Krishna Naishadham

Measurement of the microwave conductivity of a polymeric material with potential applications in absorbers and shielding

The microwave conductivity of a new material, the polymer poly-p-phenylene-benzobis-thiazole (PBT) made conductive by ion-implantation doping with iodine, is measured at 9.89 GHz as a function of temperature using the cavity perturbation technique applicable to thin films of arbitrary shape. The DC and microwave conductivities of PBT are seen to approach asymptotically the low-temperature limit predicted by Motts energy-dependent hopping model. The potential utilization of conductive polymers in microwave absorbers and electromagnetic interference (EMI) shielding is examined using layered media EM theory. >

Shielding effectiveness of conductive polymers

The plane wave shielding effectiveness of two new materials, conductive polymers polyacetylene and PBT doped by ion implantation with iodine, is evaluated as a function of frequency, electrical thickness, doping, polarization, and angle of incidence. Conductivity of the polymers, measured in previous investigations by the cavity perturbation technique, is used to compute the overall reflection and transmission coefficients of single and multiple layers of the polymers. The polymers are found to display excellent shielding effectiveness (better than 40 dB over a substantial frequency band). With recent advances in synthesizing stable highly conductive polymers these light-weight mechanically strong materials appear to be viable alternatives to metals for EMI shielding. >

Application of spectral domain Prony's method to the FDTD analysis of planar microstrip circuits

Residual reflection from absorbing boundaries that truncate the computational mesh in the finite-difference time-domain (FDTD) method introduces significant error in the characterization of transmission lines and discontinuities employed in microwave and millimeter-wave integrated circuits. We apply the least squares Pronys method to accurately estimate the complex reflection coefficient (at Murs absorbing boundary) in the frequency domain by representing the sampled voltages along a microstrip transmission line as a plane wave superposition of incoming and outgoing transverse electromagnetic (TEM) waves at each reference port. Pronys method is used to compute the frequency-dependent effective dielectric constant of a microstrip line and the scattering parameters of microstrip circuit elements, which corroborate well with published and measured results. A new method is discussed to reduce the computer memory required to store the temporal samples which are employed in the spectral processing of the FDTD data. >

Dispersion of waves guided along a cylindrical substrate-superstrate layered medium

The effects of curvature and of the electrical parameters of thin dielectric layers deposited as superstrates on a perfectly conducting circular cylinder on the modal dispersion of waves guided tangentially along the outer (superstrate) layer of a two-layer geometry are examined. To chart the propagation characteristics of the layer-guided modes relevant to the three-dimensional (dipole-excited) Greens function for this geometry, it is necessary to solve the radial eigenvalue problem for the complex azimuthal propagation constants nu /sub p/( beta ), p 1, 2, . . ., which also identify poles of the nu -dependent spectral integrand of the Greens function. Here, beta is the spectral variable along the axial direction, with the Greens function synthesized as a double spectral integral over nu and beta . The pole locations are obtained numerically by solving the dispersion equation using Davidenkos method, and are parameterized in terms of layer radius, dielectric constant, and thickness. The dispersion relation, and hence the propagation constants, are shown to reduce correctly to the corresponding results for the planar geometry in the limit where the superstrate outer radius approaches infinity. >

Efficient prediction of radiation from printed transmission-line discontinuities

An efficient full-wave space domain Galerkin method of moments (MoM) is developed to compute the current distribution and the radiation associated with arbitrarily shaped microstrip (or printed transmission line) discontinuities. Several techniques are used to increase the efficiency of the MoM algorithm so that a circuit of moderate electrical size can be analyzed in reasonable time. These include the utilization of quasi-dynamic and far-field approximations of the microstrip Greens functions where applicable, as well as the various symmetries in the problem formulation. The influence of asymmetrical currents on the radiation from some representative microstrip discontinuities is examined. Sample computational results are presented to show that the current distribution and the radiation associated with resonant size structures can be significantly higher than the regulatory limits. The MoM algorithm is validated by comparing the computed current distribution and resonant frequencies of a microstrip transmission line with analytical results derived from quasi-TEM transmission line theory. >

A rigorous experimental characterization of ferrite inductors for RF noise suppression

Miniature inductors consisting of a coil wound on a high-permeability soft ferrite core material, find useful application in the suppression of EMI at RF and microwave frequencies. These inductors can be installed as either surface-mount devices (SMD) or as discrete chokes on slugs or beads. An understanding of the high-frequency parasitic and packaging effects, such as stray capacitance, magnetic losses, resonant characteristics, etc., can be gained from an equivalent circuit description of the inductor. We present a rigorous experimental method to fully characterize the behavior of wideband SMD ferrite inductors designed for maximum noise suppression at RF. The equivalent circuit parameters of the inductor, as well as the effective permeability, are extracted in closed form from an accurate measurement of the RF impedance.

Full-wave analysis of radiated emission from arbitrarily shaped printed circuit traces

A mixed (scalar and vector) potential surface integral equation formulation, developed for microstrip antennas, is employed in conjunction with the method of moments to predict the radiated emission from arbitrarily shaped printed circuit traces. Computed currents and radiated fields for a typical trace configuration in the form of a rectangular loop loaded by low- or high-impedance lumped loads indicate good agreement with transmission line theory and/or elementary loop antenna analysis, when the trace size is electrically small. Computed results are presented to highlight the radiation and coupling due to common-mode currents. >

Minimization of reflection error caused by absorbing boundary condition in the FDTD simulation of planar transmission lines

Residual reflection from absorbing boundaries introduces considerable error in the frequency-domain parameters of open-region planar transmission line components simulated in the time-domain. Various dispersive and super-absorbing boundary conditions have been developed to minimize this reflection. In this paper, a computationally efficient method, termed as geometry rearrangement technique (GRT), is proposed to correct the dominant reflection from absorbing boundaries by superposition of two subproblems with different source or boundary locations. The computational improvement of GRT is demonstrated by the FDTD simulation of dispersion in microstrip and coplanar transmission lines. A new method is discussed to accurately estimate the boundary reflection, and then applied to correct the characteristic impedance of planar transmission lines for boundary reflection.

Feasibility of noncontacting electromagnetic despinning of a satellite by inducing eddy currents in its skin. I. Analytical considerations

The servicing of disabled satellites requires that the units be despun before any repair can be done. This paper embarks on the feasibility study of an electromagnetically coupled despinning system based on inducing eddy currents in the satellite skin by an external dc magnetic field. The eddy current power input would act in such a way as to reduce the rotational energy of the satellite. The system does not contact the disabled satellite, and therefore, eliminates the hazards associated with physically attaching equipment, or astronauts with equipment, to the satellite. Two methods are investigated for inducing the eddy currents: (1) a large-diameter current loop positioned around the satellite such that the axis of the satellite rotation is in the plane of the loop, and (2) a magnet, either bar or U-shaped, positioned close to or surrounding the satellite. Closed-form solution for-the interacting magnetic flux, in terms of which the eddy current power input may be derived, is given for the two cases. Design parameters are evaluated for the two despinning systems in terms of input power and weight requirements. >

An efficient computation of transient scattering by a perfectly conducting cylinder

The real axis integration technique is developed to compute the oscillatory infinite spectral integrals occurring in transient scattering by a perfect electrically conducting cylinder excited by a magnetic line source. Davidenkos (1953) method is used to evaluate efficiently the complex azimuthal wavenumbers, namely, the complex zeros of the Hankel function in the order plane. The computed results of the transient Greens function compare very well with those obtained by the wavefront approximation at early time, and by the singularity expansion method at late time. The real axis integration method can be used to compute the transient response for all time. >