The vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) offers a new degree of freedom for radar techniques, although to date its application to synthetic aperture radar (SAR) imaging has not yet been widely reported. In this paper, a SAR imaging model based on vortex EM waves is derived. Ku-band vehicle-borne vortex EM waves SAR experiment using phased uniform circular array (UCA) antennas is reported for the first time. A detailed comparison with conventional plane EM waves SAR imaging is given in terms of signal-to-noise ratio, and azimuth resolution. The experimental results demonstrate the effectiveness of the proposed imaging model, and indicate that vortex EM waves can be exploited in SAR imaging with higher azimuth resolution than plane EM waves. OAM, which is completely independent of frequency and polarization, is a new degree of freedom in SAR imaging. This work shows the new applications of vortex EM waves, which paves the way for the development of novel SAR systems.

The dual-active-bridge (DAB) converter is applied for power transmission and voltage conversion in dc power grid. To improve the quality of input current, an input LC filter is cascaded to DAB converter. However, due to the interaction between LC filter and DAB converter, substantial oscillation and instability occur, leading to the excessive electromagnetic interference, large voltage ripple, and power loss and even operation failure. To solve this issue, an active damping strategy is proposed in this article to reshape the small-signal input impedance by parallel virtual impedance. The input voltage is regarded as a control objective, as well as the output voltage, therefore, a triple-closed-loop control structure is established. Besides, all dynamics of controllers are considered for the input impedance modeling of DAB converter. Moreover, compared with traditional methods, an active damping method for cascaded DAB converter with LC filter is adopted to design the virtual impedance and controller, suppressing resonance of LC filter and achieve a larger bandwidth. Therefore, a more stable and rapid dynamics cascaded system composed of DAB converter and LC filter are achieved. Finally, a prototype is set up and the effectiveness and superiority of proposed strategy are verified by experiments.

In this letter, the authors put forward a symbolic approach to analyze the nonlinearities of switching power converters. This proposed method can be used to describe bifurcation and chaos behavior by means of coarse-grained symbols. First, the period-doubling structure of a symbolic series is studied in symbolic dynamics. Second, the block entropy, which is the related statistics of subsequences of a symbolic series, is introduced. Finally, two Boost converters are studied as examples to illustrate the applications of the proposed symbolic dynamics method. Via block entropy, both period-doubling and border collision bifurcations are identified and quantified, and initial value sensitivity is successfully used to distinguish chaos from periodic motions.

The smart unmanned aerial vehicle (UAV) with a mini-SAR payload provides an advanced earth observation capability for target detection and imaging. Simiar to large-scale SAR, mini-SAR also has an increasing requirement for high resolution and wide swath. However, due to the low cruising altitude of UAVs, small coverage angles in the direction range, and the insufficient operating range of mini-SAR, expanding the swath has become an urgent problem for mini-SAR. To solve this problem, this paper proposes a W-Band active phased array miniaturized SAR (APA mini-SAR), whose scanning capacity has been proven on the multi-rotor UAV platform. Many efforts, including the novel active phased array antenna scheme, the sparse triangular arrangement of antenna elements, the wideband chirp source, and the three-dimensional integration technology, have been made to develop this APA mini-SAR for the first time in the W-Band. The volume of this APA mini-SAR is 69 × 82 × 87 mm³, with a weight of 600 g. Combined with a new motion compensation strategy and the ω-k imaging algorithm, the focused image is finally obtained. Experiments have been conducted, and the results indicate that this APA mini-SAR has a resolution of 4.5 cm, the imaging swath is three times that of the traditional single-channel mini-SAR, and the operating range is increased to 800 m.

Radio frequency tag detection by radars has received much attention in recent years for its long working distance and high positioning accuracy. This study proposes a novel method that combines synthetic aperture radar (SAR) system with metamaterial-modulated tags that can simultaneously achieve tag positioning and imaging. However, the high working frequency of SAR and the noncoherence between SAR and tag signals pose great challenges to the design and focused imaging of tag. Based on the mathematical model establishment of modulated target imaging, a new transmission waveform strategy and signal processing method were developed for simultaneous focused imaging of scenes and tag that were verified through simulation. Meanwhile, a tag-designing method based on metamaterial modulation was proposed, and direct loading of modulation signals onto radar echoes in the millimeter-wave band was realized. Additionally, an experimental system based on vehicle-mounted miniaturized SAR (Mini-SAR) and metamaterial tag was established. The experimental results indicated that focused images of the tag and environment were simultaneously obtained, and the SAR system realized long-distance, accurate detection, and positioning of the tag, with a working distance of 300 m, a range resolution of 27 cm, an azimuth resolution of 18 cm, and a positioning accuracy of 1.5 cm.

In order to expand the applications of the traditional Vivaldi antenna by addressing the low gain of the traditional Vivaldi antenna, this paper combines the phase distribution of the Vivaldi antenna aperture based on the analysis of the transmission characteristics of a dielectric unit by utilizing the advantages of artificial electromagnetic material. A more reasonable method of loading a dielectric lens is proposed and utilized in combination with edge grooving. A comparison of the results shows that the antenna operates in the frequency range of 4 to 16 GHz. The low-frequency boost is approximately 3 dB, and the high-frequency boost is approximately 9.6 dB.