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In this communication, a broadband and tunable radar absorber is proposed by integrating graphene capacitor with resistive frequency-selective surfaces (RFSS). The microwave reflectivity can be dynamically controlled by changing the effective sheet resistance of graphene through an electrostatic field bias. The simulated results indicate that this structure can tune its reflectivity over a wide frequency band ranging from 2.9 to 15.9 GHz under the normal incidence, when the graphene resistance is varied from 120 to $700~\Omega $ /sq. The tuning range of the average reflectivity is from −4.6 to −15.3 dB. The physical mechanism of this tunable absorber is discussed by studying the electric field distribution and power loss density. We fabricated this absorber and measured its tunable reflectivity by applying different biasing voltages. The experimental results are in a good agreement with those of the simulation results. In addition, this absorber is also verified to have a good angular stability and polarization insensitivity, which will have broad prospects in the application of stealth technology. [ABSTRACT FROM AUTHOR]

This article presents the design and optimization of a deployable 1 m mesh reflector compatible with a 12U-class CubeSat. This antenna is designed for telecommunication and is compatible with NASA’s deep-space network (DSN) at ${X}$ -band (i.e., uplink: 7.145–7.19 GHz; downlink: 8.4–8.45 GHz) and Ka-band frequencies (i.e., uplink: 34.2–34.7 GHz; downlink: 31.8–32.3 GHz). Three right-handed circularly polarized (RHCP) antennas, both transmit and receive, are introduced here: ${X}$ -band only, Ka-band only, and ${X}$ -/Ka-band. For the ${X}$ -band-only antenna, a gain of 36.1 and 36.8 dBic is achieved at uplink and downlink frequency bands, respectively. This translates into an aperture efficiency of 72% and 62%. For the Ka-band-only antenna, a gain of 48.4 and 48.7 dBic is obtained at downlink and uplink frequency bands, which translate into an aperture efficiency of 62% and 72%, respectively. [ABSTRACT FROM AUTHOR]

This article studies both experimentally and numerically the backscattering radar cross section (RCS) and Doppler spectrum characteristics from 2-D linear and nonlinear time-varying sea-like surface at UHF band under low grazing incidence. The small slope approximation and choppy wave model are applied to solve rough surface scattering and generate nonlinear sea waves, respectively. A coherent Doppler radar operating at 340 MHz was deployed at the tip of Huangqi peninsula in the southeast coast of China to measure the echo spectrum from the actual sea surfaces. At the same time, an ocean buoy was placed in the radar coverage to monitor the sea states. The RCS and Doppler spectrum are analyzed and compared comprehensively between radar measurements and numerical predictions. After compensating the influence of wind direction, the responses of radar echo power and numerical predicted RCS to wind speed are basically consistent and are more sensitive in low sea states. The correlation of Doppler spectra between radar measurements and numerical simulations exceed 0.96 during the whole experiment. The intensity of the high-order peaks increases rapidly with the wind speed, while the intensity of the Bragg peak decreases slightly. Doppler spectrum will be shifted by an amount proportional to radial surface current. [ABSTRACT FROM AUTHOR]

The coherence factor (CF) is defined as the ratio of coherent power to incoherent power received by the radar aperture. The incoherent power is computed by the multi-antenna receiver based only on the spatial variable. In this respect, it is a 1-D CF, and thereby the image sidelobes in down-range cannot be effectively suppressed. We propose a 2-D CF by supplementing the 1-D CF by an incoherent sum dealing with the frequency dimension. In essence, we employ both spatial diversity and frequency diversity which, respectively, enhance imaging quality in cross-range and range. Simulations and experimental results are provided to demonstrate the performance advantages of the proposed approach. [ABSTRACT FROM AUTHOR]

In this article, we consider the problem of beampattern synthesis with sidelobe control using constant modulus weights. We specifically focus on two applications, i.e., notching and sidelobe-level (SLL) control (SLC). The considered problem is a challenging NP-hard due to the employed constant-modulus weights. Here, we convert the problem into an intermediate form by incorporating auxiliary variables and changing the form of the constant modulus. Then, we decompose the intermediate form into subproblems of single constraint each. Finally, we propose an iterative solution following the alternate direction method of multipliers (ADMM) and Proximal frameworks. The proposed solution solves the original problem by iteratively solving the subproblems in coordinated logical order. Importantly, it combines the convergence speed of ADMM and convergence property of Proximal methods. In addition, it always satisfies the constant modulus constraint. The solution has a significant contribution to mitigate interference using notching and to realize RadCom. Simulation results show that the proposed solution has a better performance in terms of computational speed, notching, and constant SLL than the previous works. [ABSTRACT FROM AUTHOR]

In this article, we present a new methodology in spectral domain to study a novel, complex canonical electromagnetic problem constituted of perfectly electrically conducting (PEC) wedges immersed in complex environments. In particular, we present an arbitrarily flanged dielectric-loaded waveguide that resembles practical structures in scattering analysis, radar applications, antenna’s design, and electromagnetic compatibility. The proposed method is based on the recently developed semianalytical method known as the generalized Wiener–Hopf technique that extends the applicability of classical Wiener–Hopf method to a new variety of problems constituted of different geometries and materials. In this article, the method is further extended and it is now capable of handling piecewise constant inhomogeneous dielectric layers by resorting to the application of characteristic Green’s function procedure starting from the wave equation. The method has the benefit to be a comprehensive mathematical model and to be quasi-analytical, thus allowing us to investigate the true physics of the problem in terms of field’s components. The proposed solution is also of interest in computational electromagnetics to benchmark numerical codes. Validation through numerical results is reported in terms of engineering quantities such as geometrical theory of diffraction (GTD)/uniform theory of diffraction (UTD) coefficients, total far fields, and modal fields. [ABSTRACT FROM AUTHOR]

This article presents the synthesis of isophoric (equally fed), wideband wide-scan sparse concentric ring antenna arrays (CRAs) with low sidelobe level (SLL). The technique utilizes the characteristics of Bessel functions (BFs) of the first kind for representing the array factor (AF) and propose deterministic design guidelines for SLL reduction. It is shown that higher order BFs are necessary to predict nonsymmetry in the azimuth pattern of CRA. A design process for deriving ring radii and number of elements in the rings is presented to achieve highly sparse CRAs. Numerical test case studies are presented demonstrating the design procedure and its validation with the prior literature. The effect of the mutual coupling on the array performance over wideband and wide-scan volume is also presented. The results show sparser CRAs with comparable values of peak SLL (PSLL) and −3 dB beamwidth. This work may complement the existing stochastic or other efficient optimization techniques in reducing their computational time, especially for large arrays (more than hundreds of elements). This work focuses on phased array antennas for radar applications. [ABSTRACT FROM AUTHOR]

A multi-port element approach, engineered to address practical design challenges of very high-frequency (VHF) phased array radar systems, is presented in this communication. A cornerstone of the antenna element design is its simplicity, which counteracts the mobility issues of physically large and unavoidably bulky ground-based VHF radar systems. The multi-port element introduces a third dimension in the otherwise planar array configuration. Using a nonlinear optimization scheme to realize a set of matching networks for the element, the active loading from surrounding elements in the array environment is suppressed resulting in a nondistorted embedded element pattern. The element is compared to the resembling Yagi–Uda design in terms of standard multi-port and radar metrics. The simulated results indicate that the element enables a significant suppression of grating lobes when beam steering within a limited angular sector, which in turn has an overall positive impact on the array performance. [ABSTRACT FROM AUTHOR]

The emergence of CubeSats has opened up the possibilities of advanced space missions with lower costs and faster development times. A major factor that limits the functionality of CubeSats is the absence of high-gain antennas (HGAs) that can sustain a high data-rate link for communications or provide the required spatial resolution for remote sensing. In this work, we discuss the development of one of the largest apertures at Ka-band for CubeSats—a 1 m mesh deployable offset reflector antenna, with a stowed volume of 3U ($10\times 10\times 30 \,\,\text {cm}^{3}$)—to enable precipitation radars that can achieve the required spatial resolution while meeting the stringent mechanical constraints posed by the small CubeSat volume. We detail the critical aspects of this antenna design, including RF characterization, antenna fabrication, and measurement. We also describe a novel deployment mechanism that facilitates the packaging of such a large aperture in a small volume. The antenna demonstrated a measured efficiency of 60%, with a half-power beamwidth of 0.60° at 35.75 GHz. The cost reduction afforded by CubeSats makes launching a constellation of such radar based CubeSats practical, allowing higher temporal sampling rates, which is essential for the observation of weather processes with a short-time evolution. [ABSTRACT FROM AUTHOR]

Scattering properties of millimeter waves (mmW) remain relatively unknown compared to lower frequency bands, although their study is of interest considering the increasing number of civil applications, especially in the $W$ -band. In this communication, we present an efficient measurement setup that provides scattering measurements with spatial and polarization diversities. The targeted application is foreign object debris (FOD) detection and identification for surveillance of airport runways. Most existing systems are monostatic but rapid advances in mmW technology pushes toward the deployment of multistatic radars on either side of airport runways. In the following, we investigate the contribution of multistatic measurements with a special focus on the reconstruction of glass and brake pad debris, courtesy of DGAC (French civil aviation). We also present an efficient approach for making images of FODs. For this, we process scattered field measurements with an additive (with respect to polarization) backpropagation algorithm. First images include all measured points taken from the upper hemisphere. In order to get closer to the final application, we restrict the field of view to receivers facing the transmitter. Both approaches provide very good shape reconstruction of the glass debris and the central zone of the brake pad. To the best of our knowledge, this is the first time that full-3-D measurements have been conducted on FODs in the $W$ -band. It is also the first example of a 3-D qualitative image of FOD obtained from measured data in the $W$ -band. [ABSTRACT FROM AUTHOR]

This contribution describes a realistic rain model that considers a cluster of axis-symmetric but flattened raindrops with a specified shape–size relation, a raindrop size distribution, and a distribution of orientation angle of the symmetry axis. The backscattering cross section and specific attenuation due to the rain medium are determined by the extended boundary condition method. The model is applied to data from two X-band radars to estimate reflectivity and rainfall rate, considering attenuation effects during the two-way propagation between the radar and each sampled rain volume. The results are then compared with corresponding ones from well-established attenuation-correction methods. One of these methods, which relies on unattenuated reflectivities measured by an S-band radar, has been carefully validated by a previous study. The reasonable agreement between the two methods indicates that the proposed model may be a suitable option to estimate reflectivity and rainfall rate fields based on X-band radar data, particularly when no such references are available. [ABSTRACT FROM AUTHOR]

Stealth airborne systems are low observable platforms, which bring together technologies in the field of aerodynamics, electromagnetics, and material science. Radar absorbing materials/structures play a key role in the design of such stealth airborne platforms. In this paper, a radar absorber has been fabricated based on the design of a nonperiodic metallic patch coated with graphene. The simulation results of the novel design over an ultrawideband (1–60 GHz) yield good absorption (99.93%). With the constraint of fabrication facility, a small unit cell of the structure has been fabricated and measured. The measurement results show a good agreement with the simulation results. The absorption bandwidths, thickness, and weight of the structure are some of the key elements that were considered in the design model. It has been observed that the design allows the electric potential tuning by changing the Fermi energy of the graphene layer. The significant absorption of the proposed design in ultrawideband shows that it has potential applications in the RF stealth technology. [ABSTRACT FROM AUTHOR]

Patient monitoring of vital signals such as breathing rhythm and heartbeat rate can be done remotely by the use of a radar system. This approach is advantageous since it does not require any contact with the patient. Obviously, contactless monitoring results in a more comfortable situation for the patient, and in occasions, it is almost mandatory as in the case of heavy burned or newborn patients. Moreover, additional information such movement patterns are also available. A 120 GHz frequency modulated continuous wave radar is described with special focus on the design, construction, and testing of a specific reflector antenna for the system. The system is based on a commercial radar chipset that includes its own antennas. The challenge has been to design the optimum reflector and to build it and test it in a cost-effective way. The reflector has been 3-D printed and a near-field testing technique has been implemented to assess its performance. The results show that the system is able to measure the vital signs at distances beyond 1 m. [ABSTRACT FROM AUTHOR]

Studies devoted to the application of numerical solutions of the parabolic equation to the prediction of propagation conditions of radio waves traveling above the ground surface are briefly reviewed. Emphasis is placed on the analysis of the physical irregularities of propagation of radio waves in difficult conditions (above the sea surface, including formation of evaporation ducts and in forests, including the influence of forest and terrain profiles on the radiation field of phased array antennas). [ABSTRACT FROM AUTHOR]

A conventional monopulse radar system uses three beams, namely, sum beam, elevation difference beam, and azimuth difference beam, which require different layers of weights to synthesize each beam independently. Since the multilayer structure increases the hardware complexity, many simplified structures based on a single layer of weights have been suggested. In this communication, we introduce a new technique for finding disjoint and fully covering sets of weight vectors, each of which constitutes a sparse subarray, forming a single beam. Our algorithm decomposes the original nonconvex optimization problem for finding disjoint weight vectors into a sequence of convex problems. We demonstrate the convergence of the algorithm and show that the interleaved array structure is able to meet difficult beam constraints. [ABSTRACT FROM AUTHOR]

Compact phased-array antennas embedded in flip-chip ball grid array (BGA) packages are proposed to operate over the unlicensed 60 GHz frequency band. Radiation properties are enhanced by designing novel corrugated soft-surface structures and implementing them using the low-temperature co-fired ceramic process with no extra cost and fabrication complexity. This is achieved by adjusting via land diameters of periodic grounded vias in top four layers of the package, which in fact create perfect magnetic boundaries between the phased-array antenna elements to modify the electric near-field distributions and consequently improve far-field characteristics including gain, cross polarization, sidelobe level, and beam steering. Low-loss transitions from flip-chip IC to antenna elements are designed with less than 0.1 dB amplitude and 2° phase imbalance over 50–70 GHz. Prototypes with/without soft-surface structures are fabricated to work with flip-chip ICs with/without switches. A USB-interface adapter and a waveguide-interface module with a compact vertical power combiner are proposed for the first time to experimentally evaluate the phased-array antenna while connected to the flip-chip RF IC. Significant miniaturizations and improvements are achieved over the wide frequency range of 56–67 GHz, demonstrating up to 7 dB reduction in sidelobe level, up to 4 dB reduction in cross polarization, and up to 3 dB enhancement in gain at different directions. [ABSTRACT FROM AUTHOR]

Sum ($\Sigma$) and difference ($\Delta$) beampatterns are commonly utilized to estimate the bearings of targets in the radar system. However, the nulls of $\Delta $ -pattern are not always aligned with those of $\Sigma $ -pattern, and this phenomenon would become more severe in practical scenario since the array errors exist. Meanwhile, the sidelobe level (SLL) of the array would also increase due to the unavoidable errors. The above-described factors would dramatically decrease the performance of radar in the adaptive suppression of interference. To increase the output signal-to-interference-plus-noise ratio of a radar system, this paper presents an approach to design the low SLL $\Sigma \Delta $ -patterns where the nulls of the $\Delta $ -pattern, excluding the central null, are always aligned with those of the $\Sigma $ -pattern in the presence of errors. Two schemes are addressed to obtain the covariance matrix for null-alignment constraint, which is exploited to force nulling in desired angles accordingly. In the sequel, the matrix tapering method is adopted to broaden the nulls and enhance the robustness for the possible errors. With the proposed approach, the low SLL and nulls alignment can be achieved simultaneously. Numerical examples are provided and discussed to demonstrate the effectiveness of the proposed solution, including the results based on a linearly spaced microstrip array antenna. [ABSTRACT FROM AUTHOR]

This paper discusses the impact of a wet flat radome on the performance of a line replaceable unit active phased-array antenna developed for a dual-polarized phased-array weather radar. Water formations, such as film and droplets, were fully characterized over flat and curved radome surfaces using an analytical model as a function of the precipitation rate. Numerical simulations and experimental results validated the proposed analytical model. An active dual-polarized phased-array of $8\times 2$ elements was used to evaluate the degradation of the cross-polarization component as well as the mismatch and the phase of scanned antenna radiation patterns for vertical and horizontal polarizations. In addition, a radome panel of an $S$ -band weather radar (WSR-88D) was characterized in the far field using a single radio-frequency probe. It is demonstrated that even at the $S$ -band, the water formation on the radome affects both copolarization and cross-polarization components, degrading the overall performance of the dual-polarized radar system under rain conditions. [ABSTRACT FROM AUTHOR]

This communication proposes a compact, wideband, two-port substrate-integrated waveguide (SIW) antenna with a central, double-slotted metallic plate flanked by two paired corrugations for radar applications. The proposed antenna uses a two-layer structure, with a slotted metallic plate with corrugations on the top layer and an FR-4 substrate SIW on the bottom layer. The antenna has compact dimensions of 63 mm $\times$ 54 mm ($2.32\lambda _{0} {\times}\,\, 1.99\lambda _{0}$) with a low profile of 7 mm ($0.26\lambda _{0}$). To understand the operating principle of the two-port SIW antenna clearly, modal analysis is performed for single-port SIW antenna. The antenna satisfies a −10 dB reflection coefficient from 10.15 to 11.58 GHz. Although the separation between the two central slots is minimized to 4 mm ($0.14\lambda _{0}$), the isolation performance remains excellent (less than −30 dB). The gain enhancement, with a narrow beam pattern for radar applications, is realized using constructive superposition of in-phase electromagnetic energy from the two slots. In addition, because the antenna has two ports, it also has a beam scanning function from −12° to +12° in its boresight direction. The experimental and simulated results from the proposed antenna are in good agreement. [ABSTRACT FROM AUTHOR]

This contribution presents the results of a dual-polarized radiating element designed to achieve low cross-polarization (lower than −40 dB measured for the E-and H-plane, at least −30 dB in the D-plane based on simulations) and large fractional bandwidth (18%) over wide scanning angles (±60°). The proposed design includes multiple features that enable high isolation between ports, reduction of spurious radiation, highly symmetrical radiated fields, and suppression of diffracted fields between contiguous subarray gaps. To verify the polarimetric requirements for a weather radar, simulated and measured results, including electronic scanning of the array and embedded element patterns of the antenna, are shown. [ABSTRACT FROM AUTHOR]

By using the second bandgap in mushroom electromagnetic bandgap (EBG) structures, a high isolation can be achieved between transmitting (TX) and receiving (RX) microstrip array antennas integrated with a millimeter-wave single-chip frequency-modulated continuous-wave (FMCW) radar. These EBG structures are much easier to manufacture at millimeter waves as compared to the first bandgap mushroom EBG structures. In addition to improving isolation, the EBG structures are also used to reduce radiation pattern ripples caused by edge diffractions due to surface waves. Experimental validation is performed on antenna level and on the radar system level. For the system-level validation, the antenna arrays have been integrated with a single-chip mm-wave FMCW radar using the bond-wire interconnect technology. Measurement results in the 57–64 GHz band show the TX–RX isolation of 40 dB, which is 15–20 dB better than the isolation between TX and RX arrays without EBG structures. In addition, the radiation pattern ripple caused by edge diffractions is reduced below 3 dB. [ABSTRACT FROM AUTHOR]

In this reply, we refer to comments made by a reader on our publication “Overlapped phased array antenna for avalanche radar.” The authors would like to thank the reader for reading this article. In this article, the authors provided detailed answers to the comments made by the reader. [ABSTRACT FROM AUTHOR]

We propose a complete procedure for synthesizing radiation patterns of leaky waves using binary programmable metasurface. As an example, a binary leaky-wave antenna (BLWA) is designed and fabricated using the fast procedure. By optimizing coding states of all radiation elements of the BLWA, beam forming is realized at a fixed frequency. Full-wave simulations and measurements of the proposed BLWA further prove the effectiveness and practicability of the design procedure. The presented method and BLWA may find many significant applications in novel communication and radar systems. [ABSTRACT FROM AUTHOR]

In high-frequency hybrid sky–surface wave radar, the echo characteristics of hybrid sky–surface wave propagation channel are extremely complex in spatial, time, and frequency domains, which seriously affect radar performance. The mechanisms and echo characteristics are theoretically and experimentally analyzed in this paper. Based on the hybrid sky–surface propagation model, the signal-to-noise ratio equation of the channel echo and ionospheric direct wave are initially derived. The simulation method is developed, which considers bistatic layout, ionospheric propagation, sea surface propagation, and radio noise. The echo characteristics by sweep frequency detection are simulated and analyzed. Using the newly developed hybrid sky–surface wave experimental system, the experimental results by sweep frequency detection are first achieved in China and abroad, which are consistent with the theoretical analysis, and the validity of the simulation is verified. Then, the characteristics of first-order sea clutter in different azimuths are theoretically and experimentally analyzed, and it is found that the bistatic layout and ionosphere play a significant role in the Doppler broadening and path distribution of the sea clutter. Finally, the experimental results by joint sweep and fixed frequency detections are analyzed. The echo characteristics are extremely complex with frequency. Specifically, owing to multimode echoes overlap, the spread of the sea clutter and ionospheric direct wave signals would seriously affect ship target detection in part of the operating frequency band. It is shown that the detection and diagnosis for hybrid sky–surface channel in spatial, time, and frequency domains are crucial for target detection performance. The research results on the propagation mechanisms and echo characteristics could provide important supporting function for layout design, frequency selection, target detection, and location of the radar. [ABSTRACT FROM AUTHOR]

A phase-based composite ranging system is investigated that provides accurate and absolute distance estimates for backscatter radio frequency (RF) tags by combining the range estimates of a dual-frequency, continuous-wave (DFCW) and a continuous-wave (CW) radar. The relatively low-resolution DFCW range estimate is used to resolve the wavelength ambiguity in the CW radar, and the disambiguated CW range estimate provides the final, high-resolution, accurate range estimate. Two data fusion methods for combining the DFCW and CW range estimates are proposed and their performance is compared based on measurement results with a backscatter RF tag system operating in the 5.8 GHz industrial, scientific, and medical (ISM) frequency band. Each data fusion method uses spatial averaging, as the RF tag is moved through space to reduce the overall system error. The efficacy of spatial averaging is investigated by showing that multipath and hardware imperfections cause oscillating errors in the phase used for ranging and the errors are significantly reduced through spatial averaging. It is shown that the proposed system can achieve millimeter-level ranging resolution in a typical indoor multipath environment while adhering to the bandwidth constraints of the 5.8 GHz ISM frequency band. [ABSTRACT FROM AUTHOR]

In this communication, a novel substrate integrated waveguide slot array with flat-shoulder-shaped radiation pattern is introduced for both 77 GHz automotive medium-range radar and long-range radar applications. The operational principle of the antenna is introduced, while the synthesis of the desired radiation pattern is facilitated by a mixed optimization method, including nonlinear fitting and aggressive space mapping. Experiments are carried out and the measured peak gain of the proposed array is 21.7 dBi. The demonstrated antenna presents to be an excellent candidate for 77 GHz automotive radar application. [ABSTRACT FROM AUTHOR]

This paper presents a high-isolation, low cross-polarization dual-polarized patch antenna for multifunction phased array radar applications. Its hybrid feed design has been implemented, and the vertical and horizontal polarizations are excited by a balanced-probe feed and a slot-coupled feed, respectively. Simulations and measurements have demonstrated an input isolation of 45 and 43 dB between the horizontal and vertical ports, respectively. For further improvement in the cross-polarization level, the image feed method is also implemented, and a $2\times 2$ -element array made up of designed elements with image configuration has been fabricated. The simulated and measured S-parameter and radiation patterns of the horizontal and vertical polarizations of the designed $2\times 2$ -element array are presented and the measured cross-polarization level of less than −37 dB is achieved. To examine the performance of the designed element in an array, a $3\times 3$ -element array of designed $2\times 2$ -element subarray is fabricated and tested. In the $6\times 6$ -element measurements, −35.4 and −36 dB cross-polarization levels for horizontal and vertical polarizations are achieved, respectively. Also, using the measured embedded element patterns, the cross-polarization level lower than −36 dB for scan angles up to 45° is achieved. [ABSTRACT FROM PUBLISHER]

In this paper, we devise beampattern synthesis algorithms for sparse arrays using the alternating direction method of multipliers (ADMM). Unlike the usual weighted l 1 norm, we utilize the l p norm of the array weight vector, where $0

Crossed-loop (CL)/monopole antenna has been widely used in high-frequency ground-wave radar for ocean surface remote sensing owing to its compact size. The multiple signal classification (MUSIC) algorithm is commonly adapted to achieve a favorable angular resolution for this broad beam antenna. However, the direction of arrival (DOA) estimation is always troubled by the frequently occurring distortion of antenna pattern. Previous studies are mainly focused on the calibration by using the actual pattern or other assistant algorithms. In this paper, theoretical analysis is made to illustrate the reason for pattern distortion, which draws a conclusion that the distortion is essentially due to the different influence of environmental obstacles upon the electric field part and magnetic field part. This also implies that there is nearly no distortion between the two loops, which is validated by three different experiments. Based on this conclusion, the DOA estimation performance of the MUSIC algorithm is studied by creating a numerical relationship associating the estimation error with distortion level. Furthermore, a new method using ideal pattern is proposed to improve the estimation performance by removing the amplitude information of monopole and relying on the CL processing. Both simulation and experimental results are given to prove its validity. [ABSTRACT FROM AUTHOR]

A methodology for designing a frequency selective reflector with two-sided absorption bands is proposed. This method is based on utilizing multimode resonators inside a 3-D unit cell to obtain an absorption-reflection-absorption response. Detailed analysis of this 3-D absorptive frequency selective reflector (AFSR) is provided with the aid of an equivalent circuit model to reveal the operating principle. To show the benefit of the new structure in antenna’s radar cross section reduction (RCSR), a dual-polarized 3-D-AFSR integrated with monopole and dipole antennas are simulated, fabricated, and then measured. Compared with their counterparts with normal ground plane, AFSR-backed antennas have shown a remarkable out-of-band RCSR. The −10 dB RCSR has fractional bandwidths of at least 64.7% and 41% with center frequencies of 3.4 and 8.3 GHz, respectively. The radiation characteristics of AFSR-backed antennas are nearly maintained. The realized gains are enhanced by 0.9 and 2.3 dB for monopole and dipole antennas mounted on a 3-D-AFSR, respectively. [ABSTRACT FROM AUTHOR]

In this communication, we introduce a channel model for personal radar applications where a millimeter-wave (mm-wave) massive array is required to scan the environment and to reconstruct a map of it. The analysis is based on measurement campaigns, in a corridor and in an office room, performed using mm-wave massive arrays. In such a context, we aim at characterizing the channel from both a temporal and an angular perspective by exploiting a 2D CLEAN-like technique to extrapolate the multipath components and a K-means algorithm for clustering, where centroids statistics depend on the environment contour. The obtained channel model can be exploited for mapping algorithms based on backscattered radar measurements. [ABSTRACT FROM AUTHOR]

In this paper, we present a new horn antenna that can mechanically reconfigure the beam direction and polarization by combining a traditional linearly polarized horn antenna with an anisotropic Huygens metasurface. Three layers of Jerusalem-cross structures are integrated in a dielectric substrate to form the anisotropic Huygens metasurface; the entire structure has a thickness of about one fifteenth the working wavelength. By placing the metasurface in front of a traditional linearly polarized horn antenna, the main beam of the horn antenna is steered to a predetermined direction. Furthermore, the direction and polarization of the main beam are reconfigurable by rotating the anisotropic Huygens metasurface. The measured results of the antenna confirm the simulated results. Possible applications of the presented antenna include radar detection, base-station communications, and satellite communications. [ABSTRACT FROM AUTHOR]

A frequency reconfigurable phased array element is presented. The operational band of the single port L/S-band antenna can be selected by modifying the element apertures with p-i-n diode switches. The antenna element satisfies strict requirements on its frequency band separation (2.2:1), size, feeding structure, and control lines to be integrated into a phased array system. A multiscale array topology is proposed to achieve wide angle scanning (±60°) in both operational bands of the array. [ABSTRACT FROM AUTHOR]

Subarrayed monopulse linear and conformal circular arc arrays with low-sidelobe sum and difference patterns are synthesized. For the monopulse array with a subarray partition consisting of contiguous elements to reduce the cross points, the synthesis problem is first formulated as a constrained optimization problem. Then, by defining an appropriate fitness function to convert the problem into an unconstrained one, the problem is efficiently solved by the differential evolution algorithm under successful-parent-selecting framework. The numerical results of small and large monopulse linear arrays are reported by comparisons with the state-of-the-art techniques. At last, synthesis of a monopulse conformal circular arc array is also investigated based on the same technique. [ABSTRACT FROM AUTHOR]

Snow avalanche is a natural phenomenon, which annually causes infrastructure damage and leads to human casualties in many countries all over the world. As a result, a detailed hazard mapping is required to be able to understand the avalanche risk levels. Also, the scarcity of the areas suitable for constructing infrastructures in the mountains places a high premium on providing high precision hazard mapping. This paper presents the design and development of an advanced phased array antenna for an FMCW radar to produce a high resolution image of snow avalanche in range and cross-range. The proposed antenna is a 16-element fully populated phased array with ±14° azimuth beamwidth and ±22.5° fixed elevation angle. The designed phased array antenna has 14.4 dBi gain and −20.3 dB sidelobe level. In this design, subarraying overlapping technique has been used to eliminate undesirable grating lobes by means of spatial antialias filtering. A phased array power divider is proposed to allow tapering of the amplitude of the elements to achieve low sidelobe level. [ABSTRACT FROM PUBLISHER]

A partially parallel-feeding system of traveling-wave waveguide slot array has an advantage to reduce the frequency dependence of excitation phase of slots cut on the waveguide, which results in broadband feature of gain and radiation pattern. However, a conventional narrowband post-loaded radiating slot for array element limits the overall bandwidth of the array. A broadband wall-surrounded slot on a stepped narrow wall in the waveguide is proposed in this paper. The effects of the step on the waveguide narrow wall and the wall-surrounding slot are evaluated individually by electromagnetic simulation in this paper. The proposed antenna configuration of the waveguide assembled at the waveguide center allows fabrication by plastic molding with metal plating. The antenna performance is measured to confirm the feasibility of the proposed antenna in the millimeter-wave band. [ABSTRACT FROM AUTHOR]

This communication presents a compact 94-GHz 2-D multibeam antenna using the low-temperature co-fired ceramic (LTCC) technology. A good 2-D substrate integrated waveguide (SIW) beam-forming network (BFN) for eight beams is proposed, where four-phase shifters are properly introduced to increase the flexibility of the beams. Benefiting from multilayer LTCC, the BFN is compactly implemented in a completely symmetrical structure by using two multifolded SIW Butler matrixes with 50% reduction of the longitudinal size, and four couplers placed on different layers with interlaced signal routes to avoid overlaps or crossovers. Combined with a $2 \times 4$ SIW slot antenna array, the multibeam antenna can realize eight symmetric scanning beams on 2-D with stable gains for a wide coverage. For demonstration, the two single-port antennas with different pitch-angle beams are fabricated and measured, and good agreements with the expectations are observed. The differences of the beam orientations are less than 5°, while the measured peak gains of the beams are about 8.5 dBi. [ABSTRACT FROM PUBLISHER]

A novel antireflection (AR) theory based on an anisotropic metamaterial is presented in this paper. Based on this theory, we derive the required anisotropic material parameters for perfecting matching at a particular frequency for a particular angle of incidence, including extreme angles. For a proof of concept, the anisotropic AR layers are synthesized with metamaterial structures and near perfect matching is demonstrated in simulation at 88° incidence for both transverse-electric and transverse-magnetic polarizations. For experimental validation, the AR layer is redesigned to match at 60° in order to avoid spill-over losses in the quasi-optical measurement due to the finite substrate size. Excellent agreement is observed between the simulation and measurement results. [ABSTRACT FROM PUBLISHER]

A virtual active element pattern (AEP) expansion method is presented in which each AEP in an unequally spaced array is considered to be the pattern radiated by a subarray of some equally spaced virtual elements. With the help of this method, the pattern of an unequally spaced array including mutual coupling can be efficiently evaluated by fast Fourier transform (FFT). By incorporating this idea into the iterative Fourier transform procedure, we develop a novel iterative synthesis method, which can apply the iterative FFT to efficiently synthesize unequally spaced arrays including mutual coupling. Different excitation constraints, such as phase-only control and amplitude-phase optimization with a prescribed dynamic range ratio, can be easily added into the proposed synthesis procedure. A set of synthesis examples for different antenna arrays with pencil and shaped beam patterns are provided to validate the effectiveness and advantages of the proposed method. [ABSTRACT FROM AUTHOR]

Uniform circular array (UCA) is a pioneering approach to generate radio twisted beams carrying orbital angular momentum (OAM). However, faulty antenna elements in a UCA could render the array into an imperfect UCA (IUCA), consequently creating distortion for OAM-based communication or radar systems. To analyze the effects of an IUCA on generating OAM, elementary wave function and wave transformation are derived to obtain field equations of a general circular array. Further derivation of IUCA shows that the field of an IUCA is a superposition of OAM modes, which is caused by polluted orthogonalities of OAM modes. To achieve pure OAM modes, the orthogonal matching pursuit algorithm, the convex optimization tool, and the analytical solution are employed to design a feeding network to retrieve the orthogonal property at the cost of strong gain loss. Then, the reduced constraints method is proposed for the IUCA to achieve high-purity and low-loss OAM mode. Finally, four circular arrays with different shapes are explored for limited installation space to generate twisted beams. [ABSTRACT FROM AUTHOR]

This paper presents a novel tri-band (X/Ku/Ka-band) planar antenna array with dual polarizations and shared aperture. Compared with traditional dual-polarized arrays, the proposed array has advantages of low cost, low profile, and high integration. Three types of antennas resonating at different frequencies, including the perforated patch, stacked patch, and slim crosspatch, are innovatively interleaved in the same aperture. The crosspatch fed by proximity coupling is presented as Ku-band element for its advantages of compact size, high isolation, and pure polarization. The techniques such as series feed and reverse feed are utilized to implement the six feed networks in a compact size with reduced cross polarizations. Measured results agree well with the simulations, showing three operation bands at X-, Ku-, and Ka-bands with the bandwidths of 3.6%, 6.7%, and 5.3%, respectively. The antenna also exhibits excellent radiation performance with the cross-polarization discrimination over 25 dB at the three bands. To the best of the author’s knowledge, this is the first shared-aperture X/Ku/Ka-band dual-polarized antenna array reported, which is useful for potential synthetic aperture radar applications. [ABSTRACT FROM AUTHOR]

Structured (e.g., rectangular) meshes in finite-element (FE) analysis offer several advantages, but are not often used because of the error introduced in fitting material interfaces. The nonconforming voxel FE method (NVFEM) is a hierarchical structured method that systematically reduces the geometric error in an adaptive way, but it requires large numbers of elements and therefore is expensive. By computing basis functions that take into account the interface cutting through an element, the number of elements needed is greatly reduced. The resulting method, CBF-NVFEM, is applied to the computation of the radar cross section of free-space scatterers. Results indicate that CBF-NVFEM is much more efficient than NVFEM and is competitive with FEM using unstructured meshes of tetrahedra. [ABSTRACT FROM AUTHOR]

The direct measurement of the angle rate of moving objects using a radar with a spatially diverse electric field pattern, a measurement analogous to the measurement of the range rate of a moving objects, represents, along with the measurements of range, range rate, and angle, a fourth basic radar measurement. Recently introduced and experimentally demonstrated, the theoretical accuracy of the direct measurement of angle rate is derived in this paper, and it is compared to the measurements of range, range rate, and angle in the context of the optimal signal forms for the best measurement accuracy. Signal forms achieving optimal accuracy for each measurement are discussed; example implementations of high-accuracy measurements are compared to the optimal forms; and the limitations of simultaneous measurements of pairs of measurements are derived. Combining the angle rate measurement with the three other standard radar measurements may provide future radar systems the capability to simultaneously and instantaneously measure the position and 3-D trajectory of moving objects without compromising the accuracy of any individual measurement. [ABSTRACT FROM AUTHOR]

In this communication, we present a new contribution to radar-target identification via the extinction-pulse (E-pulse) method. New E-pulses are built using exponential \beta -splines, which include in their construction free parameters that are adjusted to obtain more robust identification systems. The adjusting process is done by searching for the free parameters that maximize the discrimination capability of the system in the absence of noise. Simulation results justify the approach and show that the E-pulses obtained by maximization of the discrimination capability improve the results obtained with the classical scheme. [ABSTRACT FROM PUBLISHER]

This communication presents a dual-port reconfigurable square patch antenna with polarization diversity for 2.4 GHz. By controlling the states of four p-i-n diodes on the patch, the polarization of the proposed antenna can be switched among linear polarization (LP), left- or right-hand circular polarization (CP) at each port. The air substrate and aperture-coupled feed structure are employed to simplify the bias circuit of p-i-n diodes. With high isolation and low cross-polarization level in LP modes, both ports can work simultaneously as a dual linearly polarized antenna for polarimetric radars. Different CP waves are obtained at each port, which are suitable for addressing challenges ranging from mobility, adverse weather conditions and non-line-of-sight applications. The antenna has advantages of simple biasing network, easy fabrication and adjustment, which can be widely applied in polarization diversity applications. [ABSTRACT FROM PUBLISHER]

In the optical region, the global scattering center (SC) model is important for target recognition and data compression. However, the existing methods are difficult to be accomplished because of the requirement for large storage or time consumption. To solve this problem, a new easy-to-implement method is proposed in this paper. By employing a modified random sample consensus method, all candidate positions for the global SC model are extracted and discriminated efficiently. The clean step is unnecessary in the proposed method, which makes the procedure more efficient. Experimental results on the high-frequency-electromagnetic data demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A complete functional 60-GHz BiCMOS single-chip millimeter-wave frequency-modulated continuous wave (FMCW) radar with on-chip integrated antennas is designed, realized, and experimentally validated. The chip is configured with one transmitting and two receiving antennas. A high antenna efficiency and isolation between the transmitting and receiving antennas is achieved by using a cavity-backed on-chip monopole and by optimizing the package environment. Surface-wave losses and back radiation are reduced, and a high antenna gain is obtained with small ripples in the antenna pattern by implementing special structures on the PCB of the integrated circuit package. The experimental results from the single-chip radar show that the on-chip antennas provide a gain of 2 dBi with very low back radiation. The realized antennas achieve an impedance bandwidth of more than 50% with a corresponding simulated efficiency of 45%. In addition, the proposed approach suppresses surface waves and meets the specific FMCW radar requirements, such as a more than 25-dB isolation between the transmitting and receiving antennas. [ABSTRACT FROM PUBLISHER]

CubeSats are positioned to play a key role in Earth Science, wherein multiple copies of the same RADAR instrument are launched in desirable formations, allowing for the measurement of atmospheric processes over a short evolutionary timescale. To achieve this goal, such CubeSats require a high-gain antenna (HGA) that fits in a highly constrained volume. This paper presents a novel mesh deployable Ka-band antenna design that folds in a 1.5 U (10\times 10 \times 15 \,\textcm^3) stowage volume suitable for 6 U (10\times 20 \times 30 \,\textcm^3) class CubeSats. Considering all aspects of the deployable mesh reflector antenna including the feed, detailed simulations and measurements show that 42.6-dBi gain and 52% aperture efficiency is achievable at 35.75 GHz. The mechanical deployment mechanism and associated challenges are also described, as they are critical components of a deployable CubeSat antenna. Both solid and mesh prototype antennas have been developed and measurement results show excellent agreement with simulations. [ABSTRACT FROM PUBLISHER]

With the advancement of radar technology, detecting objects, determining the structure of the target, and estimating the direction and the speed is prominently increasing. There is no doubt that small cross section targets are hardly identified and determined. This problem demands the need for antenna pattern synthesizing with a high gain and highly reduced sidelobe level (SLL). In this paper, an optimization technique called self-adaptive differential evolution (SaDE) that can be able to learn and behave intelligently is integrated to determine an optimal set of excitation weights in the design of elliptical–cylindrical antenna array (ECAA). In this work, the SaDE-optimized hyper beam is proposed to achieve qualified radar performances. Nonuniform excitation amplitudes of the individual array elements of ECAA are performed to obtain reduced SLL, high directivity, and flexible radiation pattern which are crucial in radar applications. With increased antenna gain, small cross section objects can be accurately identified and determined even in volatile environment where there are different natures of interferences. Besides, the proposed work enables steering electronically and it allows scanning very rapidly across a wide range of elevation and azimuth planes. To evaluate the improved performance of the proposed optimization scheme, comparison is done with genetic algorithm (GA), particle swarm optimization (PSO), biogeography-based optimization (BBO), and invasive weed optimization (IWO) against different parameters. In general, the proposed work of pattern synthesis has resulted in much better reduction in SLL and first null beamwidth (FNBW) than both the uniformly excited and thinned ECAA. The results of this study clearly revealed that the SLL is highly reduced at a very directive beamwidth. The proposed antenna array offers high system availability over wide range of radar applications. [ABSTRACT FROM PUBLISHER]

This communication presents the experimental results of a ultrawideband 2–18-GHz dual-polarized Vivaldi antenna array for airborne radar measurements of snow. The antenna design is based on the previously reported all-metal flared-notch array by Kindt and Pickles for operation over the frequency range 0.7–9 GHz. An antenna array prototype consisting of $8 \times 8$ active dual-polarized elements was fabricated with precise aluminum machining and tested in the anechoic chamber. Beamsteering upto 30° was experimental demonstrated from 2 to 18 GHz. The measurement results are in a good agreement with the full-wave simulation results in both polarization configurations. Preliminary sample results from data collected using the Vivaldi array are also presented. The antenna array enables full polarimetric measurements of snow-over-sea-ice for estimating the snow-water-equivalent (SWE), as well as fine-resolution mapping of snow-air and snow-ice interfaces for estimating thickness. [ABSTRACT FROM PUBLISHER]