Here, we propose a design method for a 2-bit transmissive programmable metasurface and demonstrate the generation and dynamic control of multimode electromagnetic vortex beams. By adjusting the states of the loaded PIN diodes, the metasurface element achieves high-efficiency transmission with an insertion loss as low as 1.2 dB and precise 2-bit tunable phase control at a center frequency of 4.15 GHz. The coding schemes necessary for generating steerable vortex beams are then theoretically derived. To validate the design principle and simulation results, a metasurface prototype is fabricated. Near-field scanning measurements show that the proposed metasurface can dynamically generate vortex beams with various modes, exhibiting distinct spiral phase characteristics and annular amplitude distributions, with the mode purity of the dominant mode remaining above 0.88 for orders within ±2 at the center frequency. Far-field radiation pattern measurements further confirm that the generated vortex beams can be dynamically scanned within 0°～45°, with a scan loss of less than 3 dB. The results measured align well with the simulations. The proposed programmable metasurface for vortex beam control demonstrates strong potential for applications in radar imaging and wireless communication.
Compared with traditional radar systems, the Vortex Electromagnetic Wave (VEMW) radar based on Orbital Angular Momentum (OAM) provides additional degrees of freedom for target detection. However, most existing VEMW radar imaging and parameter estimation methods rely on a large number of OAM modes, which significantly increases the hardware cost and complexity of the system. Moreover, the introduction of high-order modes causes the main lobe of the VEMW to deviate from the region of interest while reducing the system’s robustness to noise, severely limiting its practical engineering applications. To address these limitations, this study proposes a three-dimensional target parameter estimation method based on limited OAM modes. First, a forward-looking imaging geometric model of the VEMW radar is constructed, and pulse compression technology is used to extract the target’s range information. Subsequently, the echo ratio between the sum and difference beams of dual OAM modes is constructed, and the symmetry of the Bessel function is utilized to estimate the target’s azimuth angle information. Finally, the echo signal ratio of adjacent OAM modes is constructed, and the recurrence property of the Bessel function is used to estimate the target’s elevation angle information. Furthermore, for a dual-target scenario within the same range bin, the echo signal ratio of adjacent OAM modes is constructed, and the target parameters are estimated by exploiting the phase difference characteristics of the echoes. In addition, this paper derives the Cramér-Rao lower bound for azimuth and elevation angle estimation and compares the root mean square error of the proposed closed-form three-dimensional estimation method with the theoretical lower bound. Numerical simulations and microwave anechoic chamber experiments show that the proposed method achieves reliable three-dimensional parameter estimation in multitarget and noisy environments, thereby verifying its effectiveness and engineering feasibility.
Owing to their inherent Orbital Angular Momentum (OAM), vortex electromagnetic waves display a helical wavefront phase structure. Their echoes include an amplitude component modulated by Bessel functions and a phase component modulated by the azimuth angle of the target. By utilizing different OAM modes, the azimuthal scattering points of targets can be measured differentially, enabling high-resolution azimuth imaging. However, current methods require observing targets with multiple OAM modes. The inconsistency of Bessel function terms across modes causes azimuth imaging resolution to degrade. Additionally, the Bessel function term is influenced by the elevation angle of the scattering points, resulting in strong coupling between elevation and azimuth information. When the elevation angles change, compensation becomes challenging, further reducing azimuth resolution. To overcome these issues, this paper uses single-mode vortex electromagnetic waves to observe targets. By compensating the echo signals, the azimuth information of the scattering points is shifted from the phase to the amplitude, producing the desired single-mode signal. This method diminishes the effect of elevation angle variations on the amplitude component, thereby lessening their impact on azimuth imaging. At the same time, the amplitude time-delay is employed to locate the azimuth positions of the scattering points, enabling two-dimensional range-azimuth imaging based on the single-mode signal. Simulation experiments show that the proposed method achieves azimuth resolution close to the diffraction limit even with changing elevation angles, while maintaining strong imaging performance.
To address the urgent need to identify birds and rotary-wing Unmanned Aerial Vehicles (UAVs), this paper proposes a vortex radar-based method for extracting micromotion parameters of targets. The study focused on target parameter acquisition and systematically extended target modeling and parameter extraction strategies. First, mathematical models were developed for the body motion and wing flapping behavior of birds as well as for the rotor movement characteristics and body structure of rotary-wing UAVs. Further, analytical expressions for the radial and rotational Doppler frequency shifts at scattering points were derived, and micro-Doppler features were extracted from radar echo signals to enable target parameter inversion. For bird targets, the radial Doppler frequency was estimated by extracting the spectral peak of the echo signal to obtain the flight velocity. In addition, by combining the rotational Doppler frequency shifts of the scattering points and analyzing the variations of the rotational Doppler frequency using the Short-Time Fourier Transform (STFT), the wing-flapping length was estimated. Even under low Signal-to-Noise Ratio (SNR) conditions, the estimation error of the wing-flapping length remained within 0.03 m. For rotary-wing UAV targets, an echo signal model was first constructed, and the analytical relationship between the radial and rotational components of the micro-Doppler frequency shift was derived. Using the reconstructed Doppler information and through range-time domain analysis, six structural and motion parameters were retrieved, including the Euler angles, rotor rotational speed, rotor length, and the distance between the UAV body and rotor. The estimation errors for all parameters were significantly lower than those obtained with conventional approaches based on individual Doppler features, with all parameters remaining within 2%. Simulation results demonstrated that the proposed vortex radar-based parameter extraction method enables accurate multiparameter estimation for birds and rotary-wing UAVs. The method also exhibits stable and reliable performance under low SNR conditions, confirming its effectiveness and applicability in practical engineering scenarios.
Traditional radar can detect moving targets. However, when the target velocity vector is perpendicular to the electromagnetic wave propagation direction, effectively obtaining the target’s angular motion information becomes difficult. This results in a detection blind zone for estimating the rotational motion trend of the target. Vortex electromagnetic waves carrying Orbital Angular Momentum (OAM) possess a helical phase structure and an annular radiation pattern. When interacting with rotating targets, these waves can induce a rotational Doppler effect, providing a new approach for acquiring angular motion information. Therefore, target detection methods based on the rotational Doppler effect of vortex electromagnetic waves have attracted extensive attention in recent years. This paper systematically reviews the research progress of rotational Doppler detection techniques using vortex electromagnetic waves, with a particular focus on studies in the microwave band. The reviewed studies include rotational Doppler detection methods based on phase and spectral features, methods based on OAM mode characteristics, and target micromotion detection methods based on vortex electromagnetic waves. Finally, this paper summarizes the major challenges in existing studies and provides an outlook on future research directions and potential applications.
