Terahertz (THz) imaging technology, as an emerging imaging method, has the advantages of being non-invasive, non-destructive, and high-resolution, and has achieved significant progress in recent years. This paper reviews and analyzes terahertz imaging technology from the perspectives of technical approaches and current research status. Based on the type of signal source, the technical characteristics of pulsed terahertz imaging and continuous-wave terahertz imaging mechanisms are compared and analyzed. On this basis, the paper focuses on the technical solutions with super-resolution and high-speed imaging capabilities, analyzes their current development status, and discusses their advantages in future terahertz imaging scenarios. Finally, the challenges of application-oriented terahertz imaging technology are summarized and prospected.

Interferometric imaging technology can accurately measure the tiny deformations or displacements on the surface of objects in a non-contact manner, demonstrating extensive application potential. In this paper, a reflection and transmission interferometric system based on the terahertz band is designed and constructed. This system directly reconstructs images using phase information after scanning. Experiments show that at frequencies of 164 GHz, 172 GHz, 196 GHz, and 204 GHz, the imaging quality and resolution of the Michelson interferometric system are superior to those of the direct measurement method. At 150 GHz, without using any imaging algorithm, the system can achieve half-wavelength resolution, with a contrast improvement of 50% compared to direct measurement. At 180 GHz, a Mach-Zehnder interferometric system is implemented, where the dual-path difference effectively reduces phase noise, thus proving the feasibility and advantages of the system. Terahertz interferometric technology provides a noncontact, high-resolution imaging solution for the imaging field.

A terahertz fourth harmonic mixer based on anti-parallel Schottky diode pair is introduced, and its Radio Frequency (RF) operating range covers 400 GHz to 600 GHz. The mixer use a multi-point ground structure at the RF ends, which effectively reduces the return loss of RF port compared to the traditional single point ground structure. By adopting an integrated RF-end design method, the number of matching branches is reduced by 30% during the design process compared to traditional methods, thereby enhancing design efficiency. The measured results show that the typical value of conversion loss is 20 dB and the optimal value is 14 dB in the whole working band when the Local Oscillator (LO) power is in the range of 7～13 dBm.

The quality of the insulation layer of high-voltage cables is crucial for the long-term reliability of power transmission and transformation systems. Non-uniformity in the thickness of the insulation layer can lead to localized tangential electric fields, which are prone to creating safety hazards. A reflective terahertz detection system, combined with a cylindrical coordinate scanning device, is employed to achieve full-angle scanning measurements of the insulation layer thickness of cross-linked polyethylene high-voltage cables. The non-roundness of the insulation layer thickness is quantified, and its distribution uniformity is assessed. The terahertz images intuitively display the interface wrinkle texture features of the insulation layer, which are highly consistent with the actual specimen morphology. This validates the effectiveness of terahertz imaging technology in the quality inspection of high-voltage cables, providing a new detection method and evaluation approach for the quality assessment of high-voltage cables.

Millimeter-wave holographic imaging systems based on phase imaging have been extensively applied in fields such as security screening and non-destructive testing, due to their special penetration capabilities and high-resolution advantages. While extending operational frequencies to higher bands for achieving better resolution, the problems of dense transceiver arrangements and high system complexity need to be considered. This study presents a W-band short-range high-resolution millimeter-wave holographic imaging system utilizing linear array scanning. By implementing a 25-transmitter and 100-receiver array configuration with integrated electronic circuitry design, the system achieves high-quality short-range imaging across the 85～105 GHz operational band. Experimental results demonstrate the system can obtain spatial resolutions superior to 2 mm in the horizontal direction and 2.5 mm in the vertical direction at an imaging distance of 0.5 m, confirming its enhanced performance in practical applications.

To meet the low electromagnetic scattering requirements of optical instrument windows, a metal microstructure model based on Voronoi diagrams is employed, arranging the metasurface structures in a checkerboard array. This design achieves effective reduction of Radar Cross-Section (RCS) while satisfying the transparency and imaging quality requirements of the instrument windows. Experiments shows that the checkerboard metasurface without transparentizing could achieve more than 10 dB RCS reduction in the frequency band of 11.6～17.9 GHz, and the phase response of the metasurface unit remains consistent before and after transparentizing. The designed transparent RCS-reducing metasurface has high light transmittance, can uniformly diffract stray light distribution, and possesses the characteristic of wideband RCS reduction. The research results provide a new idea for the design of low-RCS transparent windows.

With the development of Low-Earth Orbit (LEO) satellite systems, satellite-borne phased array antennas have been applied in satellite systems, providing the conditions for phased array measurement of the Direction of Arrival (DOA) of satellite-borne electromagnetic waves. In the current satellite direction-finding and positioning systems, the single-DOA positioning system is unable to locate aerial targets, and the dual-satellite Time Difference of Arrival (TDOA) and DOA joint positioning system requires synchronized measurement and calculation by two satellites with simultaneous visibility of the target to achieve positioning. To address these issues, a positioning method based on single-satellite dual-DOA is proposed. The principle and algorithm of single-satellite dual-DOA positioning are analyzed. Through simulation, the quantitative impact of different satellite intervals, direction-finding errors, and satellite position errors on the positioning accuracy of satellites is calculated. The Geometric Dilution of Precision (GDOP) of this positioning system is derived, and its feasibility is verified through simulation.

To address the issue of high computational load and long processing time in the imaging calculation process of lightning radiation sources using time-reversal algorithms in the frequency domain, a series of optimized acceleration methods for time-reversal algorithms are proposed. Firstly, the introduction of Graphics Processing Unit (GPU) parallel computing is further expanded to dual-GPU cluster computing, while adopting multi-threading concurrent programming methods of Central Processing Unit (CPU) and GPU to cover the time consumption caused by mutual waiting; secondly, the maximum energy search algorithm and phase difference filtering algorithm and other programs are written to the GPU to achieve parallel search and calculation of phase difference values; finally, the data acquisition and transmission module, GPU processing module, and positioning imaging module are integrated to achieve the integrated function from data collection to positioning imaging of lightning data. For a 500 ms segment of Very High Frequency (VHF) lightning data, the actual measurement only takes 18 minutes, which is about 703 times faster than the previous CPU single-line processing process, and the number of effective positioning points is only reduced by 9.3%.

In response to the current state and developmental requirements of HF (High Frequency) reconnaissance and localization, research work on the processing of ionospheric wave propagation multipath and polarization information has been conducted. This research investigates the simulation data of shortwave ionospheric multipath propagation delay and polarization matching, and quantitatively analyzes the relationship among the polarization state of the electromagnetic wave signal when it is received by the antenna, the polarization state when the electromagnetic wave signal is emitted, and the changes in the polarization state of the electromagnetic wave during its propagation in the ionosphere. A shortwave ionospheric multipolarization propagation model is constructed, and the research on the relationship between shortwave ionospheric multipath propagation delay and polarization matching is completed. The simulation results indicate that for shortwave signals, their polarization rate after passing through the ionosphere is only related to the angle between the propagation direction and the geomagnetic field.

A compact, high-gain ±45° dual-polarization patch antenna is proposed. A rectangular patch antenna operating in TM05 mode is investigated and modified. The out-of-phase current portions of the patch are replaced by thin microstrips, minimizing the influence of the out-of-phase currents on the radiation pattern. Considering the one-dimensional antenna array configuration required in base station applications, the thin microstrips are meandered so that the proposed antenna structure size is significantly reduced compared with that of the traditional dual-polarized antenna. A pair of differential power dividers is utilized to replace the complex feeding network of the normal antenna array, thus simplifying the structure of the antenna. The simulated S11 of the proposed dual-polarized antenna is less than -10 dB in the range of 3.4 to 3.6 GHz and the measured realized gain at the operating frequency range is greater than 11.3 dBi.

Metasurfaces have demonstrated extensive application potential in multiple fields. During the design of metasurfaces, it is necessary to optimize the components based on factors such as polarization, amplitude distribution, and phase distribution. This process typically requires the involvement of experts and is time-consuming. A method is proposed for inverse design of components that integrates high-precision ultra-wideband spectral forward prediction using neural networks and genetic algorithms. This method can simultaneously predict the amplitude and phase of a 16×16 high-degree-of-freedom discrete grid structure within the frequency range of 0.5～2 THz. The amplitude prediction accuracy can reach 0.019, and the phase prediction accuracy can reach 4.332°. The average optimization time for a single metasurface unit is 1.5 min. Two sets of 3 bit frequency-multiplexed and polarization-multiplexed metasurfaces are designed and simulated, and the simulation results validate the effectiveness of the proposed method. The method provides a new paradigm for the rapid design of components facing complex application scenarios and is of significant reference value to metasurface designers.

In battlefield communication confrontation, the rational allocation of interference parameters has always been a challenging task. Based on Deep Reinforcement Learning (DRL), this paper allocates the interference power, interference waveform, and interference target for the jammer, saving resource consumption and improving resource utilization while ensuring the effectiveness of interference. Specifically, the problem of interference parameter allocation is constructed as a fully cooperative multi-agent task. The SA (Stochastic Attention)-QMIX (Q-value based Mixing) algorithm is adopted to mitigate the issue of high decision-making dimensionality in multi-agent scenarios. By introducing the maximum entropy method and multi-head attention mechanism into the QMIX algorithm, the agents can make more effective collaborative decisions in partially observable environments. Simulation results show that when using the SA-QMIX algorithm for interference parameter allocation, compared with the traditional QMIX algorithm, it can increase the interference success rate by 5% while reducing the interference power by 1.5 dB. Moreover, the algorithm in this paper can converge faster, with the convergence speed being improved by approximately 40%.

Aiming at the problems of low efficiency and slow speed in current fall detection algorithms, a novel human posture-based fall detection algorithm is proposed. This algorithm obtains human skeletal key points information based on OpenPose and determines the human fall state based on three criteria: the descent speed of the center of gravity, the body tilt angle, and the deformation ratio of the body contour. During the experimental phase, compared with methods solely based on deep learning or wearable devices, the proposed algorithm shows the best performance, with a detection sensitivity of 98.35%, specificity of 96.79%, and accuracy of 97.11%. The experimental results verify the stability and reliability of the proposed algorithm, which has a broad application prospect.

Aiming at the problem of low accuracy in existing bird nest detection on transmission towers, a UAV (Unmanned Aerial Vehicle)-based bird nest detection system for power transmission lines based on the Lightweight Residual Convolutional Attention Network (LRCAN) is proposed. On the basis of analyzing the workflow, a bird nest detection model for power transmission lines based on LRCAN is proposed, which enables the network to focus more on the required detail features and suppress the interference of other irrelevant information. The normal convolution in the feature fusion network is modified by using depthwise separable convolution layers to reduce the number of network parameters. Simulation results show that compared with YOLOX-S, which has a similar number of parameters, the proposed model has increased the mAP (mean Average Precision) by 5.4%. Compared with YOLOX-L and YOLOX-X, which have the same level of mAP, the number of parameters in the proposed model is reduced to 1/5 and 1/10 of theirs, respectively.