[Show abstract][Hide abstract] ABSTRACT: In a previous report it was shown that the channel capacity of an in vivo communication link using microscopic antennas at radiofrequency is severely limited by the requirement not to damage the tissue surrounding the antennas. For dipole-like antennas the strong electric field dissipates too much power into body tissues. Loop-type antennas have a strong magnetic near field and so dissipate much less power into the surrounding tissues but they require such a large current that the antenna temperature is raised to the thermal damage threshold of the tissue. The only solution was increasing the antenna size into hundreds of microns, which makes reporting on an individual neuron impossible. However, recently demonstrated true magnetic antennas offer an alternative not covered in the previous report. The near field of these antennas is dominated by the magnetic field yet they don't require large currents. Thus they combine the best characteristics of dipoles and loops. By calculating the coupling between identical magnetic antennas inside a model of the body medium we show an increase in the power transfer of up to 8 orders of magnitude higher than could be realized with the loops and dipoles, making the microscopic RF in-vivo transmitting antenna possible.
[Show abstract][Hide abstract] ABSTRACT: This paper studies the challenging problem of detecting a low radar cross-section target in heavy sea clutter by proposing a physics-based sea clutter generation model. The model includes a process that generates random dynamic sea clutter based on the governing physics of water gravity and capillary waves and a finite-difference time-domain electromagnetics simulations process based on Maxwells equations propagating the radar signal. A subspace clutter suppression detector is considered to remove dominant clutter eigenmodes. The improved detection performance over matched filtering is demonstrated using sea clutter model simulations.
ICASSP 2014 - 2014 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP); 05/2014
[Show abstract][Hide abstract] ABSTRACT: A computational formulation is presented for the low frequency single-cell finite-difference time-domain (FDTD) modeling of a perfectly electric conducting (PEC) sphere. The approach is based on the fact that the scattered field from electrically small objects can be expressed in terms of an electric and magnetic dipole. These dipoles can be decomposed with respect to the dipole moments that can be defined along the discrete field components that comprise the cell wherein the PEC sphere is inscribed. The dipole moment components couple to each other, and this mechanism is quantified by a quasi analytical coupled dipole approximation (CDA). The quasi-analyticity requires to substitute the involved dyadic Green's function (DGF) terms, in the CDA formula, by their numerically computed, FDTD compatible, equivalents. The material properties of the equivalent electric and magnetic spheres are derived using the quasi-analytical CDA that leads to expressions that resemble the Claussius-Mossotti mixing formula. The theoretically derived results are supported by numerical simulations.
IEEE Transactions on Antennas and Propagation 10/2013; 61(10):5333-5338. DOI:10.1109/TAP.2013.2271311 · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we demonstrate an efficient conformal broadband dipole antenna made of a lossy magneto-dielectric material. A closed form expression of the radiation efficiency of such a dipole is given which is derived using Schelkunoff's electrically small antenna model and the duality principle. The efficiency equation includes both complex permeability (μ r = μ'-j μ") and permittivity (ε r = ε'-jε") of the magneto-dielectric. A prototypical antenna is fabricated using commercially available NiZn ferrite tiles. The measured performance is shown to be better than a non-conformal conventional whip antenna.
IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Orlando, Florida, USA; 07/2013
[Show abstract][Hide abstract] ABSTRACT: We present a quasi-static model of the electromagnetic interaction
between dipolar emitting molecule and a Plasmon-metal nano-sphere. Based
on the image theory of dielectric sphere, we model the Plasmon
nano-spheres by off-centered dipole images. The retardation effect is
taken into account by electrodynamical modifications on spherical
polarizability and dipole radiation field. The modifications of the
radiative rate, total decay rate and the quantum yield for the molecules
are derived. The image model indicates the strong distance dependence of
the enhancement on both radiative and total decay rates. The comparison
with the exact electrodynamical model and other simplified models
indicate that the off-center image provide accurate predictions on
radiative and total decay rates, even in close distances.
Proceedings of SPIE - The International Society for Optical Engineering 02/2013; 8595:08-. DOI:10.1117/12.2004164 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is shown using full-wave simulations that several of the conventional assumptions made for extracting permeability data from a microstrip permeameter are not justified. In particular, the proportionality between the measured effective permeability in the device and the true permeability of the film is not a constant. It is a function of the permeability of the film, its geometry and the dimensions of the microstrip permeameter. A model exploiting the analyticity of the function relating effective to true permeability is used to derive this proportionality function for our device and the results are confirmed using full-wave simulations. The error incurred by not using this method and employing a reference sample for calibration or by using saturation magnetization “Ms” and anisotropy field “Ha” is shown to be anywhere between 5% and 40% and possibly even more. Our measurement set up is capable of measuring films as thin as 300 nm with a relative permeability as low as 10.
[Show abstract][Hide abstract] ABSTRACT: The stability analysis is presented for the finite-difference time-domain modeling of a Lorentz dielectric. In particular this study is focused on the Young scheme that employs the polarizability and the polarization current for the representation of the Lorentz permittivity, and collocates temporally the magnetic field with the polarization current. The Routh criterion is employed for the stability limit determination. The necessary conditions that ensure stability are thoroughly derived for the general as well as for limiting cases.
[Show abstract][Hide abstract] ABSTRACT: The behavior of the finite-difference time-domain method (FDTD) is investigated with respect to the approximation of the two-dimensional Laplacian, associated with the curl–curl operator. Our analysis begins from the observation that in a two-dimensional space the Yee algorithm approximates the Laplacian operator via a strongly anisotropic 5-point approximation. It is demonstrated that with the aid of a transversely extended-curl operator any 9-point Laplacian can be mapped onto FDTD update equations. Our analysis shows that the mapping of an isotropic Laplacian approximation results in an isotropic and less dispersive FDTD scheme. The properties of the extended curl are further explored and it is proved that a unity Courant number can be achieved without the resulting scheme suffering from grid decoupling. Additionally, the case of a 25-point isotropic Laplacian is examined and it is shown that the corresponding scheme is fourth order accurate in space and exhibits isotropy up to sixth order. Representative numerical simulations are performed that validate the theoretically derived results.
[Show abstract][Hide abstract] ABSTRACT: In [Electromagnetics 23 (2003) 187], a technique for injecting perfect plane waves into finite regions of space in FDTD was reported. The essence of the technique, called Field Teleportation, is to invoke the principle of equivalent sources using FDTDs discrete definition of the curl to copy any field propagating in one FDTD domain to a finite region of another domain. In this paper, we apply this technique of Field Teleportation to the original domain itself to create a transparent boundary across which any outward traveling FDTD field produces an exact negative copy of itself. When this copied field is teleported one cell ahead and one cell forward in time it causes significant self-cancelation of the original field. Illustrative experiments in two-dimensions show that a two-layer (10-cell thick) multi-stack Radiation Boundary Condition (RBC) with a simplest Huygens’s termination readily yields reflection coefficients of the order of −80 dB up to grazing incidence for all the fields radiated by a harmonic point source (λ=30 cells) in free space located 20 cells away from the boundary. Similarly low levels of artificial reflection are demonstrated for a case in which the RBC cuts through five different magnetodielectric materials.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes an alternative expression for the in-cell capacitance of a photoelectrical cell (PEC)-mounted slot, which is the conceptual cornerstone of the Gilbert-Holland subcell finite difference time domain (FDTD) model. By treating a slightly modified electrostatic problem, the extraneous charge singularity on the PEC edges touching the cell, which is characteristic of the originally proposed model, is removed. The latter offers better physical grounds for a new expression of the capacitance and the effective permittivity used in the update equations. High resolution standard FDTD simulation results are presented in support of the new expression.
[Show abstract][Hide abstract] ABSTRACT: This paper presents the theoretical design of an artificial
dielectric exhibiting narrowband frequency selective properties in the
bulk without relying on periodic placement of elements. In this manner,
it initiates a novel approach that bypasses the drawbacks of the
traditional frequency selective surfaces (FSS), namely, unwanted
passbands, dependence on excitation angle and polarization, and
difficulties in conversion from planar to curved geometries. The key
design elements are the concentric geometry of the inclusions and the
use of Lorentzian resonant media. A discussion of physical resonant
materials is presented, substantiating the credibility of the
theoretical design. To illustrate the approach, a novel complex medium
is synthesized as an ensemble of spherical particles composed of a lossy
core coated with a highly resonant dielectric layer and embedded into a
dielectric host. The resulting structure is an amorphous substance,
lossy over its entire spectrum except for two narrow-band transparency
windows, where it may become as lossless as desired. The parameter space
of the system is thoroughly analyzed which determines the type of
constitutive materials and geometries for tailor-designing the windows
according to specifications (shape, positioning and overall
normalization). In this sense, the lossy concentric structure forms an
ideal candidate for thin absorbing films (TAFs) with extensive
applications in antenna systems, RF absorbers, and anechoic chambers
IEEE Transactions on Antennas and Propagation 02/2000; 48(1-48):107 - 116. DOI:10.1109/8.827391 · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an alternative vector potential formulation of
Maxwell's equations derived upon introduction of a quantity related to
the Hertz potential. Once space and time are discretized, within this
formulation the electric field and vector potential components are
condensed in the same point in the elementary cell. In three dimensions
the formulation offers an alternative to finite-difference time-domain
(FDTD) method; when reduced to a two-dimensional (2-D) problem, only two
variables, instead of three, are necessary, implying a net memory saving
of 1/3 with respect to FDTD
[Show abstract][Hide abstract] ABSTRACT: All dielectrics, by nature are dispersive. This frequency dependence of materials must be modelled whenever microwave structures are expected to operate over broad bands of frequency. The Analytic Continuation Model of materials meets this need. As a compact, complete, ultrabroadband representation of all physically realizable dielectrics, this model can be used for the analysis and design of realistic electromagnetic engineering structures. The model represents dispersive materials as a sum of special complex functions reducible to analytic circuits. After a brief derivation of the model, an approach for obtaining the parameters of the circuits, in the case of dispersive radiofrequency materials, is disclosed. Its use is demonstrated by obtaining a model of a carbon loaded foam that is valid over a 2,000,000:1 bandwidth. The model is then applied to the problem of synthesizing materials with prescribed frequency dependent permittivity profiles for the modification of the electromagnetic properties of structures. Illustrative examples are solved using a Dispersive Finite Difference Time Domain computer code to demonstrate the simplicity and versatility of the model.
[Show abstract][Hide abstract] ABSTRACT: Electromagnetic phenomena can be simulated by the dynamics of a mechanical system as long as the Hamiltonian of the electromagnetic and the mechanical systems coincide. In this paper we present a generalization of G.F. FitzGerald's pulleys and rubber-bands mechanical model for the interaction of electromagnetic waves with complex media. We show a direct analogy between the FitzGerald model and the electric vector potential formulation, at each stage of the extension of the original model: each mechanical observable has a unique correspondence in the vector potential formulation. This strict analogy allows further inductive developments of the mechanical model and extends the pedagogical importance of the original FitzGerald model. As a consequence very complex materials from the electromagnetic point of view, such as frequency dependent magneto dielectric materials are easily understood and implemented with simple modifications in the mechanical system. The condense node representation of the field in the vector potential formulation results in lower grid dispersion compared to other numerical techniques such as the Finite Difference Time Domain (FDTD), We describe several applications, such as classical scattering problems from dielectric, magnetically permeable, dielectritcally lossy and Debye materials. The simulations are validated with comparison to canonical solutions, or with FDTD calculations.
[Show abstract][Hide abstract] ABSTRACT: The characteristics of the plane-wave incidence on a multilayer structure with planar periodic material blocks are presented. The layered media and the implanted blocks are either dielectric or magnetodielectric. Coupled volume integral equations in conjunction with the method of moments are used to determine the displacement electric and magnetic volume current densities within the implanted periodic blocks that are due to an incident plane wave. The displacement currents are treated as secondary sources to determine the transmitted and the reflected waves. The analysis is validated through the comparison with reflectometer measurements and a low-frequency effective medium approach.It is demonstrated that the structures are suitable for both narrow-band and wideband frequency-selective-layer (space filter) applications in millimeter waves, infrareds, and optics where metallic gratings are inappropriate. The presented analytic and numerical approach establishes the basis for the analysis of wave interactions with photonic bandgap material layers.
Journal of the Optical Society of America B 10/1997; 14(10). DOI:10.1364/JOSAB.14.002513 · 1.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Effective medium theories (EMTs) along with percolation theory allow the characterization of the electromagnetic properties of binary composite mixtures at a frequency when the scale of a particle size allows the quasi-static limit approximation. These theories formulate an effective permittivity and permeability (ε^eff and μ^eff) to predict the expected electromagnetic response of binary composite mixtures of a scale size L much larger than the average homogeneous size ξ within the mixture (the correlation length of percolation theory). It can be shown that any physically realizable material's permittivity (i.e. a complex permittivity which is causal and analytic in the upper-half complex frequency plane) can be represented as a sum of series LRC circuits. From the effective permittivity described by EMTs (away from the percolation threshold) or percolation theory (near the percolation threshold), the corresponding distributed circuit models are formulated to recast the expected composite material electric response into a more familiar form. This corresponding circuit model describes the dominant contributions to the composites material response at any frequency in terms of the individual LRC circuit elements of the permittivity dispersion's model.We are demonstrating an equivalent deterministic representation (which can be recast as a distribution of particle sizes and shapes within an ordered medium) in the LRC circuit model for the effective permittivity of composites. Future considerations will include extending this permittivity model to an analogous model of the permeability dispersion. Also, incorporating within the model a description of composite mixtures at higher frequencies (for the non-quasistatic case) when effects like the skin effect in high conducting particles must be accounted for.
Physica A: Statistical Mechanics and its Applications 07/1997; 241(1-2):334-337. DOI:10.1016/S0378-4371(97)00103-9 · 1.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: All dielectric materials, by nature, exhibit a frequency dependent permittivity. This property, termed dispersion, must be taken into account when materials are employed in applications that span very broad frequency ranges; such as ultra-wideband antenna applications and the design of Low Observable structures. Therefore, an accurate and compact representation of this frequency dependence over very broadbands of frequencies is needed. Such a representation can be developed by using the analytic function properties of the permittivity and a medium set of physical assumptions. The result is a compact sum of special analytic basis functions. Because of their analyticity, dielectric data obtained over convenient portions of the Radio Frequency spectrum can be continued into other portions of the spectrum where measurements may be more difficult to perform.
[Show abstract][Hide abstract] ABSTRACT: A new class of transmission lines is proposed which approximately preserves the shape of time domain pulses without sacrificing the manufacturing advantages of standard Monolithic Microwave Integrated Circuits (MMIC) transmission lines. By incorporating an inhomogeneity of specified conductivity and shape under the strip conductor of conventional microstrip, the substrate is given and apparent frequency dependent permittivity which undoes the dispersive behavior of the real part of the microstrip's effective dielectric constant. The rules for the design of the frequency dependent substrate are given and its behavior verified by using Finite Difference Time Domain calculations. Other possible applications of these transmission lines are discussed.