This work proposes and experimentally evaluates a single layer bandpass frequency selective surface (FSS) that resonates at X-band (8-12 GHz). The metal plate of the unit cell has a half-Jerusalem cross slot of size 0.15λ₀, where λ₀ is the wavelength corresponding to 10 GHz centre frequency. The effects of unit cell parameters on filter response are analyzed through parametric analysis. The results reveal that the proposed bandpass FSS exhibits good polarization stability and angular stability at oblique angles up to 45˚. Furthermore, negligible frequency deviations in both TE and TM polarizations have also been achieved using this structure. A prototype of the bandpass FSS was fabricated on an FR4 substrate to validate the proposed design which includes 10×10 elements in a dimension of 45 mm × 45 mm × 1.6 mm. Measurements show that the bandpass FSS has a fractional bandwidth of 40% centered at 10 GHz from 8 GHz to 12 GHz. The unique feature of the proposed filter is its ability to operate in the whole X band (8-12 GHz) by tuning the filter elements.

Microwave imaging of small scatterers is an inverse scattering problem, and recently, the MUSIC algorithm has been proposed to solve this type of problem. The MUSIC algorithm, by assuming that the number of targets is a priori known, can locate the scatterers from the peaks of the well-known pseudospectrum. The noise and multiple scattering create ambiguity to detect the number of targets. Usually, information-based algorithms such as Akaike information criterion (AIC) and minimum description length (MDL) are employed for source number estimation. However, in the cases of low signal-to-noise ratio (SNR) and close targets, the performance of these methods is seriously degraded. In the present work, we propose a two-step approach to enumerate the scatterers in microwave imaging applications for cases where traditional methods fail. Firstly, the MUSIC algorithm is applied to locate all possible targets by assuming the maximum number of targets, and secondly, we can discriminate between the real and unreal targets by using a novel formula that acts as a spatial filter. The efficiency of the proposed method has been examined through various simulation tests using numerical and experimental datasets, and the results verify that the method can accurately specify the location and the number of scatterers in 2D microwave imaging applications.

The systematic design approach of a 42 GHz CW gyrotron has been extensively presented in this paper. Beam-wave interaction of the conventional tapered cylindrical cavity gyrotron is demonstrated using commercially available Particle-In-Cell (PIC) code. Beam absent and beam present cases have been considered to observe the performance of the device. Beam absent case is presented to validate the design in desired mode as well as resonant frequency whereas beam present case is demonstrated to validate and observe the beam-wave interaction behavior of the device in terms of output power. In order to optimize the dimension of interaction structure to achieve desired performance of the device, several parameters were considered. RF output power of the device is estimated with the variation of structure parameters as well as electron beam parameters to achieve better performance in terms of efficiency. Using the designed parameters, beam present analysis offers a saturated output power well above 250 kW. The particles phase space behavior along the interaction length is demonstrated to realize the energy transfer phenomena. The PIC simulation results are found in close agreement with the self-consistent single mode results. The estimated output power and efficiency support the proper design of proposed gyrotron oscillator.

In this paper, a flexible full-duplex antenna is proposed with robust performance and high isolation for 5.8 GHz using foam and PET paper. The patch of the antenna is modified by corner cut and inset feeding, while the defected ground structure is used to improve isolation between transmit and receive ports. Silver nanoparticle ink is used for printing the antenna in an inkjet printer. The fabricated version supports simulated results by showing acceptable performance in desired bandwidth. Bending tests and human body loading experiments are carried out on the fabricated antenna to demonstrate antenna's effectiveness for wearable applications. To the best of authors' knowledge, this is the first flexible full duplex antenna designed, achieving a high isolation level of -50 dB. Moreover, wide bandwidth, improved gain, radiation efficiency, low cost, easy fabrication, and robust performance make it a good option for 5.8 GHz wearable applications.

The rapid development of telecommunication systems has promoted the research of electromagnetic metamaterial absorbers. Based on the equivalent circuit theory, this paper proposes and designs a broadband absorption absorber based on electromagnetic metamaterials, which adopts a sandwich structure with an overall absorber thickness of 3.234 mm. The results show that the absorber has an absorption rate of more than 90% in the X-, Ku-, and K-bands (8.06 GHz-18.46 GHz) for the incident angle varying in the range of 0-50°. The absorption rate is higher than 90% for TE and TM mode electromagnetic waves and electromagnetic waves with polarization angle in the range of 0-50°. The absorber still has good absorption characteristics. The study shows that the absorber has small size, thin thickness, and broad angle broadband absorption characteristics.

A 4-port planar multiple-input multiple-output (MIMO) antenna system design is proposed. The antenna elements are modified meandered wideband antennas which cover frequencies from 674 MHz to 1 GHz, 1.9 GHz to 2.1 GHz, 3.175 GHz to 3.476 GHz, 4.529 GHz to 4.761 GHz and 5.254 to 5.513 GHz for long term evolution (LTE), Internet of Things (IoT), and sub-6 GHz applications and thus can be used for robotic navigation, logistics, healthcare, tracking, transportation etc. Due to very small envelope correlation coefficient (ECC) between the ports (< 0.5), the MIMO configuration can be efficiently implemented which helps in increasing the data rates. It is very compact in size and thus can be used for portable handheld devices. Since there is the problem of current localization due to common ground, the future work aims at minimizing coupling and improving the impedance matching using novel decoupling networks. These MIMO antennas are connected to a common slotted ground plane. Antenna simulation has been done using Computer Simulation Technology (CST) Microwave Studio Suite simulator. A low cost FR-4 substrate with dimensions 65 mm × 90 mm × 1.6 mm has been used for antenna fabrication, and experimental results are obtained using an anechoic chamber and a vector network analyser. ECC and realized gain of the antenna are also obtained experimentally and are almost similar to the simulated results.

In order to improve the practicability and versatility of ultra-wideband (UWB) antennas, a reconfigurable band notch antenna is proposed in this paper. It has a compact size of 18 mm×16 mm×1.6 mm. The reconfigurable band notch function is realized by two small tunable units. The tunable unit makes up of a split ring resonator (SRR), a dielectric substrate, and a varactor diode. The simulation results show that the antenna combines the functions of band notch emergence, removal and movement. The applied reconfigurable method can effectively broaden the continuous movement range of band notch. The measurement proves that the antenna has the band notch reconfigurable function, and the measured results are in good agreement with the simulation ones. The radiation patterns are measured, which are stable and consistent under two modes with and without band notch, showing omnidirectional radiation characteristics. These research results provide reference value for the design of band notch UWB antenna shielding civil narrowband communication band.

Amorphous alloy transformers (AMDT) have become the mainstream of energy-saving and environmentally friendly distribution transformers, but the problem of environmental pollution caused by their noise has become more prominent. The high magnetostriction of amorphous alloy strip and its sensitivity to stress are the main reasons for the vibration of AMDT core. Accurate calculation of the overall core vibration of transformers is the key issues in transformer noise research. This paper studies the vibration of amorphous alloy transformers under operating conditions, and establishes a three-dimensional magnetic-mechanical coupling model considering the magnetostrictive effect of the power transformer core, and the magnetic field distribution and core vibration displacement of the dry-type transformer under no-load conditions are calculated by finite element method. Combined with experiments, the mechanism of vibration generation of amorphous alloy transformer core is studied, and an iron core vibration prediction calculation based on electromagnetic field coupling analysis is proposed. The research results not only have important academic value for exploring the vibration mechanism and noise suppression mechanism of amorphous alloy transformers, but also have important significance for ensuring their efficient operation.

To further improve fault diagnosis performance, a new hybrid feature selection approach combined with whale optimization algorithm and extreme learning machine is presented in this study. Firstly, three filter methods based on different evaluation metrics are employed to select and rank 25 input features derived from gases concentration values, gases ratio and energy-weighted dissolved gas analysis. Then, feature fusion approaches are applied to aggregate feature ranks and form a lower-dimension candidate feature subset. Afterwards, the whale optimization-based extreme learning machine model is implemented to optimize parameters and select optimal feature subsets. The accuracy of the model is used to evaluate the fault diagnosis capability of the concerned feature subsets. Finally, novel subsets are determined as the optimal feature subset to establish a fault diagnosis model. According to the experimental results, the average accuracy of the proposed approach is better than that of other conventional methods, which indicates that the optimal feature subset obtained by the proposed method can significantly promote the fault diagnosis accuracy of the power transformer.

Antennas are essential devices to build everything connected in the era of information. However, the quality of communications would be degraded with the presence of raindrops on the antenna surface. Additional antiwater radomes may generate radiation loss and dispersive impedance mismatch over a broad frequency range, which is not acceptable for next-generation communication systems integrating multiple bands. Here, we report the first experimental demonstration of self-hydrophobic antennas that cover the bands of 1.7 GHz, 3.5 GHz, and 8.5 GHz through a laser-direct-writing treatment. Experimental results show that the return loss, radiation pattern, and efficiency of self-superhydrophobic antennas can be maintained in the mimicked rainy weather. Furthermore, writing hydrophobic nanostructures on both dielectrics and metals is compatible with commercial printed circuitry techniques widely used in industries. Our technique will augment the laser fabrication technology for specialized electromagnetic devices and serve as a powerful and generalized solution for all-weather wireless communication systems.

In the conventional virtual synchronous generator (VSG) dual adaptive inertia and damping control schemes, the inertia J and damping D exhibit different variation patterns in different time intervals and are mutually constrained. To address this problem, an adaptive neural-fuzzy network inference system (ANFIS)-based dual adaptive inertia and damping VSG control technique applied to the direct-drive permanent magnet synchronous wind generator (D-PMSWG) system is proposed in this paper. In ANFIS-VSG, the controller is designed on the basis of the ANFIS control principle, and the input and output data are collected by PID control. The Sugeno-type ANFIS controller model is adopted to train the fuzzy inference system (FIS) online. Moreover, the virtual inertia and damping coefficients can be dynamically adjusted in real time according to the frequency variation without taking the different variations and mutual constraints of inertia J and damping D in different intervals into consideration, so the design difficulty and calculation process can be simplified, and the accuracy of the proposed control algorithm is enhanced through training. Furthermore, when the system is subject to load changes, integrating into the grid from an islanded state, and when the output power sets value steps, the power-frequency characteristics and the anti-interference capability of the three-phase output current of VSG can be improved. Finally, the proposed control strategy is simulated and analyzed based on Matlab/Simulink simulation software, which proves the correctness and effectiveness of the proposed control algorithm.

A novel balanced-to-balanced differential-mode negative group delay (NGD) microwave circuit with excellent common-mode suppression is proposed. The proposed circuit consists of two sections of coupled lines, six transmission lines, and four open-circuited stubs. The coupled lines combined with the open-circuited stubs produce the NGD characteristic, which is connected by the λ/2 transmission lines to form a balanced structure for excellent common-mode suppression. To verify the proposed balanced circuit, a microstrip circuit prototype with a center frequency of f₀ = 1.0 GHz is designed, fabricated, and measured. When the prototype is excited in differential mode, the measured NGD time at f₀ is -3.45 ns with an NGD bandwidth of 16.6 MHz (991.7-1008.3 MHz), insertion loss of less than 2.88 dB, and return loss of more than 11.7 dB. Furthermore, the measured common-mode suppression is greater than 41 dB in the NGD band.

A very economical and compact size wearable antenna operating over Ultra-Wide Band (UWB) spectrum is investigated in the proposed work. The antenna is modelled on a thin FR-4 (0.2 mm) material that makes it flexible and well-suited for wearable appliances. The radiating patch structure is the combination of one square and two elliptical patches rotated at 45˚ and fed with a Coplanar Waveguide (CPW) to achieve a wide impedance bandwidth. The complete radiating structure looks like a flower shape, and it has a partial ground to support the radiation from the antenna over the complete UWB. The flexibility of the proposed structure is investigated by bending it along xz and yz planes using cylindrical shape foam. The peak Specific Absorption Rate (SAR) is demonstrated for 1 g and 10 g of tissues at different chosen frequencies like 3.7, 8.4, and 11.2 GHz using a three-layer phantom model. The presented antenna performance analysis and compact size confirm that it is a good candidate for wearable applications.

To shield undesirable microwave radiation to protect electronic systems and human health, microwave perfect absorbers have attracted increasing interests in recent years. However, the opaque or semitransparent nature of most implemented microwave absorbers limit their applications in optics. Here, we demonstrate a high-performance microwave absorber based on an impedance-assisted Fabry-Pérot resonant cavity with an ITO-dielectric-ITO structure without complex nanofabrication. The device features near-unity absorption (99.5% at 14.4 GHz with a 4.5 GHz effective bandwidth), excellent electromagnetic interference shielding performance (24 dB) in the Ku-band, and high optical transparency (89.0% from 400 nm to 800 nm). The peak absorption frequency of the device can be tuned by changing the thickness of glass slab and sheet resistance of ITO films. Our work provides a low-cost and feasible solution for highperformance optically transparent microwave shielding and stealth, paving the way towards applications in areas of microwave and optics.

Energetic ion or electron beams cause plasma instabilities. Depending on plasma and the beam parameters, an ion beam leads to change in the dispersion relation of Alfven waves on interacting with magnetoplasmas as it can efficiently transfer its energy to the plasma. We have derived dispersion relation and the growth rates for oblique shear Alfven wave in hydrogen plasma. The particles of the beam interact with the Shear Alfven waves only when they counter-propagate each other and destabilize left-hand polarized mode for parallel waves and left-hand as well as right-hand polarized modes for oblique waves, via fast cyclotron interaction. The collisions between beam ions and plasma components affect the growth rate and the frequency of generated Alfven waves, differently for right-hand (RH) and left-hand (LH) polarized oblique Alfven modes. For (ω + k_zv_bo > ω_bc), the most unstable mode is the LH polarized oblique Alfven mode, and it is the RH polarized oblique Alfven mode for (ω + k_zv_bo < ω_bc), which shows a polarization reversal after resonance condition. Numerical results indicate that the growth rates increase with increase in angle of propagation. The maximum growth rate values in the presence or absence of beam increase due to obliquity of wave.

Inspired by neural networks based on traditional electronic circuits, optical neural networks (ONNs) show great potential in terms of computing speed and power consumption. Though some progress has been made in devices and schemes, ONNs are still a long way from replacing electronic neural networks in terms of generalizability. Here, we present a complex optical neural network (cONN) for holographic image recognition, within which a high-speed parallel operating unit for complex matrices is proposed, targeting the real-imaginary-splitting and column splitting. Based on the proposed cONN, we have numerically demonstrated the training-recognition process on our cONN for holographic images converted from handwritten digit datasets, achieving an accuracy of 90% based on the back-propagation algorithm. Our training verification integrated architecture will enrich the further development and applications of on-chip photonic matrix computing.

Chirp sequence has been adopted in automotive applications for its simple generation and flexible integration within radar-centric systems. Besides, recent studies have shown its ability to carry data between communicating vehicles in the surroundings. Since the parameters adopted from current automotive radar sensors can differ at the transmitter side dependent on the automotive supplier, the carrier alignment of the communication receiver of one of the communicated nodes might not concur with the one in the transmitter. This paper presents a novel two-stage synchronization method for communication-assisted chirp sequence (CaCS) signals. The proposed synchronization method applies a sequence of up- and down-chirp as a preamble to estimate frequency and time offsets during the transmission. The suggested synchronization scheme supports partial chirp modulation systems and can be adapted for similar radar-centric systems that employ chirp modulation. The former stage performs a coarse synchronization, reallocates the receive carrier frequency, and corrects eventual time offsets between the communication receiver from one CaCS-node and the transmitter of another node. The carrier allocation at the communication receiver side is based on a combination of spectrum sensing via short-time Fourier transforms and image processing to estimate the transmitting signal pattern (slope, frequency offset, and delay). The latter stage, in its turn, relies on range-Doppler estimation to perform a fine correction of time and frequency offsets and compensates residual offsets of the coarse synchronization stage. Furthermore, the paper analyzes the case of a multi-user scenario with mutual interference between the signals that affects the synchronization and communication data detection. Besides, measurements are provided based on two completely unsynchronized software-defined radios to validate the proposed method. The study also illustrates the influence of the signal-to-noise ratio on the proposed method and verifies it with simulations in MATLAB. As a result, the offsets at the investigated CaCS-node are returned to recover the transmitted data correctly.

This paper presents a novel microstrip three-mode filtering power divider (FPD) with high frequency selectivity and high isolation, which integrates only a single resonator and a resistor to realize the dual functions of the power division and filtering. In order to further improve its frequency selectivity and obtain wide upper stop band, three open stubs are loaded into the input and output ports of the filter power divider. The measured and simulated results show that the range of S₁₁ < -10 dB is 1.86~2.1 GHz; the relative bandwidth of 3 dB is 17.9%; the in-band isolation is higher than 26 dB; and it has a relatively simple topology.

This paper presents an analytical method for the optimal estimation of time constants of synchronous machine from Standstill Frequency Response Testing (SSFR). We show that the analytical method is advantageous over the conventional one since the latter is based on curve fitting representing the variation of the operational inductance as a function of the frequency and provides in accurate and non-unique solutions. In fact, the analytical method applies the standard theory of linear systems to locate the values of poles and zeros in the frequency response and determines the optimal order of the equivalent circuit that can model the machine accurately. The proposed method is simple, practicable and effective. However, it needs an optimisation process based on parameter differentiation, to improve the values of time constants. Based on the measured data, realistic tests are given to show the advantages of the method.

A novel Variable Inclination Continuous Transverse Stub (VICTS) antenna element and array model is proposed in this paper. The bandwidth and gain of the element are increased by adopting a linear-gradient stub, matching structure and rectangular grating slow-wave structure (SWS). A circular array can be obtained by arranging antenna units of different lengths linearly. The array antenna uses a bow-parabolic box antenna as the line source generator (LSG) and utilizes a double-layer transition waveguide structure to realize the propagation of planar wave. Finally, a wide range of beam scanning in the elevation plane was achieved. The results of the simulation and antenna prototype test are in good agreement. Showing the impedance matching characteristics of the antenna unit and array meets the engineering requirements in the range of 12~16 GHz. The maximum gain of the antenna array is 34.3 dBi, and the maximum 3 dB beamwidth is less than 10°. It is confirmed that the designed antenna has the characteristics of high gain, narrow beam, and low profile, and realizes two-dimensional beam scanning in the range of 6~79° in the elevation plane, which meets the requirements of the Satellite Communications On-the-Move system (SOTM).