Low Frequency Operation Research Articles

Design of a Compact, Planar, Wideband, Overlapped, Bow-Tie Antenna in a Single Layer with Stable Bi-Directional Radiation Patterns

In this paper, a planar, bow-tie antenna with an enhanced bandwidth and a bi-directional radiation pattern is proposed. The concept of multi-resonance is applied by implementing the superposition of three different bow-tie components with various radii and flare angles in an overlapped fashion into a single element, resulting in a significant increase in bandwidth. The antenna has an electrical size, kr, of 1.10, calculated at the lowest frequency of operation. The proposed antenna is simulated, and a prototype is fabricated for verification by measurement. The result is a wide −10 dB bandwidth of 80.3% (1.23–2.88 GHz) from the measurements. The antenna also maintains a bi-directional radiation pattern, with a negligible difference between the forward and backward realized gains, within the entire −10 dB bandwidth. The measured realized gain values in the forward and backward directions are between 1.5 dBi and 3.7 dBi over the −10 dB bandwidth. The comparison of the measurement and simulation results shows good agreement.

Graphene-Based Thermopneumatic Generator for On-Board Pressure Supply of Soft Robots.

Various fields, including medical and human interaction robots, gain advantages from the development of bioinspired soft actuators. Many recently developed grippers are pneumatics that require external pressure supply systems, thereby limiting the autonomy of these robots. This necessitates the development of scalable and efficient on-board pressure generation systems. While conventional air compression systems are hard to miniaturize, thermopneumatic systems that joule heat a transducer material to generate pressure present a promising alternative. However, the transducer materials of previously reported thermopneumatic systems demonstrate high heat capacities and limited surface area resulting in long response times and low operation frequencies. This study presents a thermopneumatic pressure generator using aerographene, a highly porous (>99.99%) network of interconnected graphene microtubes, as lightweight and low heat capacity transducer material. An aerographene pressurizer module (AGPM) can pressurize a reservoir of 4.2 cm3 to ∼14 kPa in 50 ms. Periodic operation of the AGPM for 10 s at 0.66 Hz can further increase the pressure in the reservoir to ∼36 kPa. It is demonstrated that multiple AGPMs can be operated parallelly or in series for improved performance. For example, three parallelly operated AGPMs can generate pressure pulses of ∼21.5 kPa. Connecting AGPMs in series increase the maximum pressure achievable by the system. It is shown that three AGPMs working in series can pressurize the reservoir to ∼200 kPa in about 2.5 min. The AGPM's minimalistic design can be easily adapted to circuit boards, making the concept a promising fit for the on-board pressure supply of soft robots.

Four‐port dual CP MIMO antenna with enhanced isolation for 5G applications

SummaryIn this paper, a 4 × 4 MIMO (multiple‐input multiple‐output) antenna with metasurface (MS), which is composed of dual circularly polarized (CP) patch antennas, is proposed for 5G (sub‐6 GHz) applications. The MIMO antenna consists of four microstrip patch antennas. In order to improve the isolation of the MIMO antenna, parasitic elements are placed on the MS and layer of patch antennas. In addition, a substrate‐integrated waveguide (SIW) structure is used at the center of the MIMO antenna for this purpose. The proposed MIMO antenna is fabricated for validation tests. The antenna has overall dimensions of 115 mm × 115 mm × 3.2 mm (1.219 × 1.219 × 0.033 , where is the free space wavelength at the lowest operating frequency in 10‐dB impedance bandwidth). The MIMO antenna has 10‐dB impedance bandwidth (IBW) from 3.3 to 3.8 GHz (14.08%), 3‐dB axial ratio bandwidth (ARBW) from 3.42 to 3.69 GHz (7.59%), and 6.36‐dBi peak gain. The isolation is greater than 28.14 dB in the n78 frequency band (3.3–3.8 GHz).

Robust Multi-Band DNG metamaterial absorber for GPS (L1), ISM, and 5G application with Enhanced polarization and angle stability

This research introduces a triple-band metamaterial absorber characterized by insensitivity to both polarization and incident angles. The structure includes square ring resonators on the upper side and a continuous metal ground on the lower side, separated by an FR4 substrate. At the lowest operating frequency, the unit cell’s thickness, and length measure 0.0128λ and 0.156λ, respectively. Under normal incidence, the proposed absorber demonstrates three clear absorption peaks at 1.56, 2.43, and 3.36 GHz, achieving absorption rates of 98 %, 99 %, and 97 %, respectively. The absorption response exhibits variation with incident angles up to 70° for both TM and TE polarizations due to its high-level symmetry. This configuration attains negative permittivity and permeability suitable for operational frequencies in the L-band and S-band. The study examines the electric field, impedance, and surface current to provide additional evidence for the absorption analysis. A validated RLC circuit equivalent to the proposed structure has been confirmed using Advanced Design System (ADS). The results indicate a slight deviation, especially at frequencies beyond the resonance frequencies. The structure underwent fabrication and testing in an anechoic chamber, revealing a strong correlation between the simulated and measured results. The suggested absorber holds promise for ISM applications, radar systems, sensing technologies, and energy harvesting systems.

Passive wall thickness monitoring using acoustic emission excitation

Erosion-corrosion is a problematic damage mechanism for the oil and gas industry. To manage the risk of erosion-corrosion networks of particle impact monitoring systems have been installed on pipelines in order to detect acoustic emission from abrasive sand particles impacting the inside surface of the pipe. It would be of value if the existing network of particle impact monitoring systems were not only capable of detecting particle impact, but also sizing the remaining wall thickness. Particle impact monitoring systems are passive and are not generally equipped for excitation. This paper explores the feasibility of using passive acoustic emission transducers for wall thickness measurement, utilizing the fact that active pulse-echo measurements can be approximated by autocorrelating diffuse acoustic waves, such as those generated by particle impact. Two measurement modalities are presented: a) time-of-flight measurements and b) resonant ultrasound spectroscopy measurements. The more usual time-of-flight based measurement is limited by the fact that acoustic emission transducers typically have sensitive bandwidths limited to <1 MHz. The relatively low frequency operation limits the use to thick wall components where the component thickness ≫ ultrasonic wavelength. In thinner walled components a resonant ultrasound spectroscopy approach is required. Experimental measurements are shown that are truly passive (with no purposeful excitation at all), and semi-passive, utilizing acoustic emission from sand impact or compressed air as the excitation source. Results show very good agreement with active measurements.

Coupled stack-up volume RF coils for low-field open MR imaging.

Low-field open magnetic resonance imaging (MRI) systems, typically operating at magnetic field strengths below 1 Tesla, has greatly expanded the accessibility of MRI technology to meet a wide range of patient needs. However, the inherent challenges of low-field MRI, such as limited signal-to-noise ratios and limited availability of dedicated radiofrequency (RF) coils, have prompted the need for innovative coil designs that can improve imaging quality and diagnostic capabilities. In response to these challenges, we introduce the coupled stack-up volume coil, a novel RF coil design that addresses the shortcomings of conventional birdcage in the context of low-field open MRI. The proposed coupled stack-up volume coil design utilizes a unique architecture that optimizes both transmit/receive efficiency and RF field homogeneity and offers the advantage of a simple design and construction, making it a practical and feasible solution for low-field MRI applications. This paper presents a comprehensive exploration of the theoretical framework, design considerations, and experimental validation of this innovative coil design. We demonstrate the superior performance of the coupled stack-up volume coil in achieving 47.7% higher transmit/receive efficiency and 68% more uniform magnetic field distribution compared to traditional birdcage coils in electromagnetic simulations. Bench tests results show that the B1 field efficiency of coupled stack-up volume coil is 57.3% higher compared with that of conventional birdcage coil. The proposed coupled stack-up volume coil outperforms the conventional birdcage coil in terms of B1 efficiency, imaging coverage, and low-frequency operation capability. This design provides a robust and simple solution to low-field MR RF coil design.

Performance Analysis and Optimization of Switch Device for VLF Communication Synchronous Tuning System Based on Coupled Inductors

The very low frequency (VLF) communication system is characterized by a limited transmit bandwidth. Due to the low operating frequency, the dimensions of VLF antennas are significantly smaller than the corresponding wavelength. Therefore, VLF antennas are considered electrically small antennas (ESAs) with high Q values and narrow bandwidth. To achieve broadband VLF communication, synchronous tuning technology is commonly employed. In this study, we focus on analyzing the performance of the switching device in the synchronous tuning system using coupled inductors and IGBT as core components. By considering the equivalent circuit of the controlled source associated with coupled inductors, we propose an adaptive input impedance optimization algorithm based on the variable capacitor (hereinafter referred to as VC-ADIO) to address issues arising from coupling coefficient variations and internal parameters within IGBTs that affect primary loop input impedance. The radiated power of the VLF antenna is improved effectively.

Modeling and Experimental Study of Vibration Energy Harvester with Triple-Frequency-Up Voltage Output by Vibration Mode Switching.

Conventional wireless sensors rely on chemical batteries. Replacing or charging their batteries is tedious and costly in some situations. As usable kinetic energy exists in the environment, harvesting vibration energy and converting it into electrical energy has become a hotspot. However, the power output capability of a conventional piezoelectric energy harvester (PEH) is limited by its low operational frequency. This paper presents a new mechanism for achieving continuous triple-frequency-up voltage output in a PEH. The proposed system consists of a slender piezoelectric cantilever with two short cantilever-based stoppers. The piezoelectric cantilever undergoes a pure bending mode without contacting the stoppers. In addition, the beam switches into a new vibration mode by contacting the stoppers. The vibration modes switching yields reverses the signs of voltage outputs, inducing triple-frequency-up voltage output. Analytical and experimental investigations are presented, and it is shown that a significant triple-frequency up-conversion of the voltage output can be obtained over a wide frequency range. A peak power output of 3.03 mW was obtained. The proposed energy harvester can support a wireless sensor node.

Near field food contamination detection technique employing a compact Antipodal Vivaldi antenna

This paper presents a simple yet robust technique for identification of the presence of contaminants in food samples by detecting the variation in the transmission coefficient (S21) in a near field measurement setup. A compact step-notched antipodal Vivaldi antenna with a frequency range of operation 3 - 18 GHz, and realized gain of 12.81 dB at 16 GHz having compact dimensions 60mm×40mm×0.51mm, i.e., 0.6×0.4×0.005λ03 (where λ0 is the free space wavelength at the lowest frequency of operation) is designed for contamination detection due to its compact size, high directivity and high boresight gain for near-field measurement. An equivalent image is constructed from the S - parameters, measured at different positions and different antenna polarizations over the frequency range of 3-18 GHz for metallic contaminants, i.e., iron ball and aluminum foil. An algorithm is developed for detection of contamination from the images using the reference case of no-contamination scenario. The proposed algorithm is validated using full-wave simulation.

Attenuation Measurement Using the IF Substitution Method in the Frequency Range of 9 kHz to 10 MHz

Using the intermediate frequency (IF) substitution method, this study establishes a low-frequency attenuation measurement standard operating in the 9 kHz to 10 MHz frequency range. We designed and fabricated a low-frequency attenuation measuring instrument using a mixer that converts the input radio frequency signal into an output IF of 1 kHz. When varying the attenuation of a variable attenuator under test, an attenuation of up to 100 dB was measured. The IF output of the mixer was measured using a spectrum analyzer as the receiver. The receiver's performance was evaluated using an inductive voltage divider whose measurement capability is traceable to the electrical measurement standard. The measurement uncertainty, which accounts for uncertainty sources such as the performance of the receiver, mismatch, and mixer nonlinearity, was evaluated to be 0.005 dB to 0.058 dB (k = 2) in the attenuation range of 10 dB to 100 dB.

Design and implementation of an innovative single-phase direct AC-AC bipolar voltage buck converter with enhanced control topology

Direct AC–AC converters are strong candidates in the power converting system to regulate grid voltage against the perturbation in the line voltage and to acquire frequency regulation at discrete step levels in variable speed drivers for industrial systems. All such applications require the inverted and non-inverted form of the input voltage across the output with voltage-regulating capabilities. The required value of the output frequency is gained with the proper arrangement of the number of positive and negative pulses of the input voltage across the output terminals. The period of each such pulse for low-frequency operation is almost the same as the half period of the input grid or utility voltage. These output pulses are generated by converting the positive and negative input half cycles in noninverting and inverting forms as per requirement. There is no control complication to generate control signals used to adjust the load frequency as the operating period of the switching devices is normally greater than the period of the source voltage. However, high-frequency pulse width modulated (PWM) control signals are used to regulate the output voltage. The size of the inductor and capacitor is inversely related to the value of the switching frequency. Similarly, the ripple contents of voltage and currents in these filtering components are also inversely linked with PWM frequency. These constraints motivate the circuit designer to select high PWM frequency. However, the alignment of the high-frequency control input with the variation in the input source voltage is a big challenge for a design engineer as the switching period of a high-frequency signal normally lies in the microsecond. It is also required to operate some high-frequency devices for various half cycles of the source voltage, creating control complications as the polarities of the half cycles are continuously changing. This requires at least the generation of two high-frequency signals for different intervals. The interruption of the filtering inductor current is a big source of high voltage surges in circuits where the high-frequency transistors operate in a complementary way. This may be due to internal defects in the switching transistors or some unnecessary inherent delay in their control signals. In this research work, a simplified AC–AC converter is developed that does not need alignment of high-frequency control with the polarity of the source voltage. With this approach, high-frequency signals can be generated with the help of any analog or digital control system. By applying this technique, only one high-frequency control signal is generated and applied in AC circuits, as in a DC converter, without applying a highly sensitive polarity sensing circuit. So, controlling complications is drastically simplified. The circuit and configuration always avoid the current interruption problem of filtering the inductor. The proposed control and circuit topology are tested both in computer-based simulation and practically developed circuits. The results obtained from these platforms endorse the effectiveness and validation of the proposed work.

High power, broadband, coaxial to microstrip bi‐directional coupler in the VHF/UHF band

AbstractBroadband couplers consist usually of multiple coupled lines that are each a quarter wave‐length long. Herein, in contrast to conventional methods, a short length of a microstrip line is coupled to a coaxial line. The length of the coupled microstrip line is small compared to the wavelength at the lowest operating frequency. The coupling is weak and the microstrip line is λ/8 at the highest operating frequency. The proposed structure resembles a pair of coupled lines with a linear coupling response. The coupling exhibits a slope of 6 dB/octave over multi‐octave bandwidth. The microstrip line is connected to a lumped element compensating network at the coupled port. The compensating network is a low‐pass circuit with a slope of −6 dB/octave. The power handling of the coupler is high due to the coaxial line and the weakly coupled microstrip line. Electromagnetic simulations and analytical formulations are presented. The broadband coupler handles nearly 1 kW of input power. Due to the weak coupling of the microstrip‐coaxial coupler, the lumped element circuit needs to handle &lt;1.5 W. A high‐power coupler has been fabricated for the frequency range of 30 to 500 MHz. It exhibits a coupling response of 56 ± 0.4 dB.

Multimodal surface coils for low field MR imaging

Low field MRI is safer and more cost effective than the high field MRI. One of the inherent problems of low field MRI is its low signal-to-noise ratio or sensitivity. In this work, we introduce a multimodal surface coil technique for signal excitation and reception to improve the RF magnetic field (B1) efficiency and potentially improve MR sensitivity. The proposed multimodal surface coil consists of multiple identical resonators that are electromagnetically coupled to form a multimodal resonator. The field distribution of its lowest frequency mode is suitable for MR imaging applications. The prototype multimodal surface coils are built, and the performance is investigated and validated through numerical simulation, standard RF measurements and tests, and comparison with the conventional surface coil at low fields. Our results show that the B1 efficiency of the multimodal surface coil outperforms that of the conventional surface coil which is known to offer the highest B1 efficiency among all coil categories, i.e., volume coil, half-volume coil and surface coil. In addition, in low-field MRI, the required low-frequency coils often use large value capacitance to achieve the low resonant frequency which makes frequency tuning difficult. The proposed multimodal surface coil can be conveniently tuned to the required low frequency for low-field MRI with significantly reduced capacitance value, demonstrating excellent low-frequency operation capability over the conventional surface coil.

Lightweight and broadband metamaterial absorber based on ITO impedance film

In this paper, we present a lightweight, broadband metamaterial absorber that exhibits polarization stability and is insensitive to incident angles. The absorber is composed of one layer of indium tin oxide impedance surface, three layers of quartz fiber composite material, and one layer of paper honeycomb on a metal sheet. Our proposed absorber has a 90 % absorptivity bandwidth of 13.19 GHz, ranging from 4.83 GHz to 18.12 GHz at TEM plane wave normal incidence, covering the entire X and Ku band. We illustrate the variation of design parameters and the distribution of electromagnetic field and current to investigate the absorption mechanism. To demonstrate the effectiveness of our design, we fabricate a prototype sample and measure its absorption performance. The results show a bandwidth of 12.38 GHz, ranging from 4.91 GHz to 17.29 GHz, with a low profile of 0.095λL at the lowest operating frequency. The experimental results are in good agreement with the numerical simulation, effectively verifying our proposed design.

Realization of an ultra-thin absorber based on magnetic metamaterial working at L-band

In this paper, at the L-band, we design and verify an ultra-thin easy-realized absorber (LUTA) based on metasurface and magnetic material. The absorptivity achieves over 90% at 1–2.51 GHz (86.04% fractional bandwidth), covering the whole L-band (1–2 GHz) and the partial S-band (2–4 GHz). The total thickness of the LUTA is as low as 3.7 mm, corresponding to 0.012λ0 at the lowest operating frequency, which greatly breaks through the Rozanov limit. The excellent performance is achieved through analyzing the transmission line model of a basic absorber and then designing the metasurface layer with required impedance characteristics to make the impedance of the LUTA match with the air at the designed frequency band. The simulated absorption, power loss density, and the surface current distributions of the LUTA verify the design method, while the measured results demonstrate that the LUTA has the superiority of easy-fabrication, polarization independent, wide angle incidence, and high absorption at the whole L-band. The LUTA has great application potentials in the fields of L-band electromagnetic stealth and radar cross section reduction.

Paediatric ankle rehabilitation system based on twisted and coiled polymer actuators

Rehabilitation is crucial for children with physical disabilities arising from various conditions. Traditional exoskeletons, reliant on electric motors and rigid components, making them cumbersome, heavy, and unsuitable for use outside clinical facilities. To overcome these, researchers are turning to soft wearable rehabilitation robots (SWRRs) with artificial muscles based on smart materials like twisted and coiled polymer actuators (TCPs). TCPs offer enhanced compliance, adaptability, comfort, safety, and reduced weight—critical for paediatric use. Despite facing challenges like low operating frequencies and high temperatures, TCPs are explored as potential artificial muscles for SWRRs, due to their advantages on the force they can generate, the strain and a linear behaviour. This study details a proof of concept for a paediatric rehabilitation system for ankles based on TCPs, including the actuator characterization, mechanical design, control strategy, and human-computer-interface (HCI). The resulting device achieved a 1.4 Nm torque, a 10° range of motion in dorsiflexion within 5 s, and integrated electromyographic HCI. This research marks a promising step towards innovative, soft wearable rehabilitation solutions for children with physical disabilities.

DC Power Boosting Circuit for Freestanding‐Sliding Triboelectric Nanogenerators with High Intrinsic Impedance and Multi‐Harmonic Output

AbstractThis study presents a freestanding sliding triboelectric nanogenerator (FS‐TENG) designed for low‐frequency motions (below 2 Hz). However, for practical applications, an efficient power management strategy is required to handle the harmonic outputs of the FS‐TENG. The high‐frequency signal is not sustainable for powering low‐power electronic applications. To address the issue, a novel direct current power supply circuit (DPS) is proposed that utilizes a double charge circuit (DCC) and a comb filtering circuit (CFC) to efficiently harness harmonic sources from the FS‐TENG. The direct power supply (DPS) outperforms traditional converters by reducing the impedance of the FS‐TENG and collecting the target harmonic sources, which facilitates a continuous power supply to the load. The results demonstrate that the DPS is capable of providing a continuous DC voltage of 2.2 V to a 10 MΩ load with minimal ripple (0.039%) at a low operating frequency of 0.625 Hz. Additionally, the practical application of a self‐powered temperature sensor is demonstrated to highlight the potential of FS‐TENG and DPS for low‐frequency energy harvesting solutions in real‐world scenarios.

A diplexer antenna with extremely high isolation for co-site duplex applications

Co-site full-duplex communication system is facing self-interference (SI) issues caused by tremendous gap between Tx power and Rx sensitivity. SI suppression in the propagation domain is addressed by a co-polarized diplexer antenna with high port isolation in this paper. It shares a dual-band microstrip antenna with a small frequency ratio, at 1.5 and 2.0 GHz as an example. A shorting pin directly connects the antenna and diplexer, avoiding 50-Ω interface and impedance matching circuit. The diplexer is composed of open-loop resonators (OLRs) and step-impedance resonators (SIRs). Compact channel filters are constituted by OLRs, and SIRs are adopted to improve the filtering response of the diplexer. General guidelines of the diplexer are given through rigorous analysis. The fractional bandwidths of the lower and higher channels of the diplexer are 4% and 3%, and the fractional bandwidths of the diplexer antenna are 1.5% and 1%, respectively. The port isolation is greater than 53 dB for both channels. Size of the diplexer antenna is 0.65 λ×0.60 λ×0.016 λ, where λ is the free-space wavelength at the lowest operation frequency. Potential advantages of the proposed antenna in suppressing SI are validated using Software-Defined Radio, which is potentially useful in full-duplex communications.

Graphene-Based Artificial Dendrites for Bio-Inspired Learning in Spiking Neuromorphic Systems.

Analog neuromorphic computing systems emulate the parallelism and connectivity of the human brain, promising greater expressivity and energy efficiency compared to those of digital systems. Though many devices have emerged as candidates for artificial neurons and artificial synapses, there have been few device candidates for artificial dendrites. In this work, we report on biocompatible graphene-based artificial dendrites (GrADs) that can implement dendritic processing. By using a dual side-gate configuration, current applied through a Nafion membrane can be used to control device conductance across a trilayer graphene channel, showing spatiotemporal responses of leaky recurrent, alpha, and Gaussian dendritic potentials. The devices can be variably connected to enable higher-order neuronal responses, and we show through data-driven spiking neural network simulations that spiking activity is reduced by ≤15% without accuracy loss while low-frequency operation is stabilized. This positions the GrADs as strong candidates for energy efficient bio-interfaced spiking neural networks.

Experimental study of the plasma flow created by a low-power helicon source and approaches to increase the average energy of its ion component

The article presents the results of an experimental study of the characteristics of a low-power helicon ion source. Particular attention is paid to the parameters of the particle flow emerging from the source and the possibilities for increasing ion energy. It was found that the voltage drop at the output of the source is small (10–15 V), and the main voltage drop is localized at the grounded elements. The ion flux density non-monotonically depends on the magnetic field, while the ion energy remains constant. Applying a constant voltage to the additional electrode inside the discharge chamber does not increase the potential drop at the output of the source. Switching to a lower operating frequency or narrower output orifices allows increasing ion energy, but leads to a decrease in ion flux. At the exit from the source, in the presence of a magnetic field, a directed flow of electrons was detected, the energy of which increases significantly with increasing magnetic field induction. The presence of wave processes in the discharge chamber, which may be responsible for the appearance of an accelerated electron flow, has been demonstrated.