Mode-stirred Chamber Research Articles

Multi-physical field analysis based on a multi-material mode stirrer and interlayer for microwave processing fluid

Microwave technology is widely recognized for its efficiency in the chemical, food and energy industries, and has attracted great attention due to its ability to induce both thermal and non-thermal effects. However, the use of microwave reactors often leads to thermal runaway phenomenon. To address this challenge, a multi-material mode stirrer has been integrated with a microwave reactor structured with an interlayer, and the materials of mode stirrer and interlayer inner wall have been changed to address issues related to temperature uniformity. The combined effects of multi-material mode stirrer and different material inner walls on the microwave heating characteristics have been studied by using the evaluated coefficient of temperature variation (COV) and the mean fluids temperature. It is discovered that: (1) the effect of mode stirrer on heating characteristics depends on its material and the coefficient of temperature variation of Aluminum-Alumina mode stirrer is reduced by 44 %; (2) under the action of mode stirrer, the corundum inner wall can improve the heating efficiency, but the glass inner wall has a poor effect on it; (3) changing the structure materials of the microwave reaction kettle has little effect on the heating efficiency and the combination of Polyethylene-Aluminum mode stirrer and silicon carbide inner wall can improve the heating uniformity.

Comparison Between Shielding Effectiveness of Slotted Cavities in a Mode Stirred Chamber and Free Space

Comparison Between Shielding Effectiveness of Slotted Cavities in a Mode Stirred Chamber and Free Space

Study on the microwave heating temperature uniformity of an intelligent synergistic mode stirrer

Aiming at the intercoupling of multiple physics and the time-varying characteristics of boundary conditions in the microwave heating process, how to optimize the temperature uniformity of materials with a cooperative-mode stirrer is focused in this paper. First, the formation of standing waves can be effectively avoided by introducing the mode stirrer, thereby improving the heating efficiency of the material. Description of pattern stirrer motion processes using implicit functions and level set methods. Second, in order to optimize the temperature uniformity of the material, the stirrer is made intelligent, and the electromagnetic field distribution can be optimized by using different stirrer positions and directions, and the temperature uniformity is improved through intelligent coordination of stirrers in different positions. Finally, the finite element method is used to perform numerical simulation calculations on integer and continuous variables. The results show that the proposed method can improve the temperature uniformity of the conventional microwave heating model by 28.2%–81.44% and 23.2%∼63.91% at each level and lead hammer section respectively, and can improve the heating efficiency by 14.8%∼36.4%. According to its performance when heating different shapes and different materials, the proposed method is efficient and feasible.

Passive Antenna Characterization Through Impedance Correlations in a Diffuse Field

Ambient noise correlations allow the passive recovery of Green’s functions between two probes. Recently, the same approach has been applied to electromagnetism, but by correlating diffuse fields in mode-stirred chambers. Until now, only the correlation of S-parameters has been studied. However, it has very recently been shown that the result can be difficult to interpret. To overcome this limitation, a new approach is proposed in this article to directly estimate the self- and mutual impedances of two coupled antennas from impedance correlations. The theoretical developments presented are validated experimentally in a reverberation chamber (RC) excited by a single antenna where mechanical and source stirring techniques are combined to generate a sufficiently diffuse field environment. It is shown, with antennas of different properties, that this approach allows reconstructing with good accuracy the complex impedance matrix between two receiving antennas as well as the transmission coefficient between them. The extracted gain pattern, in good agreement with that measured in an anechoic chamber, shows the good sensitivity of the proposed passive characterization technique.

Compact MIMO System Performances in Metallic Enclosures

In this work, we present a 2 × 2 near-field multi-input multiple-output (MIMO) prototype for bit-error-rate (BER) and error vector magnitude (EVM) measurements in a metal enclosure. The near-field MIMO prototype was developed using software-defined-radios (SDRs) for over-the-air transmission of QPSK modulated baseband waveforms. We checked the near-field MIMO BER and EVM measurements in three different scenarios in a highly reflecting metal enclosure environment. In the first scenario, the line-of-sight (LOS) communication link was investigated when the mode stirrer was stationary. In the stationary channel conditions, near-field MIMO BER and EVM measurements are performed. In the second scenario, BER and EVM measurements were performed in dynamic channel conditions when the mode stirrer was set to move continuously. In the third scenario, LOS communication near-field MIMO BER and EVM measurements were performed in stationary channel conditions but now in the presence of MIMO interference. In three different scenarios, near-field MIMO BER and EVM measurements were investigated at different Tx USRP gain values and in the presence of varying levels of MIMO interference.

Finite Element Simulation and Experimental Research on Uniformity Regulation of Microwave Heating of Composite Materials.

As an attractive alternative to the traditional autoclave curing process, microwave curing has been widely used for manufacturing high-performance composites. However, the nonuniform temperature distribution during composite curing is the main problem faced by the microwave curing process, which limits its application in the aerospace industry. This paper studied the regulating effects of cavity structure and mechanical optimization methods on the uniformity of the microwave field by numerical analysis and finite element simulation, and an octagonal microwave heating device with multi-microwave generators, mode stirrers and mobile platform was developed independently and the experimental verification were finally carried out. The results showed that compared with the traditional heating device, the T800/602 carbon fiber reinforced composite laminates cured in the regulating device of microwave heating uniformity established in this paper had more uniform temperature field distribution, a more synchronous curing process and lower residual stresses.

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.

Electronic Mode Stirring for Improved Backscatter Communication Link Margin in a Reverberant Cavity Animal Cage Environment

Neuroscience research in nonhuman primates (NHPs) often requires multiday neural recordings from freely moving animals inside their home cages, making ultralow-power uplinks using wireless backscatter communication highly desirable. Previous work reveals that the channel transfer function (CTF) of a standard NHP home cage in the 915 MHz and 2.4 GHz industrial, scientific, and medical (ISM) bands resembles a resonant cavity exhibiting deep nulls throughout the cage volume, which are particularly acute for round-trip backscatter paths. In this work, we investigate a novel application of passive antenna mode stirring via switched parasitic antennas (SPAs) to reduce the magnitude and prevalence of deep nulls in the cage CTF. We present a system leveraging four cage-ceiling-mounted SPAs with two dynamically controlled impedance states each, yielding 16 total mode stirring configurations. We compare the frequency-domain power ratio measurements at 126 positions throughout the cage taken with and without passive antenna mode stirring. In the 915 MHz ISM band, the optimized SPA configuration improved the maximum two-way insertion loss in 68% of testing locations, reducing the worst case two-way insertion loss by 60.2 dB. In the 2.4 GHz ISM band, the maximum two-way insertion loss was improved in 53% of testing locations, reducing the worst case two-way insertion loss by 35.6 dB. This approach eliminates the deepest nulls in the cage volume and leads to significantly improved link margin for a backscatter-based wireless brain–computer interface (BCI).

One-time Electromagnetic Irradiation Modifies Stress-sensitive Gene Expressions in Rice Plant.

Electromagnetic energy is utilized over multiple frequency bands to provide seamless wireless communication services. Plants can well perceive electromagnetic energy present in open environment due to reasonably high permittivity and electrical conductivity of constituent tissues. Moreover, higher surface-to-volume ratio of plant structure facilitates increased interaction with the incident electromagnetic waves. To date, a few well-designed studies have been conducted inside controlled electromagnetic reverberation chambers to investigate either short duration-low amplitude or long duration-periodic electromagnetic irradiation-induced molecular responses in plants. However, as far as is known, studies investigating molecular responses particularly at the mid-vegetative stage in plants following one-time (hours-long) electromagnetic irradiation have not been reported earlier. Hence, the present study aimed at investigating molecular responses in 40-day-old Swarnaprabha rice plants following one-time 1837.50 MHz, 2.75 mW/m2 electromagnetic irradiation of 2 h 30 min duration. Controlled electromagnetic irradiation inside a simple reverberation chamber was ensured to achieve pure electromagnetic environment at 1837.50 MHz with deterministic electromagnetic power density at selected position. Swarnaprabha rice plant was chosen for this investigation since the rice variety is widely cultivated and consumed in the Indian subcontinent. Subsequent alterations in some selected stress-sensitive gene expressions were assayed using real-time quantitative polymerase chain reaction technique-significant upregulation in calmodulin and phytochrome B gene expressions were noted. This investigation was purposefully focused on subsequent molecular responses immediately following electromagnetic irradiation so that the possible effects of secondary stimulations could be avoided. Observed molecular responses strongly suggested that plants perceive 1837.50 MHz, 2.75 mW/m2 electromagnetic irradiation similar to other injurious stimuli. © 2021 Bioelectromagnetics Society.

Double pendulum mode stirrer for improved multimode microwave heating performance

This is a common way to improve the uniformity of microwave heating by using a mode stirrer. However, the conventional mode stirrer is a reciprocating periodic motion, which in some cases does not effectively improve the uniformity of microwave heating. In this study, a mode stirrer with double pendulum structure was creatively proposed. Due to the chaotic characteristics of the double pendulum motion process, the mode stirrer process is no longer a traditional periodic motion, and this chaotic stirrer process will affect the uniformity of heating. Based on the finite element method combined with the implicit function and level set method, the microwave heating process with double pendulum mode stirrer was calculated, and the multiphysics coupling simulation of structure-electromagnetic-heat transfer was realized. The heating performance under different double pendulum mode stirrers was studied and compared with the heating results of the single pendulum mode stirrer. The results show that the double pendulum mode stirrer with a specific initial position can effectively improve the heating uniformity without reducing the heating efficiency.

Uncorrelated configurations and field uniformity in reverberation chambers stirred by reconfigurable metasurfaces

Reverberation chambers are currently used to test electromagnetic compatibility as well as to characterize antenna efficiency, wireless devices, and MIMO systems. The related measurements are based on statistical averages and their fluctuations. We introduce a very efficient mode stirring process based on electronically reconfigurable metasurfaces (ERMs). By locally changing the field boundary conditions, the ERMs allow us to generate a humongous number of uncorrelated field realizations even within small reverberation chambers. We fully experimentally characterize this stirring process by determining these uncorrelated realizations via the autocorrelation function of the transmissions. The uniformity criterion parameter σdB, as defined in the IEC 61000-4-21 standard, is also investigated and reveals the performance of this stirring. The effect of short paths on the two presented quantities is identified. We compare the experimental results on the uniformity criterion parameter with a corresponding model based on random matrix theory and find good agreement, where the only parameter, the modal overlap, is extracted by the quality factor.

Diffuse field cross-correlation in a programmable-metasurface-stirred reverberation chamber

Programmable metasurfaces can endow complex scattering environments with reconfigurability. Here, we make use of these configurational degrees of freedom to retrieve the impulse response between two passive antennas via cross-correlation techniques. An ensemble of stirred chaotic wave fields in a reverberation chamber (RC) can play the role of thermal noise in passive Green's function retrieval. Instead of using a conventional mechanical mode stirrer, we generate the ensemble of RC configurations with random configurations of a programmable metasurface. We adapt the data processing of the diffuse field cross-correlation technique to this stirring mechanism, which is, given the size of the RC, nominally inefficient, and we investigate the convergence of the cross-correlated signals toward the impulse response. Finally, we apply our scheme to phaseless imaging in complex propagation environments, with potential applications in indoor context-awareness.

A New Mode Stirrer Design for the Reverberation Chamber

In this paper, a mode stirrer composed of random positioned metal plates is proposed for reverberation chamber. The designing procedure of the mode stirrer is presented. The designed stirrer is compared with the common Z-shaped stirrer in both simulation and measurement. It is shown that in general the proposed stirrer outperforms the common Z-shaped stirrer with the same sweeping volume. Nevertheless, the measurement results show that the performance improvement of the designed stirrer becomes insignificant at higher frequencies with additional platform stirring. Albeit the difference, the stirring improvement of the designed stirrer is clearly demonstrated at low frequencies, which is more important due to the inherent low mode density at low frequency.

A simulation method of coupled model for a microwave heating process with multiple moving elements

In practical chemical applications, in order to improve the microwave heating uniformity, moving elements, such as mode stirrers, turntables and conveyor belts, have been widely used as an easy way to implement. However, different moving elements may have different shapes, modes of motion and moving speed, which makes the simulation of the heating process very difficult. In this paper, a stepping algorithm based on implicit function and level set method is proposed to simulate the microwave heating process of multi-component with unequal moving speed. The accuracy of the algorithm is verified by discrete method and experiments. Moreover, the model was used to analyze the heating process with different components at different positions and speeds. The effect of different materials of mode stirrers and shapes of heated sample on heating process were also compared. The researches show that the proposed model can effectively improve the heating uniformity and heating efficiency.

A Compact, High-Gain, High-Power, Ultrawideband Microwave Pulse Compressor Using Time-Reversal Techniques

Generation of high-peak-power, microwave ultrashort pulses (USPs) is desirable for ultrawideband communications and radar/remote sensing. A variety of microwave USP generators exist today, or are described in the literature, and have benefits and limitations depending on the application. A new class of pulse compressors for generating USPs using electromagnetic time-reversal (TR) techniques has been developed in the last decade and is the topic of this article. This article presents a compact TR microwave pulse-compression cavity that has ultrawide bandwidth (BW) (5-18 GHz) and employs waveguide feeds for high-peak-power output over the entire band. The system uses a time-reversal-based pulse compression scheme with one-bit processing (OBTR) to achieve high-peak power. We present results from full-wave simulations and measurements showing compression gain exceeding 21.2 dB, 22% efficiency, and measured instantaneous peak output powers reaching 39.2 kW. These are all record results for this type of pulse compressor. In addition, we present a new analysis of variation in compression gain due to impulse response recording time and BW variation, new experimental work on the effect of mode stirrer position on compression gain, and a novel RF switch-based technique for reducing time-sidelobes while using OBTR.

Evaluation on microwave drying of waste paper towel with multi-magnetron and mode stirrer

The thin-layer drying performance for a typical solid waste (paper towel) was evaluated at hot air temperatures of 90–130 °C (0.5 m/s air velocity) and microwave powers of 200–800 W in a bench-scale microwave combined convective drying setup. Influences of hot air and radiation modes (multi-magnetron and mode stirrer) on the temperature distribution uniformity were examined. The effective moisture diffusivities of sample during pure hot air drying ranged from 6.07 × 10−9 to 1.31 × 10−8 m2/s, while those for pure microwave drying were from 3.54 × 10−8 to 1.52 × 10−7 m2/s. The apparent activation energy of sample in pure hot air drying was estimated as 23.17 kJ/mol, while that for pure microwave drying was 22.64 W/g. The effective moisture diffusivities of sample in combined drying ranged from 5.71 × 10−8 to 2.05 × 10−7 m2/s. The heating uniformity for microwave was improved effectively by the combined drying. The application of the mode stirrers and multi-magnetron radiation modes further improved the microwave heating uniformity. Midilli model can well describe moisture content of paper towel over time.

Doppler Spectrum Analysis for the Prediction of Rotating Mode Stirrer Performances in Reverberation Chamber

This paper presents how the Doppler spectrum (DS) induced by rotating mode stirrers can be used in order to assess and optimize their performances when used in reverberation chambers. Indeed, a rotating mode stirrer acts as a diffracting structure whose performances are related to the DS spread. The performances of such stirrers are shown to be predictable by analyzing their DS in free space from numerical simulations using full-wave solvers with reasonable computation times. This allows the geometry of a future mode stirrer to be optimized before being built and installed in the chamber.

Fast and Accurate Assessment of the “Well Stirred Condition” of a Reverberation Chamber From $S_{11}$ Measurements

This paper presents a method to determine the frequency where a reverberation chamber starts to be “well stirred” according to the number of stirring conditions (i.e., different positions of a mode stirrer for a mechanically stirred chamber) considered. The method, based on S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> measurements of an antenna inserted in the reverberation chamber to be characterized, requires a simple measurement setup, is fast (few minutes) while leading to an excellent level of accuracy (maximum uncertainty of 10%) as demonstrated by the measurement results shown in this paper. The method takes advantage of a reliable definition of the well stirred condition leading to threshold values theoretically justified. Clear measurement guidelines and recommendations on the post-treatment process (available on demand) are also given. The method, proposed to the community of users as an alternative calibration procedure, provides reliable information on the chamber performances in the “grey zone” between the lowest usable frequency defined by standards and the frequency when the chamber is actually well stirred.

Performance Comparison and Critical Examination of the Most Popular Stirring Techniques in Reverberation Chambers Using the “Well-Stirred” Condition Method

This paper presents a performance comparison and a critical examination of the most popular stirring techniques proposed in the literature for reverberation chambers, implemented in the same parallelepipedic cavity (excluding naturally vibrating intrinsic reverberation chambers). This yields a fair comparison (in our experimental conditions) of these techniques in terms of performance and, in particular, from which frequency, each stirring process can lead to a well-stirred reverberation chamber. This analysis is performed, thanks to a recently published method based on $S_{11}$ measurements of an antenna within the chamber for different stirring configurations (i.e., different positions of a mode stirrer for a mechanically stirred chamber). The method is applied (including some adjustments when necessary) for the three main families of stirring techniques. In order to exhaustively assess the performance of each implemented technique, experiments are performed for different quantities of absorbers inserted in the cavity in order to study the effect of the average composite qualify factor of the chamber on each stirring process.

An approach for simulating the microwave heating process with a slow- rotating sample and a fast-rotating mode stirrer

Moving elements have been widely used in microwave heating process, especially mode stirrers and turntables. However, the huge difference in speed of these two elements makes the simulation of this heating process difficult to achieve. In this work, the heating process is computed by a step-by-step algorithm based on implicit function and level set methods. In each time step, the slow- rotating heated sample is considered as static while the fast-rotating mode stirrer have rotated a certain angle. The heat source in this time step is obtained by averaging the dissipated powers with mode stirrer in different angles. Moreover, the calculation amount is reduced by transformation methods based on the symmetry of geometric structure. In addition, heating process with different time step lengths and numbers of mode stirrer angles have been computed to analyze the model sensitivity. Experiments are also carried out to validate the effectiveness of the proposed method. Simulated results are in accordance with experimental ones. The combined effect of mode stirrer and turntable on microwave heating is analyzed. This model can serve as an effective method to deal with the thermal analysis with unequal velocity moving elements during the microwave heating.