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In this communication, a dual-band shared-aperture reflectarray (RA) with a phased array is proposed for intersatellite links (ISLs). The Ka-band RA is integrated with an X-band phased array by a sharing mushroom patch structure. The high-frequency ratio allows the high-frequency RA and the low-frequency broadband phased array to operate independently to support high-speed communication of the geostationary orbit (GEO) relay nodes in the intersatellite network. To verify this concept, a prototype of the antenna is designed and fabricated, and its electrical performance is measured. The RA presents a peak gain of 30.1 dBi (28 GHz) and a 3 dB gain bandwidth of 11.4% (26.4–29.6 GHz). The phased array presents the peak gain of 21.7 dBi (9 GHz), and the relative bandwidth can reach 45.3% in the boresight direction. The scanning range can cover ±45°, and the H/E planes scan gain losses are less than 2.34 and 4.33 dB, respectively. [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]

A new approach is proposed to develop aperture-shared dual-band antennas using partially reflecting surfaces (PRSs). A fully aperture-shared dual-band antenna consisting of a low-band antenna (LBA) and a high-band antenna (HBA) right below the LBA is first investigated. The aperture sharing has little influence on the LBA but causes a severe shielding effect on the HBA. To alleviate the shielding effect, a low-pass PRS is inserted between the LBA and HBA, forming an air-filled quasi-Fabry–Perot cavity, which can make compensation for the radiation from the HBA but with little influence on the LBA. The second PRS is introduced above the LBA to enhance the bandwidth (BW) for HBA. It is shown that the fully aperture-shared dual-band antenna achieves an impedance BW ($\vert \text{S}11\vert < -15$ dB) of ~43.7% (0.68–1.07 GHz) for the LBA and ~45% (1.7–2.7 GHz) for the HBA with an average gain of ~8 dBi and stable radiation patterns. Based on the PRSs, an aperture-shared dual-band antenna subarray is finally developed for base-station applications, which covers the frequency bands of 0.68–0.96 and 1.7–2.7 GHz for 2G/3G/4G mobile communications. [ABSTRACT FROM AUTHOR]

This communication presents a freehand imaging system, which takes full advantage of a compact multiple-input multiple-output (MIMO) radar to provide the real-time results by means of freehand movements. The system is based on a commercial radar-on-chip, equipped with four receivers and two transmitters, and an optical tracking system (i.e., motion capture system) for retrieving its position and attitude. The reduced size of the radar enables its handheld operation, making possible to create a synthetic aperture by moving the scanner over the area of interest. Due to the new degrees of freedom with respect to previous implementations based on quasi-monostatic topologies, a novel calibration strategy for the system is required in order to mitigate positioning errors. After calibration, the simulations and measurements are accomplished revealing that the novel freehand multistatic setup significantly outperforms the monostatic setup in terms of scanning speed and image quality. Multimedia material is provided to illustrate this performance. [ABSTRACT FROM AUTHOR]

A novel hybrid approach to the power synthesis of mask-constrained shaped beams by means of concentric-ring isophoric arrays is presented. The basis of this procedure is to use the complex aperture distribution obtained by Elliott–Stern method as a starting point of an innovative algorithm that combines a classical density tapering method with a simulated annealing one. The proposed technique allows to outperform the previous procedures. [ABSTRACT FROM AUTHOR]

This paper presents the first demonstrator of a portable, multiview, high-resolution, 3-D, and real-time microwave imaging system. The system is based on a recently developed real-time 3-D microwave camera, which performs quasi-monostatic acquisitions, equipped with an optical depth camera, providing target surface profile information. In addition, the entire system can be arbitrarily moved along the target performing microwave and depth camera synchronized acquisitions from different views with a twofold purpose, namely: 1) enabling a coverage area much larger than that possible with a static imaging system and b) allowing for incorporation of several tilt angles (or views) to enhance capturing specular reflection imaging data to improve the overall image quality. At each scanning position, the imaging data from the microwave camera are processed to build a local 3-D microwave image. The information is then merged, using recently proposed techniques for multiview synthetic aperture imaging, to compose the global image. The synchronized optical camera depth acquisitions enable tracking the entire imager movements so that the position and attitude are known. Moreover, the data acquired by the depth camera are also use to build a complementary 3-D outer surface profile model of the target, producing a combined and realistic image of the internal and external geometries of the target. Finally, the performance of the combined system is evaluated using several examples related to hidden contraband covered by clothing (i.e., people screening). [ABSTRACT FROM AUTHOR]

This paper deals with the design of tensorial modulated metasurfaces able to implement a general radiating aperture field distribution. A new aperture synthesis approach is introduced, based on local holography and variable impedance modulation. In particular, it is shown that tensorial metasurfaces can be used to generate general radiating distribution (phase and amplitude). In addition, a step-by-step algorithm is presented. In order to validate the method, several solutions are presented at 20 GHz which implement aperture distributions able to radiate different beams with general polarization. [ABSTRACT FROM AUTHOR]

In this paper, the superluminal behavior of a radiated pulse in the near-field of an ideal dipole and an ultrawideband antenna is explained and demonstrated through simulations and measurements. The deformation of the radiated pulse in the near-field is illustrated using the ideal dipole antenna radiation as well as by a Vivaldi antenna through full-wave simulations and measurements. The apparent superluminal pulse velocity is due to the pulse reshaping of the radiated pulse in the near-field of the antenna. The pulse velocity is measured by tracking pulse peak, pulse envelope peak, and also calculating the pulse centrovelocity. The superluminal pulse peak velocity of the radiated pulse from a miniaturized Vivaldi antenna is confirmed through measurements in face-to-face and quasi-monostatic radar configurations. The effect of different values of pulse velocities on the quality of reconstructed images using synthetic aperture radar processing is demonstrated as well. [ABSTRACT FROM PUBLISHER]

In this paper, a procedure to design a fuzzy classifier to classify shell-shaped targets is proposed. The proposed procedure utilizes the radar cross section (RCS) of a shell-shaped target considering the relative orientation angle and polarization dependences in various flight scenarios based on real situations. In addition, particle swarm optimization (PSO) is applied to the fuzzy classifier to maximize its classification capability. Numerical results using three different shell-shaped targets are presented to assess the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A single-layer dual-band circularly polarized reflectarray antenna has been designed, fabricated, and tested for Ka-band satellite communications. The reflectarray antenna phasing element is composed of a concentric split-ring, in combination with a modified Malta Cross, where the technique of varying rotation angle and element size has been utilized to compensate the phase delay at 20 and 30 GHz, respectively. All the element configurations have been optimized to reduce cross-polar reflection. The aperture field method has been used to predict the reflectarray radiation performance, such as gain, radiation pattern, and cross-polarization level. A reflectarray with a circular aperture of 420 mm in diameter has been designed, manufactured, and tested for verification. A planar near-field measurement setup has been utilized to measure the reflectarray radiation characteristics. The measured results demonstrate that this dual-band reflectarray has achieved aperture efficiency of 66.5% at 20 GHz and 50% at 30 GHz, respectively. [ABSTRACT FROM PUBLISHER]