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Due to its high resolution and deep penetration depth, microwave ultrawideband radar is a promising tool for the non-destructive testing (NDT) of transportation infrastructure. Microwave radiation can also be used to reconstruct the dielectric properties of objects and therefore can be used to detect an air cavity or metallic rust in concrete. We used contrast source inversion (CSI) as one of the most promising inverse scattering schemes. To resolve the observation domain limitations of NDT, radar imaging method, also known as range points migration (RPM) method, was first incorporated into the inverse scattering algorithm based on CSI as a region of interest (ROI) estimator, which substantially improves the accuracy of complex permittivity reconstruction. In addition, the ROI optimizing scheme based on the CSI cost function is used to enhance ROI accuracy. The effectiveness of the proposed methods is validated via finite-difference time-domain (FDTD)-based numerical simulation, which assumes typical NDT model. [ABSTRACT FROM AUTHOR]

This communication presents a novel systematic design of a high-aperture-efficiency and 2-D beam-scanning nematic liquid-crystal (LC)-based reflectarray (LCRA) that operates in the sixth-generation (6G) midband (7–24 GHz). Despite a 260° phase range, the maximum aperture efficiency is 35.8% at an aperture dimension (F/D) ratio of 0.58, the highest aperture efficiency at the lowest F/D ratio among LCRAs designed to operate on the mmWave band. The proposed LC-based reflectarray unit cell (LC-RUC) has significantly low reflection loss compared to other LC-RUCs. Also, the bias topology of the proposed LC-RUC makes the LCRA capable of 2-D beam scanning. The beam-scanning range of the proposed LCRA is ±50° on the xoz plane and 0°–65° in the yoz plane at 9.55 GHz. A $2 \times 2 $ patch array antenna is designed as the feed antenna to achieve a small F/D ratio system and ease of optimization. When designing the feed antenna, the number of elements or the element spacing is considered to achieve high feed efficiency. In addition, the fabricated planar feed antenna can be easily integrated with a 3-D printed jig in which the focal length can be adjusted for optimization. The lowest side lobe level (SLL) of the proposed LCRA is −15.5 dB. [ABSTRACT FROM AUTHOR]

Angle of arrival (AoA) for target echo signal detection is crucial in vehicular radar systems, where the estimation of signal parameters via rotational invariance (ESPRIT) provides a high-resolution estimation. A subarray decomposition strategy is presented in this article to create two ESPRIT antenna arrays with enhanced echo signal strengths, applicable to multi-input–multioutput (MIMO) operations for creating a large virtual array. The concept intends to maximize the echo signal’s strength with minimum signal interferences by increasing the aperture sizes of the two ESPRIT antenna arrays. The decomposition produces two virtually overlapped subarrays to increase the subarray aperture sizes in the spatial or spectral representations. It is applied to the MIMO-based arrays for two-way radiation patterns for ESPRIT estimation of AoA. Numerical simulation examples are shown to validate the feasibility and demonstrate the applications. [ABSTRACT FROM AUTHOR]

This article presents the first demonstration of a full-waveguide feed at W-band, enabling transmit–receive duplexing and 2-D tracking of targets. The proposed device is a compact unit designed to be used as feed of a 94 GHz space-borne radar. Its duplexing concept relies on a septum orthomode transducer polarizer for transmitting and receiving sum signals with opposite circular polarizations. Additionally, the first higher order mode of circular waveguide (TM01) is allowed to propagate inside the device to extract a difference signal for tracking. By comparing received sum and difference signals in amplitude and phase, both angular directions of a target position can be derived. A prototype of the duplexer was fabricated and tested: measurements at 94 GHz show return loss and isolation above 23 dB at all ports, with reasonable performance from 92 to 96 GHz. The measured monopulse characteristic enables 2-D tracking of targets deviated by less than 8.5° from boresight. [ABSTRACT FROM AUTHOR]

Fast detection of hazardous objects in millimeter-wave personnel screening can increase the operating efficiency of secure environments. In airports, accurate automatic detection and classification of body-worn objects can reduce the number of pat-downs while keeping the transportation environment safe and secure. Since many benign objects, such as paper and leather, are lossy materials, being able to characterize them and set them aside from the secondary checks will significantly reduce the screening process time. In this article, we introduce an effective image-processing-based method for characterizing lossy materials that might be concealed under clothing at checkpoints. The method has been combined with a modified version of the previously developed algorithm that can be used specifically to characterize lossless materials. The proposed algorithm can automatically distinguish lossless materials from lossy ones and calculate their thickness and permittivities. Starting from the radar reconstructed image showing a cross section of the body, we extract the nominal body contour using the Fourier series, separate body, and object responses, categorize the object as lossy or lossless based on the depression and protrusion of the body contour, and, finally, predict possible values for the object’s permittivity and thickness. Our resulting classification is good, implying fewer nuisance alarms at checkpoints. [ABSTRACT FROM AUTHOR]

In recent years, there are increasing interests and applications on radar–communication integration for system miniaturization, cost reduction, and so on. Moreover, thanks to the common functions of hardware components including transceiver, antennas, and even basic signal/data processing, the integrated design of radar and communication systems becomes feasible. In this article, through design of proper time sequences, waveforms, and recognition algorithms, radar sensing and wireless communication can be integrated into 4-D antenna arrays simultaneously. In addition, with the high design degrees of freedom of 4-D arrays, the proposed approach can also provide a low probability of intercept capability. Selected numerical and experimental results are provided to validate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

High-performance computing-based electromagnetic (EM) emulators are used to simulate real-time complex EM wave interactions between multiple radar targets, transmitters, and receivers. The radar cross section (RCS) of the radar targets are required to be stored as a table; however, the needed storage size increases dramatically with the angle and frequency sampling density. In this article, we present an innovative approach of constructing a concise spherical harmonic-based anisotropic point scatterer model that the emulators can use as part of the computations. The point scatterer model is constructed directly from the precomputed RCS data. First, we use only the monostatic RCS data and compute the spherical harmonic-based monostatic point scatterer model by solving a linear least-squares problem which has a group sparsity constraint. Then, we further compute the spherical harmonic-based bistatic point scatterer model using the full bistatic RCS data. The problem is formulated as a bilinear least-squares problem. The problem is solved using the normalized iterative algorithm, which linearly solves two parameters in a back and forth manner. The results show that the point scatterer model can effectively represent the bistatic RCS data of a radar target. [ABSTRACT FROM AUTHOR]

Due to the asymmetry of the elements’ position distribution and polarization direction, it is a challenge to synthesize difference patterns with large slope and symmetric main-lobe on 3-D conformal beam-scanning arrays with asymmetric radiation aperture. In this article, an approach to synthesize the optimal difference patterns for 3-D conformal arrays with asymmetric radiation aperture is proposed. To determine the limitation region of the main-lobe and sidelobe for a 3-D conformal array, an initial difference pattern is generated by the joint multidimensional vector clustering method. Then, an auxiliary phase function is introduced to convert the nonconvex constraint of the symmetry of the difference beam into a convex one. Finally, an iterative convex optimization is applied to obtain the optimal difference pattern. The optimization progress is finished by extracting the vector field patterns of a simulated antenna element. Two design examples employing hemispherical conformal array (HCA) and conical conformal array (CCA) with asymmetric radiation aperture are implemented to demonstrate the effectiveness of the proposed method. Difference patterns with −20 dB sidelobe level (SLL) and large slope are obtained in both numerical and simulated results of the two design examples. [ABSTRACT FROM AUTHOR]

Target classification is of great value when the number of transceivers in a sensing system is relatively small. This article proposes a high-accuracy and efficient classification method with machine learning techniques on frequency-modulated-continuous-wave (FMCW) radar. We first establish the novel mapping relationship from physical space to range-Doppler (PS-RD) image based on wave propagation theory, by which four physical features that effectively capture the kinematic and geometrical characteristics of targets, including speed, total reflectivity (ToRe), area, and incidence angle, are extracted from range-Doppler (R-D) image. Then, a multilayer perceptron (MLP) with a single hidden layer is employed to realize the classification. Since the above-mentioned four physical features, derived from wave physics, are chosen as the input of the neural network, our classifier does not work in a black-box way. The computational complexity of the whole classifier is the same as that of a 2-D fast Fourier transform (FFT), which guarantees a real-time operation. As an example, the proposed classifier is applied to the automotive radar system, where road targets are to be classified into five categories, including pedestrian, bike, sedan, truck/bus, and other static objects. Real-world data obtained from 77 GHz FMCW radars are provided for validation, where the proposed physics-assisted classifier turns out to outperform the state of the art in automotive radar application. The overall accuracy of the real data is about 99% even with complex multiple-target cases. [ABSTRACT FROM AUTHOR]

An effective method for identifying inertia characteristics of cone-shaped space target based on deep learning is proposed. The inertia ratio is determined by the time-varying scattering fields from the cone-shaped targets. The multistatic method is introduced to reduce the evaluation time of time-varying scattering fields. The micro-Doppler spectrogram (MDS) dataset is constructed by the time–frequency analysis with numerical simulation method, point scattering model, and experimental tests. The compressed dataset is further achieved by truncated singular value decomposition (SVD). Finally, the micromotion parameter identification model is constructed to identify the inertia ratio for the cone-shaped space target. The interaction loss function and the feedforward denoising convolutional neural networks (DnCNNs) are employed to improve the identification accuracy. Parameters identification of the precession frequency, precession angle, spin frequency, and inertia ratio with both simulation and experiment datasets demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

A single layer 77 GHz dual circularly polarized (CP) antenna array is proposed based on ridge gap waveguides (RGWs) technology. The antenna consists of eight U-shaped slots, fed by two feeding networks for generation of dual CP. The proposed antenna array has been fabricated and measured. The experimental results show good agreement with the simulations. The axial ratio is below 2 dB, and the return loss and port isolation are better than 13 and 18.6 dB over 76–81 GHz, respectively. The measured gain for the integrated array is 14.8 dBi at the center frequency of 78.5 GHz. With integration of feeding networks and the radiating elements in a single layer, this antenna has potential to be used in automotive radars. [ABSTRACT FROM AUTHOR]

Over-the-horizon radars (OTHRs) utilize the refractive properties of the ionosphere to illuminate targets beyond the Earth’s horizon. Consequently, the performance of this type of radar is highly dependent on the ionosphere. Reliable models of the radar ground backscatter are required to accurately assess the ionospheric propagation conditions and, thus, the expected performance of OTHRs. The ground backscatter coefficient characterizes the amount of radiation scattered back from a surface toward a receiver per unit area. While the backscatter coefficient of the sea is well understood and may be calculated from theory if the sea state is known, the backscatter coefficient of land at high frequencies is not well understood. To calculate the land backscatter coefficients over Northern Australia, a methodology that compares observed backscatter ionograms to those synthesized using high-frequency (HF) radio wave ray-tracing techniques through model ionospheres was developed. Maps of the backscatter coefficients across Northern Australia were produced. The effects of surface properties, including topography, soil moisture, and vegetation cover on the backscatter coefficients, were investigated. A weak positive correlation between the backscatter coefficient and the soil moisture and surface roughness was observed; however, it was found that the vegetation had the largest effect on the backscatter coefficient. [ABSTRACT FROM AUTHOR]

In this study, we propose a generalized algorithm for far-field radar cross section determination by using 3-D synthetic aperture imaging with arbitrary antenna-scanning surfaces. This method belongs to a class of techniques called image-based near-field-to-far-field transformation. The previous image-based approaches have been formulated based on a specific antenna-scanning trajectory or surface, such as a line, plane, circle, cylinder, and sphere; the majority of these approaches consider 2-D radar images to determine the azimuth radar cross section. We generalize the conventional image-based technique to accommodate an arbitrary antenna-scanning surface and consider a 3-D radar image for radar cross section prediction in both the azimuth and zenith directions. We validate the proposed algorithm by performing numerical simulations and anechoic chamber measurements. [ABSTRACT FROM AUTHOR]

The conventional short-range interferometric radar sensors usually employ two separate transmitting (TX) and receiving (RX) antennas for improved TX–RX isolation. However, they are subject to increased system size and misalignment of the TX/RX radiation patterns. In this article, a portable 5.8 GHz dual circularly polarized (CP) interferometric radar sensor is presented, which leverages the proposed dual CP common-aperture antenna that reduces the size of the entire radar system. The common-aperture nature gets rid of the radiation pattern misalignment and ensures that the object can be in the optimal boresight direction. Cross polarization, i.e., right-hand CP (RHCP) for TX and left-hand CP (LHCP) for RX or vice versa, was employed based on the sequentially rotated array (SRA), which not only improves the TX–RX isolation but also adapts with the polarization change in radar sensing. Moreover, a near-field cancellation technique was proposed to further increase the TX–RX isolation in the small form factor. A portable 5.8 GHz radar sensor integrated with the proposed dual CP antenna was custom-designed. Compared to the conventional two-antenna architecture, the proposed radar sensor is reduced in size by 140% but can still achieve a high TX–RX isolation of >60 dB at 5.8 GHz. Experiments were carried and the results illustrate that the proposed radar sensor has high micrometer accuracy in detecting the predefined displacement motions and is able to precisely track the human vital signs in the office environment. [ABSTRACT FROM AUTHOR]

This article presents a simulated and experimental analysis of the effects of the evaporation duct on microwave propagation in the Irish Sea. The evaporation duct is a phenomenon that occurs almost permanently over all the world’s oceans and allows electromagnetic waves to travel beyond the horizon. Weather data logged over several years from four buoys off the east coast of Ireland has been analyzed to find the probability and strength of the evaporation duct. Signal propagation in the evaporation duct has been simulated using the parabolic equation model and compared to results obtained from an experimental setup in the Irish Sea. The best antenna heights and frequencies to maximize signal propagation at this location are also found. Results show that the evaporation duct can be used to provide high bandwidth communications beyond the horizon with an uptime of approximately 40%, and that weather data from buoys can be used to predict the performance of the communications link. While this method of communication is not one that can be relied on all the time, it could be useful to reduce dependency on expensive satellite links or provide nontime critical communications. [ABSTRACT FROM AUTHOR]

The performance of very short-range radar systems, such as ground penetrating radar for antipersonnel landmine detection, is critically dependent on the impulse characteristics of the antennas that are used. Typically, a pair of individual antennas is used in the radar, one for transmit and one for receive. These antennas are usually closely spaced for reasons of both portability and close in detectability. A variety of design techniques are used to achieve their wideband performance in terms of return loss, isolation, radiation patterns, polarization, and impulse formation. Not only the antenna elements but also their surrounds form the overall antenna structure and this ensemble becomes integral to the antenna performance and design process. Both the antenna and the surrounding structure can create late time signals or time sidelobes, which are in effect a form of self-generated clutter. Minimizing the duration of the antenna impulse response assists in reducing self-generated clutter and achieving optimum radar performance. This article considers the impact of the antenna on the system performance of the radar and shows that the clutter performance of the antenna can be the prime metric of system detection rather than the noise figure, given that the self-clutter often exceeds the latter. [ABSTRACT FROM AUTHOR]

A novel technique to design a low-cost frequency-selective surface (FSS) bandpass filter is presented in this article. Wideband polarization-independent FSS bandpass filters are predominantly made of multiple microwave dielectric substrates or noncommercially available composite materials with or without active components, contributing to a very high manufacturing cost. The presented FSS filter has neither microwave substrates, nor any active devices, while it has a large controllable operational frequency band, which can support all polarizations, due to its symmetrical configuration. To the best of our knowledge, such a polarization-independent wideband bandpass response has never been achieved by any low-cost fully metallic FSS filter. The proposed FSS filter is made of three thin metal sheets composed of an engineered metallic substrate (EMS) and a metallic orthogonal dipole resonator (ODR). The EMS is responsible for ensuring the mechanical integrity of the filter without imposing electromagnetic (EM) restrictions throughout the desired frequency band. The integration of EMS and ODRs realizes a fully controllable wideband bandpass verified thorough circuital and modal analyses. According to the predicted and measured results, the FSS filter has a large bandwidth of around 31%, extending from 8.76 to 11.96 GHz with sharp roll-offs for the normal incidence. Simulated and measured results show a low sensitivity of the FSS filter response to oblique angles of incidence for both TM and TE polarizations. [ABSTRACT FROM AUTHOR]

This article presents the design, fabrication, and testing of a modulated metasurface (MTS) antenna for monopulse radar tracking at Ka-band. The antenna consists of a circular, thin grounded dielectric layer printed by a texture of metallic patches modulated in shape and size. The patch layer can be modeled as a spatially variable capacitive impedance sheet, which together with the grounded slab contribution provides an overall, modulated, inductive boundary condition (BC). The antenna aperture is divided into four identical angular quadrants each of them radiating independent broadside beams when excited by an individual monopole launcher. Each of the four launchers excites a TM cylindrical surface wave (SW), which is progressively converted into a leaky-wave (LW) by the MTS. The four subapertures are virtually separated by properly designing the MTS modulation. To this end, the LW attenuation constant is calibrated for dumping sufficiently each individual SW, thus preventing the interaction between the adjacent regions. Therefore, the printed structure is not delimited by any physical separation, but only by a continuous change of the equivalent BCs. The monopulse-type linearly polarized $\Sigma $ , $\Delta \Delta $ , $\Delta _{\mathrm{ E}}$ , $\Delta _{\mathrm{ H}}$ beams are obtained by combining the source excitation with a simple phasing scheme. Notably, this solution does not affect the overall lightness, low profile, feed simplicity, and low fabrication cost of the structure, which constitutes an inherent advantage with respect to more traditional waveguide-based solutions. [ABSTRACT FROM AUTHOR]

This article presents a duplexing hybrid antenna architecture for short-range radar systems with high isolation and low loss between the two transmitting (TX) and receiving (RX) ports. The antenna utilizes a single slot for common resonance shared by the two TX and RX port excitations. These two ports were constructed through two parallel microstrip lines crossing over the specified shaped slot. The feeding structures of these two microstrip feedings and the separation are optimally designed to ensure the mutual coupling signals out-of-phase, and therefore, the signal cancellation can be achieved, leading to a high isolation between these two ports. In this design, the slot length is $0.92\lambda $ at 24.125 GHz, forming an equivalent short circuit at the other port. Given the initial design criteria, the design with the optimal isolation could readily be found by minimal full-wave simulations. Meanwhile, the equivalent circuit model was used to investigate the working principles and to improve the isolation bandwidth and impedance matching at the central frequency. The measured maximum isolation is 70 dB at 24.125 GHz. The RF front-end modules with TX and RX amplifiers are also integrated to validate its feasibility for the radar system application. The mechanisms to widen the isolation bandwidth are also addressed. [ABSTRACT FROM AUTHOR]

A theoretical sea echo modeling of the floating-platform (FP) high-frequency hybrid sky-surface wave radar (HSSWR) is presented. The ionosphere is considered to be a no-tilt reflecting plane moving in the vertical direction. Subsequently, the first-order sea clutter cross section with consideration of the platform sway and yaw motion is derived. Simulations are conducted to explore the influences of the ionospheric movement and the platform oscillation motion on the sea clutter power spectrum. The simulation results indicate that the platform sway motion may induce additional peaks that distribute symmetrically concerning the Bragg peaks, while the platform yaw motion and the ionospheric movement may broaden and even split the sea clutter. Experiments are conducted with the newly developed HSSWR system, of which the receiving arrays are simultaneously deployed on a FP and the shoreside. Experimental measurements concerning the frequency shift, Doppler width, and the arrival angle of the E-layer/F-layer ionosphere-reflected direct wave are compared and analyzed. The study demonstrates the feasibility of the proposed modeling and provides valuable guidelines for the sea clutter characteristics analysis and the data quality assessments of the FP HSSWR. [ABSTRACT FROM AUTHOR]

This article proposes a method which generates a set of weights to synthesize sum or difference pattern with precisely controlled sidelobe level (SLL), null, and dynamic range ratio (DRR) for arbitrary arrays. Our pattern synthesis approach reduces mutual coupling (MC) between the neighboring elements and complexity of the feeding network design. However, the formulated optimization problem is nonconvex due to the nonconvex objective function and fractional DRR constraint. To tackle it, we first introduce two sets of auxiliary variables: one for DRR constraint and the other for sidelobe and null constraints. By doing so, we then decompose the original optimization problem into three sets of subproblems characterized by the auxiliary variables and weight variables. To facilitate the subproblem with weight variables reaching its optimum values, we derive an appropriate range of step size. Finally, we iteratively solve these subproblems to obtain the solution to the original problem. Extensive experiments employing non-equispaced linear and rectangular arrays, concentric ring array, and cylinder array, are implemented to demonstrate that the developed approach can accurately control SLLs, null and DRR for arbitrary arrays. [ABSTRACT FROM AUTHOR]

A periodically loaded, low-scattering dipole suitable for the secondary surveillance radar, particularly for identification-friend-or-foe antenna, is presented along with the methods to design such an antenna. The dipole is chopped into the pieces with length that minimizes scattering at 10 GHz, i.e., at the operation frequency of the primary antenna. In addition, optimal inductive loading is found to connect the pieces together so that the formed entity can radiate at 1 GHz, i.e., at the operation of frequency of the secondary antenna. Two meander-line designs are presented to realize the required inductances, and the proposed structures are realized on a printed circuit board. Designed prototypes are analyzed through simulations and measurements, which are in good agreement. The reduction in radar cross section at 10 GHz with the inductively loaded dipoles is at best more than 15 dB, when compared to the reference half-wave dipole. As a consequence, the radiation efficiency at 1 GHz decreases by 0.4 dB with the inductively loaded dipoles. [ABSTRACT FROM AUTHOR]

We demonstrate the efficacy of periodic corrugations, fabricated as part of the radar cover to serve the dual purpose of protecting the radar circuitry and maximizing transmission in millimeter-wave (mmW) automotive radars. A 2-D array of square-base pyramids engraved on the plastic radar cover was optimized to achieve minimum reflection for a wide range of incident angles, ±40°, and across the operating frequency band of 76–81 GHz. More importantly, we present an alternative radome texture, namely, the inverse-pyramid corrugations, which results in improved structural integrity compared to the pyramidal corrugations. This novel inverse-pyramid radome structure is also much easier to manufacture and achieves similar performance as the more conventional pyramid structure. Both designs are shown to significantly reduce radar cover reflections, which is perhaps the most significant bottleneck in modern mmW automotive radar sensor performance. [ABSTRACT FROM AUTHOR]

Radar imaging of typical planet landforms can be used for planetary exploration such as landform recognition and geometry measurement. This communication realizes high-resolution imaging simulations of typical planetary landforms, such as craters, hills, and volcanoes, by using parametric scattering center (SC) models. Based on the geometric models of these typical landforms, parametric SC models are constructed from scattered solutions of the physical optics (POs) method. Because the SC models take geometric parameters as model parameters, these models can simulate the scattered waves of landforms of different sizes. These SC models are validated by the PO results. In addition, the landform types can be recognized and their geometrical parameters are further extracted by SC model matching. The extracted geometric parameters show good agreement with real landforms, demonstrating that the SC models of typical planetary landforms have potential applications in planetary geomorphologic exploration. [ABSTRACT FROM AUTHOR]

A 3-in-1 depolarizing circular scatterer for the polarimetric radar calibration is proposed in this article. The proposed scatterer is low cost, compact, planar circuit, and well suited for the radar operating in a compact range, for example, chipless RFID technology. By the virtue of its circular shape, the rear side of the proposed scatterer acts as a metallic disk with strong copolarization backscattered signals. The front side is composed of eight resonant dipoles that make it nondepolarizing and depolarizing scatterers at the inclination of 0° and 45°, respectively. The features of proposed circular scatterer are tested as reference calibration objects for single antenna-based polarimetric radar calibration techniques. Two test objects are utilized: a dihedral tilted at 45° and a depolarizing multiresonant planar structure commonly called chipless RFID tag. The performance of proposed circular scatterer is also compared with the standard reference calibration objects: a metallic disk and a dihedral tilted at 22.5°. The performance of proposed circular scatterer is comparable to the standard reference calibration objects for the calibration techniques (namely, Type 1 and Type 2). The proposed scatterer is potentially tolerant of displacement up to 1 cm and misalignment equals 2°. [ABSTRACT FROM AUTHOR]

Current artificial magnetic conductor (AMC) designs use metallic patterns on rigid substrates and focus on shapes and sizes of AMC structures, rather than on material performance, which has hindered operation bandwidth and design flexibility. Here, we introduce printed graphene AMC-based broadband and flexible microwave radar absorbers, which not only redirect but also absorb the incident wave so to broaden the operation bandwidth. Contrasting to other reported works, the phase characteristics of the AMCs are realized through the control of the surface resistance provided by printed graphene laminates. We produced a variety of AMC structures, composed of printed graphene circular ring arrays with exactly the same shape and size, but different sheet resistances. By carefully designing the sheet resistance of printed graphene laminates, the optimized anti-phase reflection cancellation between AMCs can be achieved. With printed graphene AMCs and flexible dielectric substrate, the absorber presented in this work has a broadband effective absorption (above 90% absorptivity) from 7.58 GHz to 18, is polarization insensitive under normal incident, and can work at relatively wide incident angles. Furthermore, this absorber is capable of bending easily with notable performance, which makes it ideal for applications with irregular and uneven shapes. [ABSTRACT FROM AUTHOR]

A series-parallel hybrid-fed microstrip array antenna design is presented for X-band low-probability intercept (LPI) marine radars. Contrary to traditional counterparts, the proposed antenna has improved impedance and radiation bandwidth from 9.1 to 9.6 GHz to accommodate pulse compression and frequency diversity for LPI radars. The antenna structure consists of $24\times4$ patch elements with horizontal polarization. The two halves of the structure are fed in parallel and each arm is fed in a series-series feed network to achieve low sidelobe level (SLL) of −26.5 dB. The manufactured antenna has a gain of 24.0 dBi with half-power beamwidth (HPBW) of 4.5° and 21.5° in azimuth and elevation, respectively. The measured gain flatness is ±0.3 dB over the radionavigation band and ±1.0 dB over the entire band. A novel grid baffle structure with capacitive surface impedance is proposed and implemented to provide high isolation over the target bandwidth. The maximum and average isolations between transmit and receive antennas are measured as 65 and 62 dB, respectively. The proposed antenna is very suitable for dual-mode pulsed and continuous-mode LPI marine radars. [ABSTRACT FROM AUTHOR]

In this article, a 2-D bistatic attributed scattering center (BASC) model is developed to represent the bistatic return from a perfect electric conducting (PEC) target, based on the derived analytic scattering solutions of seven canonical primitives. Because of its unified and concise form, the BASC model is useful for extracting the geometrical features of targets from synthetic aperture radar (SAR) echoes. However, estimating the parameters of the BASC model is an ill-posed inverse problem. To reduce the ill-posedness, we present a sparse method for bistatic inverse scattering that incorporates differential evolution (DE) into doubly orthogonal matching pursuit (DOMP). The experimental results not only verify the validity of the parameter inversion method but also demonstrate the applicability of the proposed model. [ABSTRACT FROM AUTHOR]

A backprojection-based synthetic aperture radar (SAR) processing approach for fast and accurate imaging of moving objects and stationary background is proposed. This divide-and-conquer backprojection approach allows for efficient detection of moving objects, estimation of their positions and velocities, and overlaying their focused images over the stationary background. Two algorithms are presented to exploit the tradeoff between the complexity and performance. The full 4-D phase space imaging algorithm provides the optimal performance under low signal-to-clutter conditions, while the adaptive detection and imaging leads to reduced complexity in more favorable scenarios. We show that in a single-channel SAR the conventional straight-line data collection path does not resolve the target’s velocity, therefore a curved path is assumed in the formulation and numerical examples. [ABSTRACT FROM AUTHOR]

High cross-polarization isolation (CPI) is crucial to the accurate polarization measurement using polarimetric phased array radar (PPAR). In this article, we propose an adaptive direction-dependent polarization state configuration (AD2PSC) method to improve the polarization isolation. Compared with the conventional fixed polarization state of radiated wave whether it is linear, circular, or elliptical polarization state, our AD2PSC approach configures the polarization state on basis of beam steering. To achieve the adaptive configuration of magnitude and phase of the dual-polarization antenna, an improved steepest descent algorithm is put forward. To facilitate the uniform representation for the polarization measurement application of PPAR, the universal expressions of intrinsic and measured backscatter matrices are derived for arbitrary polarization state. The dual-polarization dipole array is used to assess the priority of our proposed method. Compared with the conventional approaches, our approach could obtain higher CPI while being available for a larger scanning range. The configured CPI meets the specific polarization requirement for PPAR. [ABSTRACT FROM AUTHOR]

In this article, a hybrid radar configuration for automotive radar applications is proposed, with the system architecture, design, and measured results reported. The hybrid radar system consists of a vertically distributed phased transmitting array (PTA) and a horizontally aligned digital beamforming (DBF) receiving array, thereby enabling 2-D beam scanning with a reduced channel number. In contrast to time-division multiplexing (TDM) multiple-input multiple-output (MIMO) radars, in which only a single transmitter is active at a time in the TDM MIMO radar configuration due to the working principle of switching between different transmitting elements, here, all the phased array transmitters of the proposed radar work simultaneously. The demonstrated system realizes a higher transmitted power and signal-to-noise ratio, provided that the equivalent isotropically radiated power (EIRP) is not limited and the radiated power of each transmitter is equal in these two radar configurations. In addition, only 1-D DBF is required for the proposed hybrid radar because beamforming in another dimension is accomplished by the PTA, thus alleviating the baseband complexity and signal processing burden. As a proof-of-concept example for system validation, a hybrid radar prototype is fabricated and measured at around 24 GHz, without considering any specific EIRP limitations in this band, thus confirming the validity of the proposed concept. [ABSTRACT FROM AUTHOR]

Tapering an active array antenna system on transmit lowers the sidelobe level, but it adversely affects equivalent isotropically radiated power (EIRP) by reducing both power and gain. In this article, we analyze this effect. First, we propose a new taper efficiency coefficient, which allows us to assess the effect of tapering on EIRP (in contrast to the standard taper efficiency that describes the effect of tapering on gain only). Then, we present an optimization method to compute the taper weights that maximize the defined efficiency for an assumed bound on the sidelobe level. The obtained optimal weights can be used not only to taper an antenna array but also as a benchmark for other tapers. Therefore, we analyze several known tapers and compare their transmit efficiencies. As a result, we find that for some combinations of sidelobe level and array length, the proposed optimal taper provides substantially better efficiency than Dolph–Chebyshev or Taylor tapers. [ABSTRACT FROM AUTHOR]

Traveling waves can be supported by long, smooth surfaces under horizontal polarization, such as the wings or bodies of aircraft. A 3-D scattering center (SC) model for traveling waves is proposed in this work. The equivalent location of the SC and the dependence function of scattering amplitude on aspect angles is derived based on traveling wave antenna theory and modified according to real scattering characteristics. Chamber-measured data and full-wave numerical results of a series of geometries are presented to verify the validity of this model in imaging simulation. We find that the equivalent position of this SC is located along a circular arc. In consideration of its polarization characteristics and inverse synthetic aperture radar (ISAR) image signatures, the proposed SC model is promising for used in the model-based automatic target recognition (ATR) to distinguish traveling waves from other scattering components. [ABSTRACT FROM AUTHOR]

Owing to the property of range–angle-dependent beampattern, frequency diverse array (FDA) has promising application potentials in radar field. Many studies have been reported to solve the time-variant problem of FDA. However, there is a misconception between the time index within pulse and the actual time, which misleads the theoretical analysis for FDA. In this article, a novel understanding perspective is proposed to better distinguish the two kinds of time. The exact time–range and frequency–phase relationships are presented. In the sequel, the correction analysis about the operation theories of FDA considering the factor of time is presented along with the accurate understanding about the range–angle beampattern. Furthermore, based on the multiple input multiple output (MIMO) scheme with multiple matched filters at receiver, the prerequisite for exploring the range-dependent property of FDA is demonstrated. Simulations demonstrate the validity of theoretical results. [ABSTRACT FROM AUTHOR]

This article presents the concept of spread-spectrum selective camouflaging based on time-modulated metasurface. The spectrum spreading is realized by switching the metasurface between the reflective states of a perfect electric conductor (PEC) mirror and a perfect magnetic conductor (PMC) mirror, using an array of microstrip patches connected to the ground via diode switches, according to a periodic pseudorandom noise sequence. As the spectrum spreading induces a drastic reduction of the power spectral density of the signal, the level of the scattered wave falls below the noise floor of the interrogating radar, and the object covered by the metasurface is hence perfectly camouflaged to a foe radar. Moreover, the object can be detected by a friend radar possessing the spread-spectrum demodulation key corresponding to the metasurface modulation, and this detection is robust to interfering signals. The proposed system is analyzed theoretically and demonstrated by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Polarimetric radar systems are beneficial to identifying and classifying targets but require multiple transmit or receive channels with different polarizations. This leads to a high hardware effort and thus higher costs. To use a single, linear polarized radar sensor as a polarimetric system, a frequency-dependent, polarization rotating reflector which can be placed in front of the radar antenna is presented. The reflector is based on a frequency selective surface (FSS) consisting of slot-excited substrate integrated waveguide (SIW) resonators. Resonator modes are analyzed and an equivalent circuit diagram to describe the filter functionality is developed. For using the described FSS as reflective structure, the design focuses on 45° oblique incidence. Different field vectors for normal and oblique incident angles are considered and different cavity modes for these cases are analyzed. An undesired mode is suppressed by an additional plated through via hole and slot impedances are matched. Reflector designs for normal and oblique incident angles are presented for $15 GHz$ ($Ku$ -band) and afterward adapted to $35 GHz$ ($Ka$ -band). The frequency band of operation with ${12\%}$ fractional bandwidth is divided into two ${4.5\%}$ subbands which allows a frequency-dependent polarization rotation of a linear polarized electromagnetic wave. Investigations on the fabrication accuracies are presented and reflectors for both bands are manufactured. Measurements are performed with a vector network analyzer and results fit well to the simulated curves. In the band of polarization rotation reflection, the matching is better than $- 13 dB$ and dielectric losses of less than $1 dB$ are achieved. [ABSTRACT FROM AUTHOR]

This article proposes a new multipolarization antenna, and its multiple states of polarization are realized by scanning the operating frequency. The proposed antenna is based on a fixed configuration and fed by a single port. This is extremely simple and attractive in many applications, such as frequency-agile radar systems and multipolarization target identification systems, especially for those operating at higher microwave and millimeter-wave frequencies. The multipolarization antenna is realized by combining a bidirectional planar antenna with a carefully designed polarization rotating reflector. The reflector reflects and rotates the wave radiated from the bidirectional antenna. Then, two orthogonal waves of the same magnitude, including the reflected wave and the wave radiated directly from the bidirectional antenna, are obtained. Furthermore, the designed reflector helps to tune the phase difference between two orthogonal waves ranging from 90° to −180° with a monotonic continuous variation. The conceptual design of the proposed antenna is analyzed in detail, and a prototype is designed, fabricated, and experimentally tested to validate the design procedure. [ABSTRACT FROM AUTHOR]

A study to reduce the peak sidelobe level (SLL) of a radar two-way array factor is presented. It applies the condition that the first significant sidelobe of the transmitting array is in the same direction as the first null of the receiving array. Moreover, it is supposed that in the two-way array pattern, two consecutive sidelobes near the main beam become equal. Moving one step further, by combining the advantages of taper distribution and the simplicity of the feed network of uniform arrays, a two-weight amplitude distribution for the radar transmit and receive arrays is designed. The resulting peak SLL of the two-way pattern becomes less than −45 dB. [ABSTRACT FROM AUTHOR]

In this article, a method for the assessment of the performance and for the design of a 2-D-array for multiple input multiple output (2-D-MIMO) radar applications is presented. The proposed approach is based on the analysis of the ambiguity function associated with the array. Such analysis leads to the definition of an area in the 2-D-angular field-of-view of the radar, denoted as ambiguity-free region, characterized by a low probability to obtain ambiguities in direction of arrival estimation. Simulations of a basic 2-D-MIMO array are used to explain the proposed method, which is then employed as the main criterion for the optimization of a 2-D-MIMO sparse array composed of four transmitters and eight receivers. The optimization is performed through a genetic algorithm. The resulted element positions guarantee an ambiguity-free region span in the azimuth and elevation angles greater than 120° and angular resolutions of 8° and 5.9° on the azimuth and elevation angles, respectively. The optimized positions are then used to realize a 2-D-MIMO sparse array that is integrated in a radar system operating at a center frequency of 76.5 GHz. Such an array is calibrated, and comparisons between the simulated and measured ambiguity functions show the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Polarimetric multiple input and multiple output (MIMO) radar is capable of acquiring target’s high-resolution images and polarization scattering matrix (PSM). In this article, calibration of a polarimetric MIMO array of an instrumentation radar for near-field measurement is achieved at ${X}$ -band by using a rotatable double-antenna polarimetric active radar calibrator (RODAPARC) and a metal cylinder quickly and accurately. The theoretical PSM of the RODAPARC with different antenna rotation angles is derived from Huygens’ radiation electric field which is applicable to different bistatic angles. A polarimetric signal model is adopted to reduce the impact of the RODAPARC on calibration. Experimental studies are conducted on a polarimetric MIMO array which is measured and divided into quasi-monostatic and bistatic transceiver channels for assessment on the calibration performance. Promising calibration results are obtained with a polarization isolation improvement of about 16 dB and great reduction of the polarization channel gain error less than ±0.5 dB in amplitude and ±3° in phase. The calibrated polarization signatures for classic objects are consistent with the theoretical results. [ABSTRACT FROM AUTHOR]

A dual-polarized planar phased array antenna with enhanced polarization and isolation characteristics for multifunction applications is presented. A bent crossed dipole is selected as the element of the array antenna. The bend angle is optimized to widen the E-plane active element pattern of the dipole resulting in a pair of matched co-polar patterns. Contrary to common patch array antennas, this design does not require a continuous grounded substrate. This diminishes the surface wave propagation along the ground plane. Efficient techniques, applied to the element design, isolate the two polarizations from each other. Higher than 40 dB port isolation is achieved within the entire frequency range of 2.7–3.0 GHz in principal plane scanning. In order to preserve the symmetry of the array antenna, a mirrored arrangement in both orthogonal directions is applied to the array configuration. This enhances the co- to cross-polarization levels to better than 45 dB in scanning up to ±45° in the principal planes. The proposed phased array antenna with enhanced polarization purity can potentially improve polarimetric performance. [ABSTRACT FROM AUTHOR]

In this article, we present a ray optics-based iterative procedure for the design and optimization of a shaped reflector able to achieve a high roll-off sectoral pattern in the azimuthal plane and a very narrow beam in the elevation plane, as required in a surveillance application at 76–77 GHz. Three prototypes of such kind of reflector with different azimuthal half-power beamwidths were fabricated and measured, showing a fair agreement with numerical simulations. [ABSTRACT FROM AUTHOR]

We propose a compact circularly polarized series-fed patch array with enhanced radiation performance at $S$ -band. To our best knowledge, no similar single-layer structure has been designed, measured, and reported in the literature with equivalent radiation performances in terms of reduced sidelobe level (SLL) and aperture efficiency as well as compactness and simplicity. The planar array consists of a 50- $\Omega $ microstrip single feed point, offering uniform and efficient excitation of all its elements that enable a broadside beam with high-gain and low sidelobes. A feeding network for such aperture control is designed and optimized to provide good impedance matching with a two-point excitation per radiating element, which is based on 0° and 90° meander lines to enable circularly polarized radiation. A structure made by a $4\!\!\times \!\!1$ arrangement of square patches is simulated, optimized, and manufactured, providing a realized gain of about 10 dBic and an SLL below −15 dB. Very good axial ratios and high efficiencies are obtained. The proposed antenna may be of interest for next-generation far-field wireless power transmission systems and other applications including target tracking, radar, and internet of things technologies requiring efficient circularly polarized radiation. [ABSTRACT FROM AUTHOR]

This article has developed a new approach to combine transmit beamforming techniques and 4-D antenna array theory for simultaneously improving the detection and low probability of intercept (LPI) capabilities of radar. This new approach enjoys most of the advantages of multiple-input multiple-output (MIMO) and phased arrays, providing excellent detection and LPI capabilities. The essence of the proposed approach is to utilize a 4-D variable to extend the detection dimension and increase the signal uncertainty. Specifically, the flexible choices of a time-modulation scheme can greatly improve the detection and LPI capabilities of radar. The substantial improvements achieved by the proposed method compared with basic MIMO and phased-MIMO arrays are demonstrated by simulations through analyzing the corresponding beampatterns. In addition, a basic experiment with reference to transmit beampatterns of 4-D antenna arrays is presented to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Antenna arrays are frequently used for localization of active communication transmitters or passively reflecting radar targets by evaluating signal phases at spatially distributed antennas. Unfortunately, the localization performance is degraded because of mutual coupling, unknown time delays, and imprecisely known antenna positions, especially if large arrays with spatially distributed antennas are used. Furthermore, these errors are altered until the array is steadily installed at its destination, which occurs because of position misalignment and cable phase instability. Hence, an in-situ array calibration is necessary to perform faultless phase measurements. For this purpose, this article proposes a novel calibration procedure for antenna arrays, correcting both the mutual coupling and positioning errors using an incoherently transmitting beacon. Because the in-situ calibration will be affected by multipath, measurements at random reference positions within the array’s near field are used for calibration. In addition, the algorithm copes with randomly varying path losses because of unknown antenna polarizations and patterns. Calibration measurements are performed, providing physically reasonable results without using an anechoic environment. Furthermore, the calibration results are validated via indoor localization using three antenna arrays that were calibrated with the proposed algorithm. [ABSTRACT FROM AUTHOR]

In this article, challenges and limitations of using cross-polarization suppression methods for dual-linear polarization antenna arrays are presented. A simple probe-fed patch antenna and high-performance, dual-polarized hybrid-fed patch antennas are used for this article. The image feed method is used for improving the cross-polarization level and increasing the geometrical symmetry of a very large phased array antenna. It is shown that decomposing the antenna radiation pattern to its even and odd components for calculating $2\times $ 2-element subarray radiation pattern has fundamental limitations and none of four different $2\times $ 2-element and $4\times $ 4-element subarray configurations will entirely suppress the sidelobe issue of configured arrays. The accuracy of using embedded element pattern for calculating the radiation pattern of very large array antennas with identical subarrays is studied, and an accurate procedure for predicting array antenna radiation pattern is proposed. The appearance of undesired sidelobes has been examined, and optimal design for mitigating the sidelobe issue associated with the large array of identical subarrays with image configuration is presented. [ABSTRACT FROM AUTHOR]

Due to the range-angle-dependent beampattern, frequency diverse array (FDA), either in the form of linear array or planar array, has been extensively investigated in recent years. However, the conformal FDA is seldom studied. In this article, a general conformal FDA framework is first developed. Specifically, a hemi-spherical FDA is exploited to illustrate the unique advantage of conformal FDA in terms of the 3-D localization ability. Furthermore, to deal with the intrinsic defect of time-variant range–angle-coupled beampattern of the FDA, an optimized static nonlinear frequency offset (OSNFO) is proposed. The OSNFO not only decouples the conformal FDA beampattern in the range–angle dimension, but also copes with its inherent time-variant problem, resulting in a quasi-time-invariant 3-D focusing beampattern. Numerous simulation results have been provided to demonstrate the superiority and the effectiveness of the OSNFO-conformal FDA. [ABSTRACT FROM AUTHOR]

This article presents a technique designed to extend the scanning range of a probe-fed patch array whose performances are limited by scan blindness, due to a strong surface wave coupling. The current framework relates to the context of radar applications requiring a large scanning range (up to 60°) and where no inter-element spacing modification is possible as a result of strong T/R modules integration constraints. The proposed solution is based on the introduction of an additional correcting coupling within the array, by means of microstrip lines connecting the sources two by two. A thorough step-by-step description of the design methodology is introduced. It is then implemented in the case of a single-polarized ten-element patch array operating in $X$ -band. Simulation results as well as an experimental validation of the concept are presented. They assess the effective improvement of the active impedance matching and realized gain resulting from the use of connecting lines. [ABSTRACT FROM AUTHOR]

Traditional low probability of interception (LPI) design methods are mainly focused on the LPI waveform design. As a matter of fact, array design can also improve the LPI performance and is compatible with waveform design. A novel beamforming approach based on 4-D antenna arrays with pseudorandom time modulation is proposed for the effective improvement of LPI performance. The time sequences of 4-D arrays are designed as pseudorandom parameters and each element is modulated with a pseudorandom time sequence in order to hide the waveform parameters in undesired directions. A parameter estimation method including the time–frequency analysis is introduced for evaluating the performance of LPI. The numerical simulation results show that the LPI performance can be significantly improved in undesired directions by pseudorandomly modulating each element. Selected experimental verifications are performed and good agreement with the numerical results is obtained, thus validating the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this article, a new wideband cavity-backed bow-tie microstrip antenna is presented for short-range 77–81 GHz radar applications. The design is based on a subarray consisting of two single elements. Each element consists of a multilayer bow-tie patch antenna surrounded by a metallic cavity to suppress the surface waves and parallel plate modes. The subarray elements are fed through a laminated waveguide with 180° phase shift. An eight-layer build-up and a new advanced multilayer PCB technology, the so-called any-layer PCB technology, is used for the fabrication. The performance of this antenna has been verified by both simulations and measurements. A bandwidth of more than 8% is achieved. The gain is 7.9 dBi, and the radiation efficiency is 70% at 79 GHz. Beam widths of 84° in the E-plane and 48° in the H-plane are obtained, which are compatible with radar requirements for short-range applications. [ABSTRACT FROM AUTHOR]