Low Frequency Operation Research Articles

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.

Selection of an Optimal Frequency for Offshore Wind Farms

Offshore wind power has attracted significant attention due to its high potential, capability for large-scale farms, and high capacity factor. However, it faces high investment costs and issues with subsea power transmission. Conventional high-voltage AC (HVAC) methods are limited by charging current, while high-voltage DC (HVDC) methods suffer from the high cost of power conversion stations. The low-frequency AC (LFAC) method mitigates the charging current through low-frequency operation and can reduce power conversion station costs. This paper aims to identify the economically optimal frequency by comparing the investment costs of LFAC systems at various frequencies. The components of LFAC, including transformers, offshore platforms, and cables, exhibit frequency-dependent characteristics. Lower frequencies result in an increased size and volume of transformers, leading to higher investment costs for offshore platforms. In contrast, cable charging currents and losses are proportional to frequency, causing the total cost to reach a minimum at a specific frequency. To determine the optimal frequency, simulations of investment costs for varying capacities and distances were conducted.

Wide-band microwave absorber with polarization insensitivity and incident angular stability using resistive ink for stealth application

A wide-band microwave absorber using five layers of resistive ink patterns is designed with a periodicity of 0.15λL, where λL denotes the wavelength at the lowest operating frequency. Symmetrically shaped resonators are patterned on five layers of FR4 substrates with air spacers between them. The fractional bandwidth of the absorber is 140% with absorption greater than 90% attained in the operating frequency range from 3.38 GHz to 19.5 GHz. Cross-polarized absorption of the structure is found to be less than 0.1%. Absorption is stable for a wide range of incident angles up to 40∘. Absorption behavior is found to be stable for both TE and TM polarized incident waves under normal incidence. Further, the mono-static radar cross-section is analyzed and found to be reduced below -40 db in the entire operating frequency.

Evaluating the Impact of Geometric Dimensions of a Multi-Split Ring Resonator

In this paper, a metamaterial unit cell is designed and simulated based on the concept of multiple split-ring resonators (SRRs) in a compact size. The presented nested modified SRR resonator construct has the advantage of achieving more usable split gaps by increasing capacitance, resulting in a lower operational resonance frequency and improved sensitivity over typical SRR metamaterials. The impact of several split ring resonator geometric characteristics, such as the gap space, the width of the copper lines, the inner rings length, and the outer ring length on magnetic permeability and permittivity was analyzed. The modified metamaterial can be utilized for 5G mobile applications.

A new single modulating and single carrier signal based control technique for symmetrical and asymmetrical multilevel inverter topology

Abstract In literature, to generate gate pulses for any inverter topology, most of the authors are using a single modulating signal with (level-1) or (level-1)/2 carrier-based pulse width modulation (PWM) techniques and single carrier (SC) signal with (level-1)/2 modulating signal based PWM techniques. Then, while going to higher-level inverter topologies, the complexity of the PWM control approach in both cases can increase. To reduce the control complexity, few authors have reported a single modulating signal with single carrier-based PWM techniques. However, these techniques need more mathematical equations and if-else loops. This article proposes a new single-modulating and single-carrier signal-based generalized PWM control approach using floor function (FF). This technique has been verified by considering one of the single-phase conventional symmetrical and asymmetrical multilevel inverter (MLI) topologies. Generally, FF rounds the value to the nearest integer towards the negative infinity. The proposed PWM control implementation process is very simple and suitable for both high switching frequency (HSF) and low switching frequency (LSF) operations. The proposed control technique (PCT) has been experimentally verified with different steady-state and transient-state studies of results under standalone operation.

A droplet-shape inspired wideband bow-tie dielectric resonator antenna for sub-6 GHz and IoT applications

In this article, an approach to realizing a dielectric resonator based bow-tie shaped antenna for the sub-6 GHz, 5G and IoT applications is discussed that utilizes droplet-shaped radiators for its two arms. The droplet-shaped radiating element is systematically derived to obtain a wider bandwidth response by truncating the sides of an aperture-coupled cylindrical DRA operating at HE11δ mode. The proposed bow-tie dielectric resonator antenna (DRA) is a modified dipole structure that uses a balanced line to differentially feed the two radiating segments to attain an overall wideband characteristics. A wideband matching balun circuit facilitates the antenna feeding which produces equal amplitude signals with 180° phase difference at its two output ports throughout the band of interest. The measurement of the proposed antenna confirms that it can serve several 5G bands like n77 (3300–4200 MHz), n78 (3300–3800 MHz), n48 (3550–3700 MHz) and n79 (4400–5000 MHz) with an overall wide bandwidth of 42.7% in the sub-6 GHz range. This planar configuration has a realized gain value ≥7.5 dBi within 3.37–3.91 GHz and it remains higher than 6 dBi almost throughout the working bandwidth of 3.18–4.91 GHz with considerably wide beamwidth. This broadband DRA has a consistent broadsided radiation pattern with cross-polarization discrimination (XPD)> 20 dB and its radiation efficiency varies between 60%–70% within 3.2–4.8 GHz. The antenna occupies a volume 1.2λ×1.2λ×0.1λ corresponding to its lower frequency of operation and it can support many more significant applications including WiMAX (3.3–3.8 GHz), Citizens Broadband Radio Service (CBRS) (3.55–3.7 GHz) and LTE (3.41–3.5 GHz uplink/3.51–3.6 GHz downlink). All these characteristics make this antenna a suitable candidate for use in transceiver modules for indoor communication, base stations, and vehicular communication.

A Compact Dual-polarized Probe-fed UWB Antenna System for Breast Cancer Detection Applications

This article proposes an electrically small, probe-fed Ultra-Wideband (UWB) monopole antenna on a slotted truncated ground plane for breast cancer detection. The physical footprint of the proposed antenna element is 33 mm × 35 mm × 0.5 mm. This element is designed on the low-cost FR4 Epoxy substrate with a thickness of 0.5 mm. The proposed antenna has an electrical size of 0.33λ × 0.35λ × 0.005λ at the lowest frequency of operation; the radiator offers an impedance bandwidth of 8.34 GHz, which implies a fractional bandwidth of 115.5%. A compact dual-polarized antenna topology with two orthogonally placed probe-fed monopole antennas is proposed to achieve polarization diversity. The impedance characteristics of the individual radiating elements are maintained in spite of the electrical proximity of the radiators. The numerically computed and experimentally measured results of dual-polarized electrically small UWB antenna agree quite well. The proposed dual-polarized antenna topology is investigated for its utility in breast cancer detection in simulation.

A GPU-accelerated 3dBEM supporting the optimization of actively morphing flapping-foil thrusters with application to AUV propulsion

In recent years, flapping thrusters have been getting attention as an eco-friendly alternative to conventional propellers for autonomous underwater vehicles (AUVs) mainly due to their low-frequency operation, advanced maneuverability, and high efficiency. Their operating principle mimics the thrust-producing kinematics found among efficient swimmers in the aquatic environment. The present work focuses on the analysis and optimization of flapping systems with active morphing features for increased performance. The two optimization studies refer to a realistic AUV propulsion scenario for a wing thruster with active hydrofoil-section adjustment, spanwise bend and twist morphing capabilities. A gradient descent approach based on sequential quadratic programming (SQP) is used for optimal tuning of design variables (planform shape, kinematics) targeting efficiency maximization under thrust and effective angle of attack constraints. For the evaluation of each candidate thruster the unsteady boundary element solver 3dBEM is developed and used for wing performance prediction. The solver exploits GPU parallel computation techniques to reduce computational time and enable fast optimization. Results indicate that the optimal thruster with chordline/spanwise bending is 25% more efficient, whereas the same result for the wing with spanwise bending and twist profiles is up to 8.8%. The 3dBEM solver can support the preliminary design process of morphing flapping-foil thrusters.

Common-Mode Voltage Reduction of Modular Multilevel Converter Using Adaptive High-Frequency Injection Method for Medium-Voltage Motor Drives

This study proposes an adaptive high-frequency injection method (AHFI) aimed at mitigating common-mode voltage (CMV) on the AC side and alleviating current stress on power semiconductor devices within each arm of a medium-voltage motor propulsion system designed for modular multilevel converters. By adjusting the quantity of high-frequency components injected into each arm according to the fluctuation coefficient, the amplitude of injected high-frequency CMV and circulating currents can be reduced across medium to rated motor speeds. This approach enhances the start-up performance of medium-voltage motor drives while diminishing CMV effects on the motor side, resulting in decreased total harmonic distortion (THD) in the three-phase output waveforms. Furthermore, the effectiveness of the proposed AHFI method in SM voltage regulation and circulating current control under low-frequency operation is thoroughly analyzed. The validity of this method is established through comprehensive mathematical scrutiny and time–domain simulations performed using MATLAB/SIMULINK software, along with real-time simulations conducted employing the real-time simulator OPAL/RT via hardware-in-the-loop simulation (HILS).

A low-frequency ultrathin metamaterial absorber using magnetic material

In this paper, an ultra-thin metamaterial absorber (UMA) in the S-band is designed. The overall structure adopts a three-layer design. The top layer is a magnetic material, and the middle layer of the UMA is composed of a centrally symmetrical open square ring (four edges are opened) and a circular pattern placed at the center, which is etched on the FR4 dielectric substrate by printed circuit board technology. The absorptivity of the absorber is more than 90% in 1.73–4.04 GHz under normal incidence, and the fractional bandwidth is 80.1%. The total thickness of the absorber is about 3.4 mm, corresponding to 0.02 λL at the lowest operating frequency. Moreover, it also has good polarization insensitivity and wide-angle incident characteristics. The absorption mechanism of the UMA is analyzed from the aspects of power loss density and surface current. Finally, a sample was fabricated and measured. The measured results are in good agreement with the simulation results, which verifies the effectiveness of the design method.

A modular multilevel converter with active power filter (APF-MMC) under low-frequency operation

A modular multilevel converter with active power filter (APF-MMC) under low-frequency operation

Amplitude methods for forming frequency-independent radiation characteristics of multibeam reflector antennas

The trend towards expanding the operating frequency ranges of radio-electronic equipment has led to an increasing interest in recent years in the study of ways to constructing multi-beam frequency-independent reflector antennas, which requires systematization of the principles and methods of their construction. When constructing reflector antennas, methods based on the selection of the directional properties of the feeds and methods based on the selection of the frequency-selective properties of the reflectors are used. Methods based on the choice of directional properties of feeds are divided into methods using horn feeds and methods using feeds in the form of cluster arrays with intersecting apertures. Horn feeds are used to create single-beam reflector antennas. Using cluster arrays, it is possible to form a plurality of beams of frequency-independent partial radiation patterns. Using the example of a hybrid reflector antenna, the possibility of forming nine beams in the azimuthal sector of 20° with a beam width of 3.2° ± 0.45° in the frequency band from 2 to 8 GHz is shown. The level of side lobes outside the working sector in the operating frequency band does not exceed –35 dB, the slope of the direction-finding characteristics varies from 4.8 to 9.8 dB/deg. Among the methods based on the choice of selective properties of reflectors, there are methods using frequency-selective surfaces and methods using corrective coatings on the working surface of the reflector. Frequency-selective surfaces are made in the form of wire meshes with variable pitch, in the form of a set of parabolic plates and in the form of a set of square waveguides with a side size ranging from half-wave at the upper frequency in the middle part of the reflector to half-wave at the lower operating frequency at the edges of the reflector. The disadvantages of using frequency-selective surfaces are the complex and expensive design of the reflector, low antenna efficiency, high side lobes and the need to use a large absorbing screen. When implementing the method using corrective coatings, the separate functions of a frequency-selective surface and a radio-absorbing screen are combined in one structural element – a frequency-dependent corrective coating applied to the working surface of the reflector. Among the advantages of this method, it is worth noting the simple design of the reflector and the possibility of its operation at any polarization, since the reflection characteristics of the coating are practically independent of the type of polarization. A common disadvantage of reflector antennas based on the choice of frequency-selective properties of reflectors is distortion of the shape of the main lobe of partial patterns and a larger range of scatter in the pattern width in the operating frequency band, which prevents the formation of linear direction-finding characteristics and reduces direction-finding accuracy. Due to the low efficiency of multibeam mirror antennas built using amplitude methods for forming frequency-independent radiation characteristics, their use as transmitting antennas is impractical. The completed review and identified features of methods for constructing frequency-independent radiation characteristics can become a methodological basis for the design of broadband multi-beam reflector antenna systems for advanced radio-electronic equipment.

Front‐to‐back ratio improvement of a compact base‐station antenna for 5G NR applications

AbstractThe front‐to‐back ratio (FBR) of a compact dual‐polarized antenna is improved for 5G new radio (NR) base‐station applications. A rim made of partially reflective surfaces is introduced to improve the FBR. A meta‐surface and a parasitic element are employed to lower the antenna height. The dual‐polarized base‐station antenna with improved FBR features a compact size of , where is the wavelength in free space at the lowest operating frequency. The FBR achieved over the 5G NR n28/n82/n83 bands (0.69–0.96 GHz) for return loss >15 dB is higher than 26 dB. The antenna gain is higher than 7 dBi and the isolation is higher than 30 dB.

Low-Power Capacitively Coupled AC Amplifiers With Tunable Ultra Low-Frequency Operation

Low-Power Capacitively Coupled AC Amplifiers With Tunable Ultra Low-Frequency Operation

Optimal Design of a Submodule Capacitor in a Modular Multilevel Converter for Medium Voltage Motor Drives

This paper proposes an algorithm for determining the optimal capacitance by utilizing a mathematical model of a submodule (SM) capacitor in a modular multilevel converter (MMC) specifically for medium voltage motor drives (MVDs). By approximating the voltage fluctuation of the SM capacitor during low-frequency operation, it is feasible to ascertain the minimum capacitance required for the SM capacitor, ensuring that its voltage fluctuations remain within an acceptable limit that is predefined as a specified value. Moreover, the study considered the injection of both a high-frequency common-mode voltage (CMV) and a circulating current to alleviate the SM voltage fluctuation during the acceleration of motor drives. The effectiveness of the proposed method is validated through verification using time-domain simulation results obtained using the MATLAB/SIMULINK software and real-time simulation results acquired using the OPAL-RT simulator platform.

A modified modular multilevel converter to reduce the second order ripples in the submodule capacitor voltage: Design and analysis

SummaryThe modular‐multilevel converter (MMC) faces the problem of second order ripples in the sub‐module DC capacitor voltage during the low frequency operation at the load side. To sort out this problem, this paper demonstrates a three‐phase modified MMC that includes a novel circuit placed between upper and lower arms of each phase, where load is connected through proposed circuit. The aim of the proposed circuit is to reduce the magnitude of the ripples in each sub‐module's DC capacitor voltage; moreover, it also uses a less number of components compared with the conventional circuits. In order to keep the capacitor voltage at its rated value and to prevent the development of common mode voltage, the proposed circuit uses the high frequency‐based power transfer control channel for balancing power between the upper or top and lower or bottom arms. Furthermore, the mathematical analysis, operating principle, and proposed control is explained in detail for the conventional MMC and modified MMC. The simulation results have been obtained for the modified MMC with resistive load by using MATLAB and Simulink. An experimental test results have been verified with simulation results for the modified MMC topology.

Continuous operating elastocaloric air-cooling device

<p>Elastocaloric cooling has shown significant potential as an alternative to traditional vapor-compression technology because it relies on nonflammable, non-toxic, and zero-global warming potential solid refrigerants (usually NiTi and TiNi-based shape memory alloys). However, the uniaxial loading mode commonly used in existing elastocaloric cooling devices is often accompanied by a lower operating frequency. Additionally, using water as the heat exchange medium presents challenges in further cooling the air with cold water. These issues will hinder the application of this type of refrigerators in space cooling scenarios. Here, we report a continuous operating tension-based elastocaloric air-cooling device that utilizes the bundles of NiTi wires. The driving mode based on the cam structure realizes the alternating loading-unloading cycle of multiple NiTi bundles, facilitating continuous the cooling process. Our air-cooling device achieves a temperature decrease of 4.4 �� in continuous air flow and provides a cooling power of 21 W. These results demonstrate the possibility of applying elastocaloric technology in space cooling and provide a technical reference for designing household air conditioners using this technology.</p>

Miniaturized High‐Performance Metasurface Antenna Embedded in Cross‐Shaped Strips

A miniaturized high‐performance metasurface (MTS) antenna with cross‐shaped strips is designed, which can be applied to wireless communication systems. The resonant frequency of the main mode of the proposed MTS is reduced by embedding a 2 × 2 cross‐shaped strip array in the gaps of the 3 × 3 circular MTS array, resulting in a lower operating frequency and miniaturization. The main mode of the MTS and the probe mode are excited simultaneously by the L‐probe structure, resulting in a dual‐mode resonance for a broadband performance. The entire design mechanism of the antenna is explained analytically through characteristic mode analysis (CMA) method. Additionally, the cross‐polarization levels in H‐plane are effectively suppressed by a metal via positioned at the MTS center. Measured results show a wide impedance bandwidth of 32.6% (reflection coefficient ≤ −10 dB) with an MTS size of 0.44λ0 × 0.39λ0 (where λ0 is the free‐space wavelength of 6.6 GHz). The achieved peak gain is 7.1 dBi, with a 2 dB gain bandwidth of 36% (5.25‐7.55 GHz) and a low cross‐polarization level below ‐20 dB.