Mode-stirred Chamber Research Articles

Statistics of the quality factor of a rectangular reverberation chamber

The mean and standard deviation of the theoretical quality (Q) factor of a rectangular reverberation chamber are considered. Different averaging methods are investigated for deriving alternative expressions for its bandwidth-averaged value. Alternative basic assumptions are made regarding the distribution of the total energy density inside the cavity across the participating eigenmodes, thus providing alternatives for the assumption of equipartition of excitation energy. The physical reasons for such possible departures are explained on the basis of the stored and dissipated modal energy. For a given volume-to-surface ratio of a rectangular cavity, the theoretical arithmetic average Q (unlike the harmonic average) exhibits an explicit asymptotic dependence on the aspect ratios of the cavity. In the asymptotic high-frequency limit, the first-order dependence of the arithmetic Q on inverse frequency is governed by the imbalance between the TM and TE quality factors and by the aspect ratios of the cavity. Simulation results indicate better agreement between actual and smoothed theoretical arithmetic averages, particularly at lower frequencies, in comparison with those for the harmonic mean values. An expression for the distribution function of the arithmetic Q is formulated based on its statistical moments. We furthermore analyze the Q of a chamber with dynamically varying walls but constant average mode density. Such a chamber may serve as a model for mode stirring using flexible walls. The existence of a mode bunching effect which varies with tuner state but stabilizes with increasing frequency is shown. Effects of continuous dynamics of the cavity deformation on Q are discussed.

The Role of Oxygen on the Stability of Crevice Corrosion

Crevice corrosion can initiate in a spontaneously active metal/electrolyte system even with some appreciable oxygen concentration in the crevice electrolyte, as indicated by the measured instantaneous milliampere current and active potential values at the bottom of the crevice. However, for crevice corrosion to occur, the oxidant concentration has to be below a certain level, set by the metal/electrolyte system and the experimental conditions. Similarly, the oxidant concentration needs to be increased to some higher level to terminate an on-going crevice corrosion process. Reinitiation of crevice corrosion (on a passive crevice wall) is much more difficult than the original initiation. This is because the passive crevice wall provides a small ionic current which generates only a modest IR voltage that cannot shift the potential far enough on the crevice wall to reach the active region. Oxidant reduction at the anodic sites on the crevice wall resulted in a significant reduction in the ionic current, and hence in IR. This decrease in IR caused to increase to a value in the passive region which resulted in passivation of the entire crevice wall and termination of the crevice corrosion process. Conversely, removal of the oxidant caused to increase and to shift negatively back to the active region resulting in reactivation of crevice corrosion. © 2002 The Electrochemical Society. All rights reserved.

A model for the number of independent samples in reverberation chambers

AbstractAntenna efficiency measurements in reverberation chambers are done by averaging a number of sampled power levels, measured while the mode stirrer rotates. The number of independent samples is the number of stirrer positions that give uncorrelated fields in the chamber, and thereby uncorrelated power levels. A larger number of samples gives a better accuracy of the measured value. This report presents a proposed theoretical model for the number of independent samples of a reverberation chamber, thereby giving a better understanding of how to optimize a chamber design. Measured data from chambers with three different types of stirrers are compared with the use of the model. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 25–28, 2002; DOI 10.1022/mop.10220

Parallel Computing in Microwave Heating Analysis

The use of parallel computing in the finite element analysis of microwave heating applicators is discussed. Numerical results for a multiple feed cavity at 896 MHz and a cavity with a mode stirrer at 2.45 GHz are presented, and it is shown that for the two structures parallelism is most effectively introduced at different levels in the analysis.

FDTD calculation of quality factor of vibrating intrinsic reverberation chamber

The finite difference time domain (FDTD) method is evaluated in order to compute the quality factor of a vibrating reverberating enclosure using cavity theory. The computed results are compared to those obtained for a conventional mode-stirred chamber.

Electromagnetic measurement in a mode‐stirred chamber

This paper describes an electromagnetic-field distribution measurement technique used in a mode-stirred chamber. The technique is based on the electromagnetic infrared method developed at ONERA. The paper shows the electric-field distribution characteristics (average, min, max, standard deviation, and spatial distribution) at discrete frequencies. The efficiency of the method is demonstrated by showing that, with a few sets of measurements, the electromagnetic-field distribution in a mode-stirred chamber can be evaluated. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 28: 417–421, 2001.

Operation of electromagnetic reverberation chambers with wave diffractors at relatively low frequencies

The dependence of the modal density on chamber geometry is analyzed, for general unstirred reverberation chambers at relatively low frequencies of operation. An expression for the lowest usable frequency is derived based on the chamber mode density. The use of curved wave diffractors is investigated as a method for increasing the average mode density. The specific effect of corner, edge, surface and volume diffractors on the mode density and quality factor of a nominally rectangular shielded room is analyzed as a function of diffractor size, density, and operating frequency. This paper complements earlier studies on field statistics of undermoded reverberation chambers and the performance of acoustic-type diffusers in electromagnetic reverberation chambers.

Electromagnetic measurement in a mode-stirred chamber

Electromagnetic measurement in a mode-stirred chamber

Effect of local stir and spatial averaging on measurement and testing in mode-tuned and mode-stirred reverberation chambers

The effect of local averaging in mode-tuned and mode-stirred reverberation chambers is investigated, under the assumption of a wide-sense stationary statistical cavity field. The analysis is based on the characterization of the tuning or stirring process in the spectral spatial domain and in the spectral stir domain. The variance function, scale of fluctuation, optimum sampling rate, and normalized spectral bandwidths are computed for each case, based on the modeled power spectral-density function. This second-order analysis enables the effect of local averaging on the point-interval correlation, mean and standard deviation of the maximum test level, upward threshold crossing frequency, time to first passage and mean excursion length to be quantified. The theoretical results for quantifying EUT reliability are illustrated and compared with measured data. The results provide guidelines for the maximum tolerable sensor aperture and stirrer step sizes for mode tuning, and sampling rate and sampling width for mode stirring in chamber calibration and EMC emissions and susceptibility testing.

Young Red Spheroidal Galaxies in the Hubble Deep Fields: Evidence for a Truncated Initial Mass Function at approximately 2 M middle dot in circle and a Constant Space Density to z approximately 2.

The optical-IR images of the northern and southern Hubble Deep Fields are used to measure the spectral and density evolution of early-type galaxies. The mean spectral energy distribution is found to evolve passively toward a mid-F star-dominated spectrum by z approximately 2, becoming more sharply peaked around the 4000 Å break. We demonstrate with realistic simulations that hotter elliptical galaxies would be readily visible if evolution progressed blueward and brightward at z>2, following a standard initial mass function (IMF). The color distributions are best fitted by a "red" IMF, deficient above approximately 2 M middle dot in circle and with a spread of formation in the range 1.5<zf<2.5. Traditional age dating is spurious in this context; a distant elliptical can be young but appear red, with an apparent age greater than 3 Gyr independent of its formation redshift. Regarding density evolution, we demonstrate that the sharp decline in numbers claimed at z>1 results from a selection bias against distant red galaxies in the optical, where the flux is too weak for morphological classification, but is remedied with relatively modest IR exposures that reveal a roughly constant space density to z approximately 2, with 32 and 16 elliptical galaxies detected above and below z=1, respectively. We point out that the lack of high-mass star formation inferred here and the requirement of metals implicates cooling flows of preenriched gas in the creation of the stellar content of spheroidal galaxies. Deep-field X-ray images will be very helpful in examining this possibility.

Investigation of the field uniformity of a mode-stirred chamber using diffusers based on acoustic theory

This paper presents the combination of acoustic diffusors with a mode-stirred chamber to enhance the field homogeneity. Simulations with a TLM-based time-domain computer program show a more homogenous field in this advanced mode-stirred chamber as compared to a normal mode-stirred chamber. The efficiency of the diffusors is illustrated in comparison with a rectangular reference room. The simulations are verified by measurements in a mode-stirred chamber combined with two different types of diffusors. The two types of investigated diffusors are the maximum length diffusors and the quadratic residue diffusors. We also investigate the combination of both.

Qualitative Measurement of Heating Uniformity in a Multimode Microwave Cavity

Qualitative tests of heating uniformity in a domestic multi-mode cavity were made using thermal paper. The power distribution in the empty cavity was non-uniform. A stationary four-bladed mode stirrer modified the distribution; rotation at 5 rpm improved uniformity. A cylindrical quartz reactor and alumina-silica insulating cylinder were placed in the cavity, concentrating the field in the reactor and insulation. The field was stronger at the bottom of the empty reactor. Stainless steel sheathed thermocouples in the reactor perturbed the field slightly. When the reactor was filled with granular activated carbon the field was concentrated at the top of the reactor.

A reflection coefficient derivation for the Q of a reverberation chamber

Reverberation chambers (also called mode-stirred chambers) are multimoded cavities that are used for radiated emissions or immunity measurements. A reflection coefficient method is used to derive the quality factor (Q) of reverberation chambers of arbitrary shape. The results are applicable to walls of general materials, but reduce to the previous result for highly conducting walls with small skin depth. The reflection coefficient method is also used to derive the decay time of reverberation chambers.

Shocked quartz from the late Eocene: Impact evidence from Massignano, Italy

This paper reports the discovery of shocked quartz grains within a well-dated upper Eocene marlstone at the Massignano outcrop (Ancona, Italy). The shocked quartz reaches a maximum concentration of 1.6 ± 0.4 grains per cubic centimetre of bulk rock. A modest Ir anomaly is also coincident with the shocked quartz, and together they strongly support an impact origin. Moreover, this impact layer has a precise interpolated radiometric age of 35.7 ± 0.4 Ma. Although impact evidence found throughout the world indicates that at least two large impacts did occur during this time, the actual number, timing, and environmental effects of these impact events remain unclear. The late Eocene was a critical time in Earth history because of significant climatic, biotic, and environmental crises.

The effects of intrinsic test fixture isolation on material shielding effectiveness measurements using nested mode-stirred chambers

The paper evaluates the effects of test fixture isolation when using nested mode-stir chambers for conducting electromagnetic shielding measurements. The nested chamber technique is used by both government and industry to evaluate the electromagnetic attenuating properties of materials as varied as infrared sensor windows to the composites used in the hulls of new ships. Two cases are considered: (1) the test aperture provides little inherent isolation between the two chambers, and (2) the aperture provides reasonably good isolation. It is shown that in the former case, data taken in this fashion can exhibit significant deviation from the latter. A correction factor for the former case is developed. >

Effect of Bandwidth on Uniformity of Energy Distribution in a Multi-Mode Cavity

ABSTRACTThis paper reports on the results of experimental studies of the effect of bandwidth on the uniformity of energy distribution in a multi-mode cavity as well as on the development of a theoretical model to, at least qualitatively, describe the effect of varying the processing frequency. Using a mapping method reported in earlier work, Microwave Laboratories and Oak Ridge National Laboratory personnel have graphically measured the effect of processing bandwidth on heating uniformity. These results can be compared to the effects of other attempts to improve uniformity e.g., “mode stirrers.” Additional insight into the effect of bandwidth on time-averaged energy distribution within the cavity can also be gained from the simple theoretical model, which is presented for comparison with the experimental results. Finally, this paper uses the presented results to comment on the scalability of the variable frequency microwave processing methodology for large-scale applications.

Polymer Curing Using Variable Frequency Microwave Processing

ABSTRACTEpoxy resin was cured by microwave heating using a variable frequency furnace. Curing behavior and uniformity were characterized by visual inspection, hardness, and differential scanning calorimetry. Frequency sweeping was used as a means of mode stirring to minimize power nonuniformities in the furnace cavity. Cure times were reduced by half and with the appropriate post-cure treatment (also done in the microwave furnace) the resin had a glass transition temperature of 165–175°C. Small amounts of additives such as carbon black and various oxide pigments were tested to determine if they would improve microwave coupling and further reduce cure time. There was little or no effect at a standard concentration of 1 wt% pigment.

Statistical model for a mode-stirred chamber

The probability density functions for one- and three-dimensional fields in a mode-stirred chamber are derived and verified with chi-square goodness-of-fit tests on experimental data. Each of the three components of the field in the chamber is Rayleigh distributed, which is the same as chi distributed with six degrees of freedom. Each component of the power density is then exponentially distributed. Experimental data confirm these distributions, though unexpected high values, or outliers, were consistently found. Maximum-likelihood estimators of the functions' parameters are derived, and their accuracy is determined as a function of the amount of data. These results are applied to estimating chamber Q. The amount of data required for a given accuracy is determined. >

Untuned cavity resonator for use with pulsed infrared and far-infrared lasers

Absorption spectra, which are unaffected by sample scattering and reflection, can be obtained by placing the sample in an overmoded, high-Q resonator. As the wavelength of an exciting source is scanned, the changes in cavity Q caused by the corresponding variations in sample absorptivity are measured. When a cw microwave source is employed, a mechanical mode stirrer is often used to obtain the high level of intracavity field uniformity that is required. The advantages of this technique can be extended to higher frequencies if means can be devised to produce sufficient field uniformity when the cavity is excited with sub-microsecond pulses from a continuously tunable infrared or far-infrared laser. We demonstrate theoretically that an appropriately baffled spherical cavity whose surface is a uniform diffuser can satisfy this requirement without the intervention of mechanical mode stirring. Experimentally, we demonstrate that a uniform diffusing surface for these wavelengths can be made by coating the cavity wall with sub-millimeter size metal shavings mixed with metal-filled epoxy. A trade-off exists between the uniformity of the angular diffusion produced by these surfaces and their net reflectivity, which determines the cavity Q. Our results indicate that an adequate compromise is feasible.

Heterogenized bimetallic clusters: their structures and bifunctional catalysis

Tailored bimetallic catalysts have been prepared by using SiO 2-bound RhFe, PtFe and IrFe carbonyl clusters as the precursors. The Fe-containing cluster-derived catalysts exhibited strikingly high activities and improved selectivities for CH 3OH and C 2H 5OH formation in CO + H 2 reaction and for C 4 alcohols in propene hydroformylation, compared with those from the homometallic metal clusters. EXAFS and transmission electron spectroscopy, coupled with 57Fe Mössbauer studies on Fe 2Rh 4, Fe 3Pt 3 and Fe 4Pt carbonyl cluster-derived catalysts suggest that the RhFe and PtFe bimetallic particles have less than a 10 Å diameter, with Rh-Fe 3+-O and Pt-Fe 3+-O bondings. The role of Fe 3+ may be to anchor the ensembles of Rh and Pt atoms to prevent sintering. The iron promotion toward oxygenates is proposed to be associated with the heteronuclear activation of CO with the adjacent Rh-Fe 3+ Pt-Fe 3+ and Ir-Fe 3+ located at the metal cluster/SiO 2 interfaces, enhancing the migratory CO insertion into M-H and M-alkyl groups, reflected in the marked increase of C 1 + C 2 alcohols in CO hydrogenation. A series of bimetallic catalysts has been prepared from hexanuclear carbonyl clusters such as Rh 6− xIr x (CO) 16 ( x = 0–6) which were presynthesized inside NaY cages. EXAFS, FTIR for CO vibrational modes, and 129Xe NMR studies demonstrated that their derived bimetallic catalysts consist of bimetallic particles of less than 10 Å, uniformly distributed inside NaY zeolites and with homogeneous metal compositions similar to their precursor carbonyl clusters. The butane hydrogenolysis activities are remarkably suppressed by increasing a modest Ir content in the Rhir bimetallic catalysts. The activities and selectivities towards scission of the central C-C bond of butane are discussed in terms of breaking down the Rh ensemble size and electron-deficient sites of the Rhir particles inside NaY, based on the 129Xe NMR studies.