Mode-stirred Reverberation Chamber Research Articles

A Novel Reverberation Chamber for In Vitro Bioelectromagnetic Experiments at 3.5 GHz

In this article, a mode-stirred reverberation chamber (RC) was designed and proposed for the first time as a cell culture incubator for in vitro electromagnetic waves exposure of adherent cells in tissue culture plates (TCPs). Typical cell incubators require specific conditions, such as temperature of 37 °C and humidity rate of 95%, which are challenging conditions for an RC. The chamber was characterized as an RC through an innovative experimental methodology based on the measurements of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> parameter of the emitting antenna. The proposed RC is adapted for in vitro bioelectromagnetic experiments for simultaneous exposure of up to 10 TCPs under highly homogeneous exposure conditions at 3.5 GHz, i.e., the mid-frequency band of the 5G telecommunication networks. Experimental results showed that the specific absorption rate (SAR) in the exposed samples extracted from temperature measurements was similar (an acceptable maximum variation lower than 30% was observed) in reason of the homogeneity and the uniformity of the field within the chamber. Specifically, measured SAR values were around 1.5 and 1 W/kg per 1 W incident, in 6-well or 96-well plates used for biological exposure, respectively. To validate our system, numerical simulations were performed. Overall, we showed that experimental and numerical SARs are in good agreement with differences <30% considering the standard deviation.

A Novel Bandstop Filter Based on Two-Port Coaxial Cavities for the Installation of Metallic Mode Stirrers in Microwave Ovens

Metallic stirrers are commonly used in mode-stirred reverberation chambers but are less common in microwave ovens, because the use of a metallic axis creates a coaxial port in the cavity wall where the axis is introduced, which may result in significant microwave leakage levels. However, due to electromagnetic, mechanical, temperature, or chemical requirements, metallic axes must be used in some cases. In this paper, we present a high power coaxial filter that enables the use of metallic stirrers in microwave applications. The filter employs two-port coaxial cavities that are analyzed to determine their design parameters. These cavities are coupled via coaxial lines to achieve proper attenuation levels. The cavities were designed using commercial electromagnetic (EM) software, and a filter prototype was validated using vector network analyzer (VNA) measurements. The simulated and measured results show that a three-stage configuration can achieve attenuations greater than 70 dB at the 2.45 GHz ISM band, allowing the use and external handling of the metallic axis inserted into microwave ovens with negligible microwave radiation leakage.

Diffuse field cross-correlations: Scattering theory and electromagnetic experiments.

The passive estimation of impulse responses from ambient noise correlations arouses increasing interest in seismology, acoustics, optics, and electromagnetism. Assuming the equipartition of the noise field, the cross-correlation function measured with noninvasive receiving probes converges towards the difference of the causal and anticausal Green's functions. Here, we consider the case when the receiving field probes are antennas which are well coupled to a complex medium-a scenario of practical relevance in electromagnetism. We propose a general approach based on the scattering matrix formalism to explore the convergence of the cross-correlation function. The analytically derived theoretical results for chaotic systems are confirmed in microwave measurements within a mode-stirred reverberation chamber. This study provides fundamental insight into the Green's function retrieval technique and paves the way for a new technique to characterize electromagnetic antennas.

A Novel Mode-Stirred Reverberation Chamber Design for 5G Millimeter Wave Bands

A novel mode-stirred reverberation chamber (RC) is designed and analyzed for millimeter wave bands. The size of the proposed RC with dimensions of 0.9 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.9$ </tex-math></inline-formula> m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times1.1$ </tex-math></inline-formula> m is suitable for user equipment (UE) tests. Two Z-shaped mode-stirrers are applied to generate a uniform field. To fulfill the communication and test requirements for user equipment, reference, measurement and link antennas with a large bandwidth are utilized, which can support both the frequency range (FR) 1 and FR2 bands of the 5G spectrum. The lowest usable frequency (LUF) and mode density were first calculated and examined. Then, the power transfer function (PTF) from 2 GHz to 40 GHz was obtained to verify the path loss. Three measurement campaigns were carried out to verify the accuracy of the RC. Firstly, the transmission power of the passive antenna for a conductive test was compared with the RC over the air (OTA) total radiated power (TRP) for single-tone signals. Then, signals with bandwidths of 100 MHz and 400 MHz were applied. A maximum power difference of only 0.8 dB was found in the above two experiments. Furthermore, the TRP for the same commercial 5G UE in both RC and compact antenna test range (CATR) systems was measured and compared. To the best of the authors’ knowledge, this is the first work to provide TRP results for 5G millimeter wave bands in a RC. The test results show that the TRP values for the two systems are almost the same with a maximum deviation of 0.5 dB, while the time required to carry out tests using the RC is lower than that for the CATR system.

Efficient assessment of well-stirred operation of reverberation chamber using coupling transfer gain functions

ABSTRACT This study presents a new efficient alternative procedure for the assessment of the well-stirred (ws) condition of a mode-stirred reverberation chamber (RC) using newly developed coupling transfer gain functions. Coupling transfer voltage and current gain functions of the RC environment are developed from the ABCD parameters of the measured two-port S parameters in an RC. The proposed procedure involves the determination of the frequency corresponding to the well-stirred operation () of RC through the assessment of RC EM-field distribution and checking of theindependent samples requirements for a particular stirred environment. The estimated of the RC using proposed method provides a faster way for the determination of the over-modded condition of RC when the placement of the two-antennas in imperative in RC. The minimization of antenna loading facotr results in more accurate esitmation of f wso using proposed procedure than only transfer characteristics.

Exposure Assessment in Millimeter‐Wave Reverberation Chamber Using Murine Phantoms

This study deals with the design and calibration of the first mode-stirred reverberation chamber (RC) in the 60-GHz-band adapted for in vivo bioelectromagnetic studies. In addition to the interface for electromagnetic and thermal dosimetry, the interfaces for lighting and ventilation were integrated into the RC walls while preserving acceptable shielding. The RC with mechanical and electronic steering capabilities is characterized in the 55-65 GHz range. To this end, murine skin-equivalent phantoms of realistic shape were designed and fabricated. Their complex permittivity is within ±12% of the target value of murine skin (6.19-j5.81 at 60 GHz). The quality factor of the RC loaded with an animal cage, bedding litter, and five murine phantoms was found to be 1.2 × 104 . The losses inside the RC were analyzed, and it was demonstrated that the main sources of the power dissipation were the phantoms and mice cage. The input power required to reach the average incident power density of 1 and 5 mW/cm2 was found to be 0.23 and 1.14 W, respectively. Surface heating of the mice models was measured in the infrared (IR) range using a specifically designed interface, transparent at IR and opaque at millimeter waves (mmW). Experimental results were compared with an analytical solution of the heat transfer equation and to full-wave computations. Analytical and numerical results were in very good agreement with measurements (the relative deviation after 90 min of exposure was within 4.2%). Finally, a parametric study was performed to assess the impact of the thermophysical parameters on the resulting heating. Bioelectromagnetics. 2020;41:121-135. © 2020 Bioelectromagnetics Society.

On the Uncertainty Quantification of the Quality Factor of Reverberation Chambers

Experimental characteristics of the normalized standard deviation and confidence intervals of the quality factor Q of a mode-stirred reverberation chamber are compared with theoretical results. Fully empirical (model-free) statistics based on spectrally averaged S-parameter data show good agreement with semianalytical estimates obtained from the theoretical Fisher-Snedecor distribution of Q with empirical distribution parameter M. For increasing frequency, the distribution of the boundaries of the sample confidence intervals rapidly tends toward Gauss normality. The location of the boundaries agrees closely with theory, while their spread reduces upon spectral averaging. Distributions and confidence intervals for the maximum and minimumQ are also obtained.

Accurate Characterization of Reverberation Chamber Resonant Modes From Scattering Parameters Measurement

This paper presents a novel approach to accurately extract the resonant modes of a mode-stirred reverberation chamber (RC) from measured transmission coefficient. This approach neither requires measurements with a fine stirrer rotation step nor assumes the pole continuity over a stirrer rotation. First, the number of modes to be extracted by the matrix pencil method needs to be overestimated in order to obtain most of the true (physical) modes with high accuracy. As this overestimation also implies the extraction of many spurious modes, the second step consists in automatically filtering these spurious modes based on a time-window increasing technique postprocess without a priori information. Performances of the introduced technique are first shown on a didactic example, especially in the presence of noise, and its limitations when increasing the modal overlap are investigated. Results obtained from measurement performed in a chaotic RC are then presented. The introduced approach allows extracting all the true cavity modes from measurement data while keeping the percentage of spurious modes below 3.5%.

Measuring the shielding properties of flexible or rigid enclosures for portable electronics.

Heaviside, in volume 1 of Electromagnetic theory, considered shielding of conducting materials in the form of attenuation. This treatment is still significant in the understanding of shielding effectiveness. He also considered propagation of electromagnetic waves in free-space. What Heaviside (1850-1925) could never have imagined is that 125 years later, there would be devices we know as mobile phones (or cell phones, handies, etc.) with capabilities beyond the dreams of the great science fiction writers of the day like H. G. Wells (1866-1949) or Jules Verne (1828-1905). More than this, that there would be a need for law enforcement agencies, among others, to use electromagnetically shielded enclosures to protect electronic equipment from communicating with the 'outside world'. Nevertheless, Heaviside's work is still fundamental to the developments discussed here. This paper provides a review of Heaviside's view of shielding and propagation provided in volume 1 of Electromagnetic theory and develops that to the design of new experiments to test the shielding of these portable enclosures in a mode-stirred reverberation chamber, a test environment that relies entirely on reflections from conducting surfaces for its operation.This article is part of the theme issue 'Celebrating 125 years of Oliver Heaviside's 'Electromagnetic Theory''.

Pulse Jitter, Delay Spread, and Doppler Shift in Mode-Stirred Reverberation

A time-domain characterization and stochastic model are developed for analyzing the evolution of the early randomization of fields inside mode-stirred reverberation chambers. The chamber is excited by a pulse amplitude modulated RF signal and stirred mechanically by a rotational stirrer. Phase noise (jitter) in the arrival times of a received pulse train is found to contain a systematic contribution that depends on stir speed and stir sense, as well as a random contribution governed by the rate of field depolarization. For the systematic component, a model of the phase delay for a uniformly orbiting scatterer inside a cavity is derived. For the random component, a nonhomogeneous Poisson model for arrivals of wave fronts in a nonstationary stir process exhibits good agreement with the empirical distribution of the measured jitter. At increasing stir velocities, the rate of field mixing increases less rapidly. The excess delay approaches asymptotically an Erlang distribution, in the limit of a homogeneous Poisson process for uniformly distributed late arrival times.

Use of a Cavity Loaded With a Conductive Liquid to Simulate In-Vehicle Wireless Multipath Channels

In this paper, we describe how we can finely adjust the electromagnetic losses inside a mode-stirred reverberation chamber (MSRC) using a device containing a variable quantity of conductive liquid. This device can be used for several applications such as decreasing the lowest usable frequency in MSRCs for electromagnetic compatibility (EMC) tests or controlling the root-mean-square (rms) delay spread inside a configured MSRC to reproduce the wireless multipath channel of different electromagnetic (EM) environments such as those inside vehicles. In this study, we will focus on wireless environment simulation applications. The rms delay spread is determined inside a cavity provided with apertures and a stirrer and loaded with a conductive liquid. It is compared to that obtained inside an automotive vehicle for different positions of a couple of wireless communication antennas. The comparisons show that, in our case, the static wireless multipath channel of a complex EM environment of a vehicle may be simulated inside such a cavity.

Probability Density Function of the Quality Factor for Reverberation Chambers Operating With Hybrid Stirring Including Frequency Stirring

In this letter, we deal with the probability density function (PDF) for the quality factor Q of reverberation chambers operating with combined mechanical and frequency stirring techniques. It will be shown the Q PDF of step-tuned and mode-stirred reverberation chambers (RCs), respectively, where the mechanical and frequency stirring are combined, and how it is affected by the width of the frequency sweep. The results for source-stirred RCs, where the frequency stirring is combined with source stirring, are implicitly obtained as well. By the results, possible effects on the measurements due to combinations of stirring techniques, including the frequency stirring, are also shown.

Low-amplitude, high-frequency electromagnetic field exposure causes delayed and reduced growth in Rosa hybrida

It is now accepted that plants perceive high-frequency electromagnetic field (HF-EMF). We wondered if the HF-EMF signal is integrated further in planta as a chain of reactions leading to a modification of plant growth. We exposed whole small ligneous plants (rose bush) whose growth could be studied for several weeks. We performed exposures at two different development stages (rooted cuttings bearing an axillary bud and 5-leaf stage plants), using two high frequency (900MHz) field amplitudes (5 and 200Vm−1). We achieved a tight control on the experimental conditions using a state-of-the-art stimulation device (Mode Stirred Reverberation Chamber) and specialized culture-chambers. After the exposure, we followed the shoot growth for over a one-month period. We observed no growth modification whatsoever exposure was performed on the 5-leaf stage plants. When the exposure was performed on the rooted cuttings, no growth modification was observed on Axis I (produced from the elongation of the axillary bud). Likewise, no significant modification was noted on Axis II produced at the base of Axis I, that came from pre-formed secondary axillary buds. In contrast, Axis II produced at the top of Axis I, that came from post-formed secondary buds consistently displayed a delayed and significant reduced growth (45%). The measurements of plant energy uptake from HF-EMF in this exposure condition (SAR of 7.2 10−4Wkg−1) indicated that this biological response is likely not due to thermal effect. These results suggest that exposure to electromagnetic field only affected development of post-formed organs.

Estimate of the Probability Density Function of the Quality Factor of Mode Tuned, Source Stirred and Mode Stirred Reverberation Chambers

The probability density function (PDF) of the quality factor Q of well-stirred mode tuned, source stirred, and mode stirred reverberation chambers (STRCs, SSRCs and MSRCs, respectively) is derived. The approach is based on the volumetric number of independent field samples, and a new physical model is introduced to estimate the ratio between the stored energy and the dissipated power, so that the stirring speed is taken into account for MSRCs. The ergodicity of the spatial-correlation function of the energy density in the chamber volume is initially assumed; in other words, the independent sample number of the energy density, which contributes to the Q , is assumed to initially be constant for each field distribution across the volume. Then, this assumption is relaxed and the theory is enhanced to the case where the size of the independent volumetric samples is considered random, as well as their number. By invoking the central limit theorem, the Q PDF is achieved. It is shown that the Q PDF is basically normal, and its standard deviation decreases with the increasing of the independent variables that contribute to the Q itself. The Doppler effect tends to increase the standard deviation of the Q PDF.

Over-the-Air Testing of MIMO-Capable Terminals: Evaluation of Multiple-Antenna Systems in Realistic Multipath Propagation Environments Using an OTA Method

A new over-the-air (OTA) testing method is required for evaluating multiple-antenna systems in realistic multipath propagation environments. Antenna design and propagation channels are the two key parameters that ultimately determine the multiple-input, multiple-output (MIMO) device performance. As antennas are inherently included in OTA testing, it is important to also consider realistic channel models for MIMO device-performance evaluation. This article shows that the multiprobe anechoic chamber (MPAC) setup is capable of emulating realistic and accurate multipath environments, making it a suitable method for testing terminals equipped with multiple antennas. In comparison to the mode-stirred reverberation chamber (RC) setup that represents an isotropic multipath environment, the MPAC method is capable of generating arbitrary spatial channel models with polarization characteristics.

Probability Distribution of the Coherence Bandwidth of a Reverberation Chamber

A theoretical probability distribution and associated statistics for the coherence bandwidth of an ideal mode-stirred reverberation chamber are derived. The stochastic model assumes and exploits the ergodicity of a dynamic wave chaotic cavity by expressing the coherence bandwidth in terms of the random effective excitation bandwidth and by replacing spatial averaging of transmitter-receiver locations with stir (ensemble) averaging. The theoretical model is validated through comparison with the empirical cumulative distribution function (cdf) extracted from measured S-parameter data from a real chamber, and through simulation using analytical calculations for a fictitious wall-stirred chamber. The results are particularly relevant to the improvement of transmission quality and uncertainty quantification of wireless multipath propagation.

Design and Experimental Validation of a Mode-Stirred Reverberation Chamber at Millimeter Waves

The purpose of this paper is to analyze the behavior of a mode-stirred reverberation chamber at millimeter waves. An experimental chamber with 0.423m × 0.412m × 0.383m dimensions has been built taking into account mechanical, electromagnetic, and measurement constraints for operation in Ka, U, and V bands. Its design warrants a relatively high quality factor (9 ×104 at 60 GHz), which is confirmed by measurements using frequency stirring. The statistical behavior of the experimental electromagnetic field inside the chamber in the 58.5-61.5-GHz range is consistent with that of a classical reverberation chamber. To the best of our knowledge, this is one of the first reverberation chamber designed and optimized around 60 GHz. It can be used as a test facility for electromagnetic interference and compatibility measurements at millimeter waves as well as for bioelectromagnetic applications.

Coupling Between Multipath Environments Through a Large Aperture

A key issue in measuring the shielding of buildings against wireless communications signals is presented. We use a mode-stirred reverberation chamber (RC) to emulate the EM coupling through windows or doors. This is achieved experimentally by leaving the RC door open. The room hosting the RC is identified as an external semi-reverberant space, and thus used to generate the stochastic field exciting the internal RC through the door opening. Two methods for the transmission parameter measurement and shielding effectiveness estimation are compared: the first one consists in moving receiving antenna within a volume, the second one makes use of a rotating stirrer. The comparison confirms the interchangeability of the two techniques provided a proper averaging factor is introduced. This argument supports a quasi-ergodic hypothesis in imperfect and semi-open reverberating environments. The measured aperture SE is compared to a theoretical value given by an ideal reverberating environment.

Evaluation Method for the Probability Distribution of the Quality Factor of Mode-Stirred Reverberation Chambers

An original experimental method for determining the empirical probability distribution function (PDF) of the quality factor (Q) of a mode-stirred reverberation chamber is presented. Spectral averaging of S-parameters across a relatively narrow frequency interval at a single pair of locations for the transmitting and receiving antennas is applied to estimate the stored and dissipated energy in the cavity, avoiding the need for spatial scanning to obtain spatial volume or surface averages. The effective number of simultaneously excited cavity modes per stir state, M, can be estimated by fitting the empirical distribution to the parametrized theoretical distribution. The measured results support a previously developed theoretical model for the PDF of Q and show that spectral averaging over a bandwidth as small as a few hundred kHz is sufficient to obtain accurate results.

Average Number of Significant Modes Excited in a Mode-Stirred Reverberation Chamber

Although the number of significant modes is intuitive, this concept has never been clearly defined, mainly because of the unbound number of modes involved in modal overlap. In the present paper, we show that, for a perfect stirring process, the effect of modal overlap can be modeled as an equivalent filtering formulation. By introducing the statistical-bandwidth concept, we show that the electromagnetic field statistics due to an infinite number of modes can be summarized by a finite number of significant modes. The case of the electric-energy density in an mode-stirred reverberation chamber has been considered and a new expression of its variability has been established. The good agreement found between the new expression and the experimental and simulation results support the several concepts introduced in this paper.