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In this letter, a radio frequency identification (RFID)-based location and tracking system that uses receiver signal strength (RSS) measurements is presented. The system is intended to be deployed at industrial scenarios or warehouses involving heavy machinery and vehicles in motion such as forklifts, and/or low visibility areas, aiming at preventing accidental collisions with workers. For this purpose, RFID tags are attached to helmets of the workers. The UHF reader antennas are located in the vicinity of the area where the workers are creating a coverage area where workers close to the vehicle can be detected and tracked. To deal with the propagation effects that degrade the RSS values, a propagation model based on on-site measurements together with an accurate characterization of the radiation pattern of the RFID reader antennas has been proposed. Validation with measurements in an scenario resembling the industrial environment is presented. [ABSTRACT FROM AUTHOR]

This article presents a portable system for freehand antenna diagnosis and characterization based on amplitude-only data. The amplitude-only samples are acquired by moving a handheld probe, which is tracked by a motion capture system, in front of the antenna under test (AUT) aperture. The acquired measurements are processed using the phaseless sources reconstruction method to compute an equivalent current distribution on the AUT aperture. Finally, the radiation pattern of the AUT can be obtained by evaluating the corresponding radiation integrals. Unlike previous work, the use of amplitude-only data avoids the need of a phase reference, paving the way to the diagnosis and characterization of antennas under operational conditions. This fact, together with the handheld capabilities, makes the system very convenient for measurements of already deployed and onboard antennas. Moreover, these amplitude-only acquisitions also simplify the required hardware. The system has been validated through measurements in a wide frequency range from Ka-band up to 300 GHz. Despite that one cannot expect the same degree of accuracy that can be achieved under laboratory conditions (including an anechoic environment and highly accurate positioners), the system shows excellent capabilities to detect malfunctions, such as wrong amplitude/phase distributions, as well as a fair estimation of the far field. [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]

This article presents a simple, cost-effective system for complex measurements of the field radiated by an antenna. In particular, the proposed architecture is based on an array of antenna probes connected to a set of phase difference detectors and a power detector. Phase difference measurements are processed to recover the phase of the antenna under test. The main novelty of the proposed system is that it does not require regular sampling, making it suitable for in situ antenna measurements. The influence of parameters, such as the number of antennas composing the array (and hence the number of phase difference detectors) and the positioning errors, is analyzed. The proposed architecture, implemented with off-the-shelf hardware, has been assessed for antenna measurement and diagnostics. A discussion of the issues related to the implementation of the system has been included, as well as the proposed solutions to improve its accuracy. Finally, diagnostics and radiation pattern measurement results are compared with those calculated with other phaseless techniques. [ABSTRACT FROM AUTHOR]

This article presents an agile system for antenna diagnosis. The system is operated by means of freehand movements of the probe antenna avoiding the need of bulky positioners and thus providing portability to the system. For that purpose, the probe antenna position, which is required by the antenna diagnosis algorithm, is retrieved by means of an affordable and quickly deployable motion capture system. The probe position and the acquired field are used to characterize the antenna under test by computing an equivalent currents distribution on the antenna aperture and its far-field pattern in real-time. To achieve that, an ad hoc workflow is designed including a method for spatially balancing the acquired field samples. Although the system is not intended to provide an accuracy comparable to the one achieved by anechoic chamber facilities, it opens a new horizon of possibilities for in situ agile characterization of antennas enabling the detection of faulty elements or radiation pattern deviations. Results at Ku- and Ka-bands, supported by attached multimedia material, are presented to illustrate the capabilities of the system. [ABSTRACT FROM AUTHOR]

This paper introduces three methodologies to improve synthetic aperture radar (SAR)-based techniques used in ground-penetrating radar (GPR) systems. They consist of: 1) equalization of the frequency response of the transmitting (Tx) and receiving (Rx) antennas; 2) processing of SAR images using partially overlapped frequency subbands; and 3) imaging domain clustering. The goal is to combine ground penetration capabilities of lower frequency bands with the resolution achieved when increasing the overall frequency band, resulting in enhanced detection and imaging capabilities. Validation of these techniques has been done at three levels: first using simulations, next by means of measurements in a controlled scenario, and, finally, using a portable setup deployed in a realistic scenario. [ABSTRACT FROM AUTHOR]

This paper aims at contributing to the specialized literature on the experimental characterization of metasurfaces (MTSs) reflectivity. The growing demand for angularly stable MTSs reinforces the relevance of the results under oblique incidence. Generally, reflection coefficient phase measurements are more challenging than the amplitude ones. Thus, an artificial magnetic conductor (AMC) is used to analyze the critical aspects for enlarging the angular margin that can be properly measured. The importance of the retrieving methodology along with the proposed data postprocessing is highlighted among the involved issues, according to the obtained results. The suitability of monostatic versus bistatic measurement setups for comparison with plane-wave simulation is studied. The aforementioned AMC is experimentally characterized using both setups providing relevant conclusions. The tradeoff between the manufactured prototype dimensions (to resemble the simulated infinite one) versus the required anechoic chamber size (to fulfill far-field (FF) conditions, i.e., the plane-wave incidence for the considered frequencies) is tackled. Indeed, a thorough literature revision shows that most authors do not have this key aspect into account. The relevance of the resulting error from comparisons between the near-field (NF) retrieved measurements versus plane-wave simulation results is unveiled. This is even more critical when the angular stability is analyzed, since the scattering patterns of the AMC are not conformed at NF distances, which makes the errors even greater. Consequently, noteworthy conclusions concerning the characterization of AMCs in particular and MTSs in general regarding FF or NF measurements and angular stability analysis will be presented. [ABSTRACT FROM AUTHOR]

This article presents a simple, cost-effective system for complex measurements of the field radiated by an antenna. In particular, the proposed architecture is based on an array of antenna probes connected to a set of phase difference detectors and a power detector. Phase difference measurements are processed to recover the phase of the antenna under test. The main novelty of the proposed system is that it does not require regular sampling, making it suitable for in situ antenna measurements. The influence of parameters, such as the number of antennas composing the array (and hence the number of phase difference detectors) and the positioning errors, is analyzed. The proposed architecture, implemented with off-the-shelf hardware, has been assessed for antenna measurement and diagnostics. A discussion of the issues related to the implementation of the system has been included, as well as the proposed solutions to improve its accuracy. Finally, diagnostics and radiation pattern measurement results are compared with those calculated with other phaseless techniques. [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 presents a technique for the acceleration of gradient-based algorithms that employ finite differences in the calculation of the gradient for the optimization of array antennas. It is based on differential contributions, which takes advantage of the fact that when an array is optimized, each element is analyzed independently of the rest. Thus, the computation of the gradient of the cost function, which is typically the most time-consuming operation of the algorithm, can be accelerated. A time cost study is presented and the technique is implemented, as an example, in the generalized intersection approach algorithm for array optimization in near and far fields. Several syntheses are performed to assess the improvement of this technique. In the far field, it is compared with periodic and aperiodic arrays using different approaches for the computation of the gradient, including the analytic derivative. A reflectarray is also optimized in the near field with the goal of improving its quiet zone. The technique of differential contributions shows the important reductions in the time per iteration in all three syntheses, especially in that of aperiodic arrays and near-field optimization, where the time saved in the evaluation of the gradient is greater than 99%. [ABSTRACT FROM AUTHOR]

A scalar calibration method for broadband off-axis holography phaseless antenna characterization and diagnostics is presented in this communication. With the proposed method, the need of vector calibration, inherent to off-axis holography setups, is bypassed by means of an intermediate step in which a well-known antenna, whose phase can be analytically modeled, is used to retrieve the phase of the reference antenna by applying the off-axis holography method twice. The method is especially suited for near-field characterization of high-gain millimeter- and submillimeter-wave antennas in which phase acquisition is a difficult task, requiring complex and expensive instrumentation and steady conditions. The phase of the field is retrieved at each point of the acquisition surface for a set of frequencies, and, consequently, the technique can be applied independently of the acquisition geometry. Validation examples are presented in the W-band. [ABSTRACT FROM AUTHOR]

In this paper, a method to estimate the attitude of an object using uniform arrays of radio frequency Identification (RFID) tags is presented. This technique is based on using the multiple signal classification algorithm with the phase measurements of the signals backscattered by the RFID tags of the array. Two array topologies have been studied (uniform linear arrays and uniform circular arrays). The performance of the system has been assessed by means of simulations and measurements in a controlled environment and in an indoor scenario. Results show that, after calibration, the attitude can be estimated with an error of a few degrees for a large range of orientations as long as the signals sent by the RFID reader do not imping the tags at grazing angles. [ABSTRACT FROM PUBLISHER]

A novel method to improve the angular stability of artificial magnetic conductors (AMCs) is presented. The study is carried out from the viewpoint of designers facing common design specifications (resonance frequency, bandwidth, and commercially available dielectric). AMCs resonating at 5.8 GHz are designed based on square loop and patch-shaped unit cells, introducing lumped components on a later stage aiming to improve the angular stability. Both an equivalent circuit model and electromagnetic simulations are used to provide a clear justification of the presented results. Moreover, some guidelines to choose the proper lumped components’ values, using analytical expressions and an AMC's equivalent circuit model, are provided. [ABSTRACT FROM AUTHOR]

This paper describes the characterization of reflectarray unit cells using support vector machines (SVMs) to obtain fast and accurately the full matrix of reflection coefficients, which is used for an analysis of dual-polarized reflectarrays, demonstrating the performance of the model. First, a surrogate model of the reflectarray unit cell is obtained using SVMs. To this end, a set of random samples of the reflection coefficient matrix with a full-wave method of moments based on local periodicity (MoM-LP) is used to train the SVMs. To efficiently obtain the surrogate model, a novel strategy to accelerate the training process is presented, remarkably reducing computing time. Next, the model is tested against a different set of samples, obtaining an excellent agreement between the SVM model and MoM-LP simulations for all reflection coefficients, including the cross-coefficients. The surrogate model is then used for an efficient analysis of three reflectarrays with pencil beam for point-to-point communications, isoflux pattern for global Earth coverage, and a shaped beam for local multipoint distribution service application, showing excellent agreement in both copolar and crosspolar patterns between the SVM and MoM-LP simulations. Finally, the analysis is accelerated by a factor larger than three orders of magnitude using SVMs instead of MoM-LP. [ABSTRACT FROM PUBLISHER]

A 2-D frequency scanning array operating in the 220–320-GHz frequency band is presented. The array is composed of 64 open-ended rectangular waveguides arranged in an 8\times 8 configuration. The feeding circuit comprises two waveguide-based phase shifting networks, which provides the required phase shift between adjacent radiating elements when the working frequency is modified. The array was manufactured over brass sheets through a mechanical procedure and experimentally characterized, providing beam steering along a 50 ^\circ \times 45 ^\circ space region while the working frequency is swept between 220 and 320 GHz, with a maximum −10-dB secondary lobe level and a measured 17-dBi gain at 275 GHz. [ABSTRACT FROM AUTHOR]

This paper describes a 2-D frequency scanning array, operating in the 240–310-GHz frequency range, which is intended to be used in electromagnetic imaging applications. The proposed antenna combines eight slotted waveguides that are parallel fed through a frequency-dependent phase shifting network. As the working frequency is swept in the considered range, the inherent frequency-dependent behavior of the slotted waveguide causes beam scanning in one plane, while the progressive phase shift introduced by the feeding network provides beam steering in the perpendicular direction. The influence of the particular negative effects that suffer the employed structures on the array performance is carefully analyzed to determine the viability of this approach. A prototype has been implemented and experimentally characterized. The manufactured antenna presents a measured 20-dBi minimum gain and it is able to scan a 60 ^\circ \times 20 ^\circ space region, which would be large enough for a great number of imaging applications. [ABSTRACT FROM AUTHOR]