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The occurrence of passive intermodulation (PIM) is inevitable in high-power microwave devices. It interferes with wireless communication and radar systems, resulting in severe deterioration of receiving performance. So far, narrowband PIM analyzers have been realized for different business bands. However, they are incapable of testing wideband systems. To solve this problem, rack-mounted analyzers piled with multiple narrowband analyzers were implemented. While such solutions significantly increase the volume, cost, and power consumption, they cannot provide a wide, continuous bandwidth for PIM localizations. Aiming to solve this problem, we propose a novel solution for realizing ultrawideband PIM analyzers. Based on the principle of cooperative radio frequency and baseband (RF-BB) nulling, we demonstrate an experimental prototype working within a continuous frequency band covering three octaves, exhibiting promising potential in PIM testing applications. [ABSTRACT FROM AUTHOR]

In this communication, we present a low-cost chipless radio frequency identification (RFID) tag with a high data capacity and a wide reading range based on time-domain scattering modulation. The proposed tag consists of a slotted square patch antenna with a switchable resistor and a mechanically driven slider etched with a barcode-like metallic wire array. Encoding is achieved by the presence and absence of the metallic wire on the slider, and tag identification is obtained by sequential bit reading. A special encoding strategy is applied to improve its insensitivity to the motion velocity of the slider. For a proof-of-concept demonstration, a prototype designed to operate at 2.1 GHz was fabricated and measured, and the experimental results verified the effectiveness. Compared with the methods based on time-domain reflectometry, spectral signature, and resonator-based sequential encoding, the proposed approach provides a substantially enhanced data capacity within a narrow spectrum without sacrificing its reading range and tag size, due to the design of nonresonant code cell. Such a mechanically triggered chipless RFID tag exhibits promising potentials in some practical applications, such as personal ID and credit cards. [ABSTRACT FROM AUTHOR]

Capable of remote presentations of human heart activities, the Doppler cardiogram (DCG) can be detected by an agile Doppler radar sensor. However, as a recently developed technology, its performance needs sustained improvements for the future practical applications. In this work, a continuous-wave, ${V}$ -band Doppler radar sensor was implemented to improve the discrimination of the receiver, for detecting micrometer-level details of heartbeat motions conducted to the chest skin. Besides, new algorithms were derived to achieve high linearity, to demodulate the Doppler phases and retrieve the fine details of DCG waveforms. Experimental measurements to different subjects showed significantly enhanced robustness and accuracy. The obtained results imply a promising potential of DCGs in wide application scenarios such as medical wisdom and personal healthcare. [ABSTRACT FROM AUTHOR]

Microwave imaging based on the inverse scattering problem has attracted many interests in microwave community. To obtain a satisfactory imaging quality, in most cases, a scattering-free environment is desired. In this work, to provide such an environment, we experimentally demonstrate a compact microwave imager integrated with a board-integrated, multiport vector network analyzer and a miniaturized, cylindrical anechoic chamber dedicatedly designed for inverse scattering-based microwave imaging. Based on the principle of electrically induced transparency and the effective medium equivalence, an inhomogeneous, ultrathin, dual-angle absorbing surface was designed, simulated, and implemented, to simultaneously absorb incident and scattered fields. Experimental imaging verified its effectiveness. The proposed imager would help the promotion of practical imaging, showing the potential in on-side imaging. [ABSTRACT FROM AUTHOR]

Recently, microwave imaging based on randomized illuminations has demonstrated promising potentials. The key to realize such imaging is to implement an imager with a reconfigurable, randomized aperture. In this work, we discuss the theory and the guidelines for implementing such an imager and demonstrate an experimental prototype working in the 24-GHz ISM band. It is realized with a board-integrated, space-fed, 400-element phased array capable of realizing independent 1-bit phase modulations for each element. The fields transmitting through this aperture can be deterministically randomized by simply controlling direct current bias voltages. Assisted by optimized system calibration and standard $\ell _{1}$ -norm regularization, experimental imaging with a notably improved quality can be obtained. The proposed solution can be used in wide applications, especially suitable for the massive personal security screenings in common public, government, and military areas. [ABSTRACT FROM AUTHOR]

In the past decades, continuous Doppler radar sensor-based biosignal detections have attracted many research interests. A typical example is the Doppler heartbeat detection. While significant progresses have been achieved, reliable, time-domain accurate demodulation of biosignals in the presence of unavoidable direct current (dc) offsets remains a technical challenge. Aiming to overcome this difficulty, we propose in this article a novel demodulation algorithm that does not need to trace and eliminate dynamic dc offsets based on approximating segmented arcs in a quadrature constellation of sampling data to directional chords. Assisted by the principal component analysis, such chords and their directions can be deterministically determined. Simulations and experimental validations showed full recovery of micrometer-level pendulum movements and strongly noised human heartbeats, verifying the effectiveness and accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

Combining the advantages of the phase tunability provided by active frequency selective surfaces (FSSs) and the beam-steering scheme used in conventional 1-D Fresnel-zone-plate (FZP) antennas, we report in this communication a 2-D beam-steering FZP antenna that needs only 180° phase tunability. Compared with transmit arrays, while the substrate layers are reduced by a half, a phase compensation strategy is used to enhance antenna specifications. To verify the proposed approach, a radome implemented based on an active, reconfigurable FSS was designed, simulated, and experimentally implemented. With a simple three-layer structure, the implemented FZP antenna works within an ultrawide bandwidth ranging from 5 to 6.3 GHz, corresponding to a 21% relative bandwidth. With a gain up to 19.3 dBi, the scanning range reaches ±45° in both horizontal and vertical planes. This cost-effective, simple-structured antenna can be potentially used in diverse RF and microwave applications. [ABSTRACT FROM AUTHOR]

Personal devices such as smartphones and tablets are rapidly becoming personal communication, information, and control centers. Apart from multitouch screens, human gestures are considered as a new interactive human–smart device interface. In this work, we propose a noncontact solution to implement hand gesture recognitions for smart devices. It is based on a continuous wave, time-division-multiplexing (TDM), single-input multiple-output (SIMO) Doppler radar sensor that can be realized by slightly modifying existing RF front ends of smart devices, and a machine-learning algorithm to recognize predefined gestures by classifying deterministic Doppler signals. An experimental setup emulating a smartphone-based radar sensor was implemented, and the experimental results verified the robustness and the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

To date, 3-D radar imaging of moving objects in a multipath coordinate space close to antennas remains a technical challenge. In this work, we demonstrate the implementation of such 3-D motion imaging based on a continuous-wave, multiple-input multiple-output (MIMO) Doppler radar sensor (DRS). Working with a time-division-multiplex (TDM) scheme, this portable, cost-effective DRS is a bistatic system with one transmitting and one receiving channels, and a radio frequency (RF) switch in each channel toggling between two antennas. Assisted by algorithms specifically derived for the TDM-MIMO architecture, highly sensitive 3-D imaging performed in a multipath indoor environment was experimentally implemented. Numerical simulations and experimental demonstrations verified the effectiveness and accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

Besides the prohibitive cost, complexity, and baseband scale, the development of millimeter-wave (mmWave) digital-beamforming fifth-generation (5G) frontends is facing a physical-layer challenge of thermal dissipation. This is caused by the massively integrated power amplifiers (PAs) and the extremely compact volume due to millimeter-scale wavelengths. In this work, we theoretically analyze and experimentally demonstrate a reconfigurable planar Fresnel lens, based on which a new-concept mmWave beamforming frontend can be implemented with significantly reduced number of PAs. It overcomes the inherent shortcomings of analog beamforming solutions relying on massive use of power dividers and phase shifters and can replace the convex lens used in the recently proposed hybrid beamforming solution. The implemented Ka-band lens is simple-structured, cost-effective, and easy to control, providing multiple modes for beamspace massive-multiple-input multiple-output (MIMO) communications. Experimental results verified its potential in different mmWave 5G systems such as base stations and point-to-point links. [ABSTRACT FROM AUTHOR]

Measurements of dielectric constant are of significant importance. Existing methods, contact or noncontact, require the sample under test (SUT) with a specified dimension. It can be the cross section of a rectangular waveguide or a regular-shaped reference SUT. In this work, a new approach extended from microwave imaging is proposed to remotely measure the dielectric constants of 2-D SUTs with arbitrary cross sections. An algorithm based on scattering integral equations is derived for an efficient parameter retrieval. Experimental measurements based on a time-division bistatic multiantenna system were conducted. It is verified that by deriving an inverse scattering problem-based algorithm, the dielectric constants of such SUTs can be effectively retrieved from the scattering data detected by a measurement setup previously designed for microwave imaging, regardless of the complexity of their cross sections. The proposed approach can be readily applied to various 2-D samples such as dielectric section bars. It can also be conveniently extended to 3-D samples in the future. [ABSTRACT FROM AUTHOR]

In this communication, we experimentally demonstrate a frequency- and beam-reconfigurable lens antenna based on an active second-order bandpass frequency-selective surface (FSS). The FSS lens is composed of three nonresonant metallic layers separated by two thin dielectric substrates. By integrating microwave varactors in FSS unit cells, the center frequency of this bandpass FSS can be tuned from 4.0 to 5.8 GHz by a direct-current (dc) bias voltage. Based on this wideband tunability, the phase-correction technique is used to implement a microwave Fresnel lens. The experimental results show that the proposed 1-D lens antenna can achieve frequency-reconfigurable beam steering within a 1 GHz bandwidth. With advantages of low cost and concise structure, the proposed reconfigurable lens antenna can be potentially used in a wide range of applications. [ABSTRACT FROM AUTHOR]

Induction logging through casing (ILTC) is of significant importance to the evaluation of residual oils. Traditional induction logging fails to work in cased wells due to the shielding effect of metallic casings. To date, the feasibility of ILTC remains an open question. In this article, we propose an ILTC based on the detection of lateral waves that carry the formation information. Based on the analytical equations derived to describe the hybrid modes of total magnetic fields that can be detected inside cased wells, numerical analyses aiming to describe the behavior of lateral waves were carried out in both frequency and time domains. The obtained results comply with full-wave simulations and existing scaled experiments. Our results indicate that if an ILTC tool can be properly configured to work in a region dominated by lateral waves, and the casing effect and the primary field of the lateral waves can be effectively compensated, it is feasible to implement an ILTC to detect formation conductivities ranging from 0.01 to 5 S/m. [ABSTRACT FROM AUTHOR]

Spatial Kramers–Kronig (KK) relations have been recently proposed to achieve perfect absorption without using any gain elements or highly anisotropic materials. However, such perfect absorption is limited to the surface of an infinitely long planar slab in the Cartesian coordinate, and the cases of surfaces with bends or sharp corners are still challenging. In this article, we apply conformal transformation optics methods to design spatial KK media, which can achieve smooth perfect absorption both in cylindrical spaces and in polygonal spaces with sharp corners. Moreover, it is demonstrated that we can also control the scattering direction of the designed polygonal spatial KK media to realize invisibility. Our work opens new avenues for perfect absorption devices based on transformation optics and spatial KK relations. [ABSTRACT FROM AUTHOR]

Most medical instruments invented to measure human heart activities, such as electrocardiograms (ECGs), rely on contact electrodes. This causes discomfort and limits application scenarios. Remote acquisition of $\mu \text{V}$ -level bioelectrical cardiac signals through ECG measurement is theoretically challenging. Based on the analysis of magnetic resonance imaging to the volume change of human hearts, we found that a single radar sensor can be used to remotely detect a Doppler cardiogram (DCG) at a distance up to 1 m, by retrieving Doppler signals induced by combined atrial and ventricular motions conducted to the skin of the back chest. This DCG can provide all the timing information of the P-wave, QRS-waves, and T-wave carried in ECGs. The implemented miniature remote sensor could be an ideal portable instrument for patients, such as burn victims, extending application scenarios to personal healthcare, battlefield rescue, and others. [ABSTRACT FROM AUTHOR]

The analytical description of dielectric multipactors requires an accurate physical model. In this article, a nonstationary statistical theory for the analysis of single-surface dielectric multipactors is established by defining an emission-to-emission transition rate and involving all the probability densities related to the emitted secondary electrons. Under the same conditions, the fluctuation of electrons in the evolution process of dielectric multipactors was calculated by the proposed theory and simulated by a commercial particle simulation tool developed based on the Monte Carlo analysis. The calculated results comply well with the simulated ones. Our results also indicated that to obtain an adequate accuracy, at least two random variables, the emission velocity and zenith angle, need to be considered. [ABSTRACT FROM AUTHOR]

In electromagnetics, anechoic chambers are used to obtain artificial analogs of free space. Such chambers are electrically large due to the inherent angle-dependent nature of absorbing materials. Existing models of perfectly matched layer (PML) imply that angle-independent reflectionless absorptions can hardly be achieved without involving gain media. In this paper, we mathematically derived an unconventional PML model, based on which the angle-dependent nature of absorbing materials can be manipulated to construct electrically small anechoic chambers. As an example, a semiclosed cylindrical chamber was experimentally implemented with an inhomogeneous artificial surface as thin as 0.012 wavelength. The inner diameter of this chamber is six wavelengths. The spatial voltage standing wave ratio (VSWR) experimentally measured inside this chamber complies with the theoretical analysis and full-wave simulation. With the aid of the dielectric 3-D printing technology, high-performance, microsized anechoic chambers can be constructed for scientific research and technology development in the future. [ABSTRACT FROM AUTHOR]

Recently, microwave imaging under random field illuminations has attracted many interests. This paper theoretically analyzes and experimentally implements a qualitative microwave radar imaging system suitable for 2-D object. It can be customized on the demands of the desired object size and imaging resolution. Based on random illuminations generated by a minimum number of antenna elements, an analytical equation can be used to reconstruct the image of the object in near real time. The simple-structured and cost-effective imaging system can be implemented in microwave, millimeter-wave, and even sub-THz frequency bands, having promising potentials for various microwave imaging applications. [ABSTRACT FROM AUTHOR]

In the past years, significant progresses have been achieved in signpost-based indoor navigation and positioning systems. In this paper, inspired by recent bionic researches on long-distance animal geomagnetic navigation, we propose an autonomous local navigation and positioning system, based on an artificially established magnetic gradient for in-grid or desktop applications. Without using any magnetic-field strength information, the proposed navigation and positioning can be accomplished by matching a spatial angle included by the direction of the vector magnetic field and a direction defined in the navigation plane using an algorithm derived based on the Kalman filter. Simulations and experiments verified the effectiveness of the proposed approach. This method can be used to improve the positioning precision of signpost-based systems. It can also be used in new applications of autonomous in-grid or desktop navigation and positioning for automatic robot arms. [ABSTRACT FROM AUTHOR]

Noncontact detection of human vital signs based on miniaturized Doppler radar systems (DRSs) can be widely used in healthcare and biomedical applications. Although significant progresses have been achieved, a reliable wireless vital signs detection in the presence of large-scale random human body movements remains a technical challenge. In this paper, based on a comprehensive use of the high-dynamic-range radar architecture and linearized Doppler phase demodulation algorithms, we further introduce matched filters to retrieve the respiration and heartbeat spectra completely concealed under the wideband noise floor caused by large-scale body movements. Along with an existing 5.8-GHz system, a single-board integrated DRS operating at 24 GHz was designed to verify the effectiveness of the used architecture and algorithms. Experimental results comply with the theoretical expectation. The obtained results imply the potential to implement practical bioradar systems for human noncontact vital signs detections. [ABSTRACT FROM AUTHOR]

Many radio frequency measurement systems require multicarrier signals with coherent phases between carriers with different frequencies. A typical example is the signal generator which is used in multicarrier multipactor (MM) testing systems. In this paper, we propose a new approach for the generation of coherent multicarrier signals based on a state-of-the-art phase-locked loops technique. Different from existing systems that rely on a heavy use of commercial microwave instruments, this generator can be implemented with on-shelf chips and devices. As an example, an ultrahigh frequency band generator is implemented and how to implement the in-system calibration of this generator in an experimental multipactor testing system is demonstrated. Based on the proposed approach, performing a quantitative experimental research on MM becomes affordable for an ordinary microwave laboratory, which may promote the in-depth experimental study in this area. It can also be used in a wide range of applications such as MIMO and phased array systems. [ABSTRACT FROM AUTHOR]

Passive intermodulation (PIM) potentially occurs in all high-power passive microwave devices. It can severely deteriorate the receiving performance of wireless communication and radar systems. To date, the simultaneous localization of multiple PIM sources in practical microwave devices is still a technical challenge. In this paper, we indicate that if a reference PIM source can be introduced into the standard PIM testing system, and the amplitude and frequency-dependent phase difference between the reference and the real PIM products can be used to construct a k-space multicarrier signal, PIM localization algorithms for both wideband and narrowband devices can be proposed based on the inverse k-space Fourier transform and inverse optimization of the partial multicarrier signal. Simulation and experimental results validate the effectiveness of the proposed approaches. These methods can solve the difficulty of multipoint PIM localization, and can be widely used in the design, production, testing, and troubleshooting of high-power microwave systems. [ABSTRACT FROM AUTHOR]