small mechanical imperfections at the probe aperture on the measured reflection coefficient. The mechanical flaws in the probe can lead to significant errors, thus probes for dielectric spectroscopy of breast tissue have to be carefully manufactured.     

Hopping conduction in “pure” polypropylene

A recently developed theory of steady-state conduction in high-density polyethylene is applied to “pure” polypropylene (PP) in the temperature range 50–93°C. Morphological changes occur in PP, including a disordered-amorphous to monoclinic-amorphous transition between 50 and 80°C, where, with increasing temperature T, free volume increases, and decreases with decreasing amorphous fraction. The latter competing processes lead to large increases in hopping site separation, λ, in the transition region, followed either by saturation or a maximum in λ vs. T. We speculate that segmental and/or main chain molecular motions lower apparent activation energies, are “pinned“ by applied field, and impeded by dangling bonds in regions surrounding the surfaces of crystallites. Our analysis is semi-quantitative only, because the latter mechanisms have not been adequately quantified, and the relative contributions of each are unknown. Measurements were carried out on heated and cooled disordered-amorphous, and 106°C, 17-h annealed, 43% crystalline samples. Hopping distances, obtained from measured current vs. applied field characteristics, ranged from 1.2 to 5.2 nm. Apparent activation energies up to 1.80 eV were obtained from In (I/T) vs. (1/T) plots. Remarkable plateaus in the temperature range of the transition were observed in these plots, implying some carrier conduction with near zero activation energy. Possible explanations for the latter, and the electronic nature of the carriers are given. X-ray and density flotation measurements enabled crystallinity determinations. © 1994 John Wiley & Sons, Inc.