Prasad K. Kadaba

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. >

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. >

Dielectric relaxation of acetonitrile and propionitrile in benzene and carbon tetrachloride solutions

Abstract Dielectric permittivities and losses have been measured at wavelengths of 3 × 104, 15.0, 10.0, 3.23, 1.95, 1.25, 0.60, and 0.40 cm at various temperatures between 24° and 60° C for benzene and carbon tetrachloride solutions of acetonitrile and propionitrile. The far-infrared spectra in the range of 10–150 cm−1 of dilute solutions of both the nitriles have been investigated. The effect of the far-infrared absorptions on the microwave data has been discussed. A single relaxation time has been obtained for acetonitrile at each temperature whereas the dielectric data of propionitrile have been analyzed in terms of two coupled relaxation processes. The thermodynamic parameters for dielectric relaxation and viscous flow have been reported.

Feasibility of noncontacting electromagnetic despinning of a satellite by inducing eddy currents in its skin. II. Design implementation

For pt. I see ibid., vol. 31, no. 4, p. 2471-7 (1995). The servicing of disabled satellites requires that the units be despun before any repair can be done. In a companion paper, we have presented the analytical considerations pertinent to the design of an electromagnetically coupled despin system based on inducing eddy currents in the satellite skin by an external dc magnetic field. Two configurations, namely, a current loop and a magnet, either permanent bar or U-shaped electromagnet, have been examined as the source of eddy current power input to the satellite. In this paper, we conduct parametric studies on the despin system designs, proposed in the earlier investigation, in order to comparatively evaluate these designs from the standpoint of reducing the despin time to a reasonable value. Computed results on the despin time and the eddy current power input indicate that times of the order of orbital daylight or less can be achieved with only moderate expenditure of power. >

Nonresonant absorption and dispersion of CF4Helium compressed gas mixtures

Abstract Nonresonant absorption of 20%, 30.7% and 55.5% mixtures by volume of CF4 in helium has been recorded at 28°C at a frequency of 1648 MHz ± 1 MHz at pressures up to 33 atmospheres. The data deviate considerably from the Debye equation for nonresonant absorption, and a sudden onset of extreme dielectric loss is observed at 33 atmospheres total pressure for the 30.7% mixture and at 18.5 atmospheres total pressure for the 55.5% mixture. The dielectric loss for the 20% mixture remains approximately constant within the range of pressures studied. The Clausius-Mossotti function is density dependent for the mixtures and pressures studied in contradiction to the classical Lorentz theory. The unusual dielectric behavior of the gas mixtures may be due to octupole interactions or to dimerization of CF4.

Relaxation mechanism in some dipolar mixtures

Abstract The dielectric constant and loss factor of the following mixtures have been studied near room temperature as a function of frequency in the microwave region: (1) nitrobenzene and o-nitrotoluene in n-dodecane in concentrations of 0.1, 0.2 and 0.3 mole fraction of each of the polar components; (2) binary mixtures of chlorobenzene and bromobenzene in various relative proportions. The static dielectric constants and the refractive indices of the above mixtures have also been measured. In the case of the ternary mixture, the value of the distribution parameter, α, has been found to increase somewhat in direct proportion to the concentration of the polar solutes. The loss-factor-frequency curves for this mixture seem to be a simple sum of the experimental curves for the corresponding single component mixtures. The Cole-Cole plots for the binary mixtures of halobenzenes show normal behavior. The dielectric data for both the mixtures have also been interpreted in terms of two partly superimposed Debye regions. The results for the mixtures studied are consistent with the assumption that the relaxation processes are those of molecules rather than larger liquid regions as proposed by some earlier workers for systems which they studied.

On the performance of tubular surface coils in nuclear magnetic resonance imaging and spectroscopy

Surface coils are important devices in the clinical application of nuclear magnetic resonance imaging and spectroscopy (NMRIS) because they have higher signal-to-noise ratios than body or head coils in superficial regions. This paper describes our theoretical and experimental study of the performances of tubular surface coils, aiding the effective application of such coils to NMRIS. We present formulas for the RF magnetic (H/sub 1/) fields produced by tabular surface coils placed over layered media, and for the self- and loaded-impedances of these coils. The calculated results show the dependence of the coil performances on the coil design parameters and the characteristics of the sample under test. We include the calculated results for the H/sub 1/ field phase shifts in conductive samples. >

On the application of moment methods to electromagnetic nondestructive evaluation

The application of moment methods (MMs) to eddy-current testing problems for nondestructive evaluation (NDE) is considered. A general formulation for the MM that can be used to analyze NDE problems is derived, and calculated results and experimental data obtained from eddy-current testing of an artificially made sample are presented. Good agreement between the calculated results and the experimental data confirms the validity of the method and shows that the MM can be used as an alternative to the finite-element method (FEM) and the boundary-element method (BEM) in NDE. >

Determination of dielectric constant in the microwave region by a four-terminal network method and analysis of the relaxation mechanism in some primary alkyl amines

Abstract The dielectric constant and loss of the following primary amines have been measured by a four-terminal network method at 10,3,2 and 1 cm from 0° down to the freezing points of the compounds in some cases. The experimental values agree very well with previous results obtained by Surbers method. Also measured were the static dielectric constant and the refractive indices of these compounds. The results have been analyzed in terms of a distribution of relaxation times between two limiting values with the aid of an IBM 360 computer for better accuracy. The results are consistent with the interpretation of a large relaxation time due to molecular rotation and a smaller relaxation time due to intramolecular motion.

Relaxation models for moist soils suitable at microwave frequencies

Abstract Relaxation models based on modifications of the Debye equation have been used in analysis of the complex dielectric constant results on various types of soils at microwave frequencies. A computer-based algorithm has been developed to provide accurate analysis through complex curve fitting. Results of this study suggest that Bergmann modification of the Debye equation seems to fit experimental data on 12% moisture content soils. Below 1 GHz one may need to consider phenomena such as residual surface effects and intermediate forms of bound water.