In this work, an ultra-wideband microwave metasurface absorber with optically transparent and flexible properties is proposed. The metasurface is composed of a reflective backplane and a microwave absorption layer sandwiched between two dielectric substrates. The impedance matching curves of the microwave absorption layer are deduced based on the impedance matching theory, which is quite helpful and useful to improve the accuracy and efficiency of the broadband optimization design. Simulated results show that absorption higher than 90% can be achieved in the frequency band ranging from 5.8 GHz to 27.3 GHz, which covers the radar wavebands of C, X, Ku and K. The relative bandwidth reaches up to 130%, thus realizing ultra-wideband absorption while the thickness of the metasurface is only 0.12 times the upper-cutoff wavelength. For the TE (transverse electric) wave incidence, the metasurface maintains good performance when incident angle θ ≤ 50°, while for the TM (transverse magnetic) wave incidence, the absorption higher than 90% can be still achieved in a broad frequency band when θ ≤ 60°. It can be seen that using double-layer dielectric substrate in the metasurface not only greatly expands the microwave absorption bandwidth, but also improves the oblique incident properties. In addition, the metasurface is insensitive to polarization since its unit cell is symmetrical. Moreover, by rationally designing materials, the metasurface in this work is optically transparent and flexible, thus quite suitable for window radar stealth and equipment conformal stealth.

For the issue of stealth and electromagnetic protection of weapons and equipment in strong electromagnetic pulse (EMP) environment, a transparent double-sided ultra-wideband metasurface is proposed to realize wave absorption/ absorption-scattering integrated function in two directions. The interactions between backscattered waves produced by the meta-atoms are manipulated and optimized simultaneously by hybrid far-field scattering pattern analysis and genetic algorithm to achieve ultra-wideband destructive interference. Both simulation and experimental results demonstrate that the polarization-independent -10 dB radar cross section (RCS) reduction performance can be achieved for more than 128% of the fractional bandwidth (4-18.25 GHz) at forward incidence, and the absorption more than 90% in the range of 2.75-12.75 GHz (fractional bandwidth more than 129%) at backward incidence. The scattering and absorbing characteristics of the metasurface are still well maintained in the range of 0°-50° oblique incidence. The method of this paper has an important prospect in related applications of strong EMP stealth and protection technology under the integration of information equipment in the future.

The unconditionally stable (US) Chebyshev (CS) finite-difference time-domain (FDTD) method is extended for solving the problems of long-time simulation or harmonic resonance using a piecewise calculation scheme. First, the CS differential matrix is derived from the inversion of integral matrix with the differential characteristic of the CS polynomials. Then, the 2-D CS FDTD formula with initial values is derived based on the CS differential matrix, highlighting the merits of a closed interval for the CS basis functions and the 0th-order CS polynomial. In addition, the time–frequency support of CS functions for order selection is discussed. Finally, based on the above derivation, a piecewise calculation scheme is proposed to simulate an entire time, where the electromagnetic field is reconstructed piecewise with the Clenshaw law. Numerical examples for the 2-D ${\rm TE}_z$ case show that the proposed method agrees well with the conventional FDTD method with the relative difference lower than -50 dB. This represents a higher accuracy than the associated hermite (AH) FDTD method and reduces the memory compared with the original CS FDTD method.

Vortex beam generators, particularly those operating in transmission mode, are extremely important in modern communication systems because they are believed to be an effective way to extend the capacity of a communication channel. However, current approaches have suffered from complex configurations, fixed operation modes, and low efficiency, particularly in a transmissive case. In this study, we propose a new strategy for improving the transmissive amplitude and bandwidth of metasurfaces by optimizing microstructures with a non-uniform thickness based on the transfer matrix method (TMM). As a result, our designed meta-atom can achieve a high transmission of greater than 0.9 within a wide frequency interval of 12.6–16.2 GHz. As a proof of concept, we designed a vortex beam generator with topological charge $l = 1$ using the designed meta-atom. We conducted near- and far-field experiments to characterize its performance, indicating that our meta-device can realize a pure vortex beam with an efficiency of higher than 82.1% at a central frequency of 15 GHz. Our findings will lay a theoretical foundation for studies on metasurfaces with a non-uniform thickness and provide a way to design high-performance meta-devices with broad bandwidths.

The characteristics of two natural positive cloud‐to‐ground flashes are analyzed in detail with the broadband very high frequency (VHF) mapping system. The bidirectional leader processes at the lightning initiation are observed, with negative leaders propagating upward and positive leaders propagating downward. The downward positive leaders can be divided into two stages, with scattered and slowly extending sources in the first stage and more concentrated and faster sources in the second stage. The overall extension speeds of the first stages are 1.8 × 10⁴ and 2.7 × 10⁴ m/s for the two flashes, and the speeds of the second stages are 7.8 × 10⁵ and 1.0 × 10⁶ m/s, respectively. The received peak power of the positive leaders is substantially lower than that for the negative leaders. The frequency characteristics show that the second stage emits more high‐frequency radiation than the first stage, while there is no significant difference in low‐frequency radiation content.

A low-frequency lightning location system was established and operated. The time-reversal approach in the frequency domain (TRFD) is applied to the system for three-dimensional (3-D) lightning source location finding. The TRFD method uses back propagation of the signals detected at each site and determines the source location automatically using the focusing point of the back-propagated waves. Three cases are reported to showcase the capability of the method. The first case is the 3-D localization of an intra-cloud flash over the network. Both TRFD and time-of-arrival (TOA) method provide LF lightning sources that are in consistent with very high frequency (VHF) sources. The lightning sources located by TRFD method reveal more details and are more continuous compared to those obtained by TOA method used in this paper. The second case concerns the localization of an artificially triggered lightning flash with a horizontal error of 30 m by TRFD. The third case is a multi-stroke cloud-to-ground flash that shows location differences between four return strokes (RSs) being less than 60 m. Furthermore, a special case was used to demonstrate the ability of the proposed method in locating RS grounding points by combining two successive RSs when the TOA is not available.