Abstract To overcome the limitations of traditional crack propagation testing methods, such as high costs and the need for specialized equipment, this study presents a crack length prediction method that combines modal analysis numerical simulation with frequency response testing. The impact of crack length on natural frequency and mode shapes is investigated by constructing a finite element geometric model of the titanium alloy tensile specimen using modal analysis theory. Modal testing combined with digital image correlation (DIC) is then used to conduct experiments on the vibration transmission characteristics of the cracked tensile specimen under random loading conditions, validating the relationship between crack length and real-time natural frequency. The results show that the proposed method predicts crack length with an error margin of no more than 13.39%, and the experimental data are in good agreement with the simulation results. This confirms that cracks reduce the stiffness of the structure, resulting in changes in the natural frequency. Additionally, the effect of preload levels on the natural frequency is minimal, and the natural frequency decreases progressively as the initial crack length increases.

Vibration-based energy indicators have been widely studied for structural damage identification, offering an advantage over traditional methods by avoiding reliance on modal parameters. However, their accuracy is highly sensitive to the selection of appropriate frequency bands. Conventional techniques typically require prior knowledge of damage states to determine the optimal frequency range, limiting their applicability in unsupervised damage identification. To overcome this limitation, this article proposes a structural damage identification method based on the energy intensity ratio of specific frequency band components. By analyzing the regularities of frequency band information changes caused by damage base on modal parameter perturbation theory, a damage information entropy function is constructed to accurately select sensitive bands without supervision. Using the optimally selected frequency bands, equivalent energy features related to acceleration, velocity, and displacement are constructed. The precise identification of damage is then achieved using the energy intensity ratio indicator. The feasibility, accuracy, and robustness of the proposed method are validated through numerical simulations of high-rise building and two typical experimental cases of frame structures. The comparative analysis and detection results indicate that, compared to traditional full-band energy indicators, the proposed method significantly improves the accuracy of damage localization and the sensitivity to damage degree changes, demonstrating promising prospects for engineering applications.

Acoustic interference patterns (AIPs) typically exhibit a regular striation structure in shallow water. However, internal solitary waves (ISWs) commonly found in the ocean can cause severe distortion in AIPs, thereby presenting significant challenges to acoustic signal processing methods reliant on AIPs. Grounded in coupled mode theory, this paper elucidates the mechanism of AIP distortion induced by ISWs. Specifically, ISWs introduce several time-varying "coupled" components while maintaining the time-invariant "adiabatic" components. Leveraging the distinct temporal characteristics of these two classes of components, this paper proposes a time-averaging processing (TAP) method. This method averages the AIPs from multiple snapshots to preserve the time-invariant "adiabatic" components while suppressing the time-varying "coupled" components. Consequently, the TAP method restores the distorted AIP to its striation structure. The efficacy of the proposed method is substantiated through validation using simulated data. Moreover, the TAP method provides a foundation for enhancing the performance of underwater acoustic signal processing methods reliant on AIPs.

An ultra-wideband eight-port multiple-input-multiple-output (MIMO) antenna suitable for fifth-generation smartphones based on characteristic mode theory is proposed. The total size of the MIMO antenna is 154 mm × 74 mm × 1.5 mm. By exciting the TM10 mode, the bandwidth of |S11| < −10 dB covers 3.23-6 GHz. This bandwidth fully covers the N77/N78/N79 and wireless local area network bands for 5G applications. The antenna achieves a port isolation of better than – 15 dB and an envelope correlation coefficient of less than 0.02 without any decoupling structure. The total efficiency reaches approximately 90% over the operation band. Furthermore, simulation results show that the antenna performance remains almost unchanged under practical usage scenarios, such as handheld operation. Measurement results show that the S-parameters and radiation performance agree well with the simulated results.

ABSTRACT The summer monsoon onset in the Arabian Sea (AS) is closely linked to the northward propagation of the first monsoon intraseasonal oscillation (FMISO). Based on the moisture mode theory, this study investigates the underlying mechanism driving the FMISO's northward movement. Originating in the equatorial western Indian Ocean, the FMISO evolves eastward and northward across the AS, manifesting as a southeast‐northwest‐oriented convective belt that ultimately triggers the AS summer monsoon. The FMISO's northward propagation is governed by meridional contrast in moist static energy (MSE), with accumulation occurring north of the convection center and dissipation to the south. MSE budget analysis reveals that the zonal MSE advection, dominated by the moisture component, serves as the primary driver of MSE tendency. Physically, during the monsoon transitional period, the seasonal moisture evolution in the northern Indian Ocean has formed a mean eastward moisture gradient in the AS. Its interactions with FMISO‐induced easterlies (north of the convection center) and westerlies (south of the convection center) enhance moisture to the north and reduce it to the south, thereby propelling the FMISO northward. During the FMISO's northward propagation, notable positive sea surface temperature anomalies appear to the north of the convection center. By altering the air‐sea turbulent heat fluxes, these anomalies account for approximately 35%–40% of the positive MSE tendency, though their signal is obscured in net turbulent heat flux measurements due to complete cancellation by the pronounced negative effects of surface wind speed. This detailed exploration of FMISO dynamics potentially advances our understanding of the monsoon onset process in the AS.

To meet the demand for high efficiency in modern broadband communication systems, this paper presents a novel continuous-mode Doherty power amplifier design method based on integrated high-order filter prototypes. By deeply merging the filter structure with the output matching network, broadband impedance transformation and harmonic suppression are simultaneously achieved within the 1.6–2.2 GHz frequency range. This approach resolves the bandwidth limitations and efficiency degradation caused by the conventional separation of matching and harmonic control stages. Using a CGH40010F GaN transistor, the impedance space was determined through load-pull analysis, and the design flexibility was enhanced by applying continuous Class-F mode theory. The implemented amplifier demonstrates a saturated efficiency of 68–72%, a 6 dB back-off efficiency of 58.9–64.9%, a saturated output power exceeding 45 dBm, an in-band gain greater than 11.2 dB, and a return loss better than −15 dB. The proposed method offers an effective solution for the design of high-performance broadband power amplifiers.

Design of a flexible, highly isolated multiband f-meta MIMO antenna for 5G sub-6 GHz applications using the theory of characteristic modes

ABSTRACT We argue that S is in a position to know that p iff S can know that p. Thus, what makes position‐to‐know‐ascriptions true is just a special case of what makes ability‐ascriptions true: compossibility. The novelty of our compossibility theory of epistemic modality lies in its subsuming epistemic modality under agentive modality, the modality characterizing what agents can do.

Purpose Multi-source disturbances present a major challenge to the control systems of hypersonic vehicles, making their rapid estimation and compensation imperative for performance assurance. In response, this study aims to present a novel fixed-time robust control strategy grounded in sliding mode theory. Design/methodology/approach Guided by an analysis of the vehicle model’s dynamics, a control-oriented framework comprising multiple decoupled subsystems is developed. Then, a fixed-time control structure based on filters, observers and controllers is constructed. In this structure, the filters are used to soften the reference signals, while the observers are used to estimate the lumped disturbances arising from the flexible state variables, parameter perturbations, actuator faults and other contributing factors. Finally, the controllers incorporating disturbance compensation are used to drive the velocity and altitude tracking of reference values. Furthermore, the stability of the closed-loop system is analyzed based on Lyapunov theory. Findings A stable control system with fixed-time convergence has been developed for hypersonic vehicles, demonstrating strong robustness against multi-source disturbances and satisfactory velocity and altitude tracking performance. Compared to the robust control system based on disturbance observer, the developed control system has obvious advantages in the fuel equivalence ratio. Originality/value A key advancement presented in this study is a fixed-time control strategy that synergistically integrates filtering, extended state observers and sliding mode control. Such a synthesized strategy opens up new possibilities for maintaining robust flight control of hypersonic vehicles amidst challenging and rapidly changing operational environments.

The emission of light when an electron passes by a periodic photonic structure, called Smith-Purcell radiation (SPR), creates new types of adjustable light sources. Here, we investigate directional or asymmetric SPR from a high-index dielectric double-grating structure. By introducing a lateral shift between the two gratings, we break the symmetry and enable directional emission through interference between coupled Bloch modes, thus without the need for a mirror-type structure as in previous designs. Using full-wave simulations and eigenmode analysis, we identify the modes responsible for radiation and describe their coupling properties. A coupled mode theory (CMT) model is developed to capture the transition between symmetric and asymmetric regimes, offering fast and intuitive insight into the emission behavior. This work highlights a compact and efficient approach for controlling SPR directionality in dielectric photonic structures.

Abstract Dynamic manipulation of the magnitude and sign of nonreciprocal radiation is widely used in heat circulator, nonreciprocal solar cell, and nonreciprocal radiative cooling device. However, the effect of a magnetic field not being perfectly perpendicular to the incident plane on nonreciprocity remains unclear at present. To address this fundamental and critical issue, the effect of the magnetic field rotation angle on the nonreciprocal emitter with magnetophotonic crystal structures is comprehensively analyzed. The magnitude and sign of nonreciprocity can be dynamically manipulated via the rotation angle. The sign of nonreciprocity would be changed near the rotation angle of π /2. The devices exhibiting reciprocal behavior carry the rotation angle of π /2. Moreover, the nonreciprocal absorption and emission spectra can be dynamically manipulated by the rotation angle without additional introduction of graphene or phase change materials. The strong irreversible radiation is attributed to the optical Tamm plasmon polaritons at the interface between the photonic crystal and the metal. Coupled mode theory is adopted to elaborate the potential physical mechanism and further verify the validity of calculations. The generalized results that cover the case of multi-band nonreciprocal radiation prove the design flexibility and reliability. This work is believed to be helpful to improve the energy conversion efficiency of wavelength-selective thermophotovoltaic systems.

In this work, a dual key-shaped antenna for 28/38 GHz mmWave applications is designed, analyzed, and tested using the Theory of Characteristic Modes. The antenna is printed on a 0.254mm thick substrate and has a compact footprint of 10 × 12mm², making it suitable for modern space-limited 5G devices. It supports two operating bands centered at 28GHz and 38GHz, with measured fractional bandwidths of 12.5% and 21.05%, respectively. At the lower band, the first four characteristic modes dominate the radiation, while Modes 2, 3, 4, 6, and 10 mainly shape the higher-band performance. To further enhance gain and cover wider practical needs, the design is extended into a four-element linear array with an overall size of 19.75 × 26 × 0.254mm³. The array provides peak gains of 10.5 dBi at 28GHz and 11 dBi at 38GHz, while maintaining more than 75% efficiency across both operating bands. A prototype of the antenna and array was fabricated and tested using in-house measurement facilities, showing good agreement with the simulated results. The proposed antenna system meets the key performance requirements for 5G mmWave communication and offers a compact and efficient solution for future 28/38 GHz wireless devices.

ABSTRACT In this study, we present the design and analysis of a mode filter based on the use of a 3-waveguide coupler. The coupler is assumed to have a multimode central guide surrounded by two similar single mode waveguides. A theoretical model is developed based on the coupled mode theory and using the coherent tunneling adiabatic passage approach. The model shows that the coupler can be characterized by an effective coupling length that accounts for the coupling of each of the outer guides with the central guide. The estimated coupling length is in good agreement with the numerically calculated one using the Semivectorial Beam Propagation Method SV-BPM. The filter function is also well explained by the supermode interference in the structure. Based on this model, the design of a 3-waveguide mode filter built on SiN multilevel technology is introduced. An optimized design of the filter showing a crosstalk better than −40 dB at a wavelength of 1550 nm is demonstrated. This opens the door for new 3D integrated optical circuits in which the transition from single-mode to multi-mode in the vertical direction can be handled.

Abstract Rescher (1962) showed that “most” could not be reduced to the universal and existential quantifiers, effectively disproving the idea that semantics can be indirect truth-definition by paraphrase into first-order quantification theory. But a truth-definition, according to a footnote added in 1984 to “Truth and Meaning” must be a modal theory, supporting counterfactuals about the truth-conditions a speaker's merely possible utterances would have. A subjunctive truth-definition, with predication supplemented by two complex predicate-formation primitives devices and a propositional function abstraction operator, both adapted from Stalnaker's work, yields a theory of semantic form that is directly Tarskian. The truth-conditions of every expression in the object-language are given by an expression in the meta-language. The theory involves no paraphrase. We apply that theory to comparative predicates, including the quantifiers “much” and “many,” their comparative “more,” and their superlative “most.” “More” is a dual quantifier of which “most” is the general case. “Most” is a most interesting adverbial predicate, with applications beyond modifying a propositional function to yield a quantifier phrase.

Mathematical theory for the interface mode in an acoustic waveguide bifurcated from a Dirac point

Abstract Quasi-bound states in the continuum (QBIC) has been demonstrated as an effective platform for enhancing light absorption in monolayer graphene. However, the absorption wavelengths of graphene metasurfaces typically shift as the asymmetry parameter is altered, and achieving robust graphene light absorption at a specific wavelength remains to be explored. Herein, we demonstrate that perfect absorption of graphene with wavelength stabilization can be realized by exciting the symmetry-protected QBIC (SP-QBIC) in metasurfaces with area compensation. Coupled mode theory enables reliable modeling of the absorption of the graphene metasurface with area compensation. The far-field multipole decomposition and near-field distribution reveal that the resonant absorption from the SP-QBIC is predominantly driven by the magnetic dipole mode. In addition, the absorption efficiency is robust to the variation of the structural parameters. The absorption intensity at the operating wavelength can be modulated by tuning the Fermi level of graphene or the azimuthal angle of the incident light. Our findings offer a route for the dynamic regulation of the absorption of graphene, facilitating the development of multifunctional switchers and absorbers based on QBIC.

End-fire radiation characteristics and high gain performance of a miniaturized printed monopole antenna based on theory of characteristic modes

A window-embedded broadband vehicle-mounted antenna for frequency modulation (FM) broadcast application is proposed. Antenna miniaturization at sub-gigahertz frequencies remains challenging due to the inherently long wavelengths, which impose strict constraints on compactness, bandwidth, and structural weight. A promising strategy to alleviate this problem is to use the vehicle itself as an effective radiator to enhance the bandwidth and maintain good radiation performance. In this work, the potentialities of the radiation patterns offered by the vehicle are analyzed by using the characteristic mode theory (CMT). A compact T-shape coupling element, with dimensions of 0.2λ0 × 0.08λ0 × 0.01λ0, is employed to simultaneously excite multiple significant characteristic modes, thereby broadening the operating band. Both simulated and measured results validate that the proposed antenna can cover the FM broadcast operating band from 87 MHz to 108 MHz, with the 1:10 scaled prototype achieving a maximum measured gain of 7.4 dBi at 950 MHz. The proposed antenna and design strategy have advantages in radio broadcasting, radio navigation, and military and law enforcement communication systems for its low-cost, compact, and easy conformal structure.

This paper puts forward a novel multi-agent deep reinforcement learning (MADRL) control framework based on deep deterministic policy gradient (DDPG) algorithm for tracking control of 2 degree-of-freedom (DoF) helicopter system. To handle the nonlinear dynamics and uncertainties in the helicopter system, we formulate a model free data-driven approach which exploits the potentials of reward shaping technique to realise an adaptive and cooperative control policies for efficient trajectory tracking. Specifically, in this study to ensure safe and efficient RL learning without violating the hard constraints of the 2 DoF helicopter, we harness physics informed reward shaping (PIRS) technique which augments domain specific knowledge with data-driven learning. For estimating the pitch and yaw velocities of the helicopter, this study adopts a super twisting observer (STO) based on the second-order sliding mode theory. The key distinguishing features of STO is that it can ensure finite-time convergence with minimal chattering. The efficacy of the proposed MADRL augmented with STO is experimentally validated on a laboratory scale 2 DoF helicopter for several realistic test scenarios including bounded disturbances and uncertain dynamics. The experimental results highlight that the proposed framework can offer better tracking and robustness features compared to conventional state feedback control techniques.

Perfect sound absorption (PSA) plays a vital role in various acoustic applications. Acoustic unidirectional guiding resonances (AUGRs) are a novel class of topologically enabled guided resonance that only radiates toward a single side. The symmetries and unidirectional features of AUGRs reduce the number of ports to one that simultaneously adds or drops energy. As a result, we theoretically show that with the addition of thermos-viscous losses, AUGRs can be used to achieve unidirectional PSA with single-sided illumination without the need for any backing mirrors. The good agreement between the results of temporal coupled mode theory (TCMT) and full-wave simulations confirms our findings. In the parameter space spanned by frequency and incident angle, topological spectral phase singularities (SPSs) appear and are associated with topological charges, which confirm the robustness of the PSA phenomenon against variations in structural parameters and physical conditions. Our proposed structure may contribute to the practical applications of advanced absorbers and sensors.