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This article presents a design for a very low-profile passive reflector device, which presents a large radar cross section (RCS) to forward looking automotive radar. With a total height of less than 0.25 mm, the reflector can be embedded in the paint of road markings, enabling the markings to be detected and identified with radar. This will allow radar to supplement several advanced driver assistance systems (ADAS) currently delegated only to camera sensors, and to improve reliability in poor-visibility and inclement weather conditions. The proposed reflector is designed for operation at the 77-GHz automotive radar band. Fabricated prototypes exhibit measured RCSs of about −13 dBsm, which is large enough to provide more than 20 dB of signal to background for typical underlying asphalt surfaces. It is also demonstrated that spatial patterns of several reflectors can be resolved by radar and used to encode different markings. Finally, with the goal of advising future development, a study of the effects of dielectric materials and water on reflector performance is conducted.

This article reports on polarimetric radar backscatter responses of different types of surfaces at high angles of incidence (80°–89°) at J-band frequencies (221–231 GHz). This study is motivated by the desire to understand the radar phenomenology of target scenes for the next-generation automotive radars. Measured data were collected for paved and unpaved surfaces under different conditions (dry, wet, ice- and snow-covered). At these frequencies, dry road surfaces are electrically rough and sizes of the subsurface aggregates are comparable to the signal wavelength giving rise to significant surface and volume scattering. The measured backscattering coefficients of all surfaces exhibit over 30 dB dynamic range across all datasets and significant depolarization (between −12 and −4 dB). For paved surfaces, the dynamic range narrows to 16 dB and the depolarization ranges between −9 and −4 dB. The data represent the first reported observations of radar response of surfaces at high incident angles at J-band frequencies. They demonstrate the radar’s potential for discriminating between different types of road surfaces needed for improved safety of driverless cars. Also, based on the measured data, a semi-empirical model for the backscattering coefficients of the paved surfaces at high incident angles is presented.

Maintaining calibration of phased array antennas in situ is a challenging problem that is still considered open. A critical step in the array calibration process is characterizing the channel imbalances between elements, so that they can be equalized. In this article, we propose a novel method for monitoring channel imbalances at elements in such arrays mounted on flying platforms using the backscatter from distributed targets within the field of view. The motion of the platform can be used to measure the backscatter of the scene in multiple positions. Synthetic aperture radar (SAR) images can be formed from these measurements using a common transmitter/receiver module and a paired receiver/transmitter module. The individual corresponding pixels of the coregistered images can be used to determine the amplitude and phase variations (channel imbalances) across the array elements. The measured phase difference between the backscatter from two coregistered images can be averaged over all pixels to provide a much better estimate of the array channel imbalances in the presence of noise. Changes in channel imbalances can be monitored in situ without the need for signals-of-opportunity or strategically placed calibration targets. This could be used in the development of more elaborate array calibration algorithms, or to maintain a factory calibration during flight. The proposed method is validated under controlled conditions by forming images of a random rough surface with a two-antenna system. Average channel imbalances in amplitude and phase over frequency are recovered within a small fraction of a dB and a fraction of a degree, respectively.

A coplanar waveguide (CPW) composite resonator for dielectric characterization of thin films at ${W}$ -band is presented in this article. The sample holder is fabricated using gold evaporation on a fused silica wafer to minimize the resonator losses. The composite resonator is composed of two inductively coupled short-circuited CPW resonators to improve the quality factor of the sample holder. The technique is suitable for complex dielectric characterization of very thin materials (a few hundreds of micrometers), such as vehicular paint due to the high sensitivity of the CPW resonator to the superstrate material. An inversion algorithm is developed to calculate the real part of the sample’s permittivity from the shift in the resonant frequency and the imaginary part from the change in the quality factor of the composite resonator after being loaded by a thin film of the unknown material. The CPW sample holder is fed by 1-mm end launchers to simplify the measurement process and overcome the complexities associated with probe station measurements. The proposed technique is validated through measurements of known Rogers substrates, polycarbonate, parylene-C, and SU-8 polymer. Then, it is used to measure and report few vehicular paint samples and clearcoats.

In this letter, a novel 1-bit transmit- and reflect-array (TRA) antenna is presented for B5G beam scanning applications. Each unit includes a patch-ground-patch layered structure with two embedded PIN diodes and delay lines to serve as binary phase shifters. By properly loading the positions of the two PIN diodes, the sandwiched structure can not only operate in receive-transmit mode, but also work in receive-reflect mode, which greatly increases the beam scanning coverage. All possible four states of the two diodes are fully utilized to allow for 1-bit reflection phase shifter and 1-bit transmission phase shifter. A 12 × 12-elements TRA is fabricated. A ±50° beam scanning range is obtained in reflection and transmission modes. The measured broadside gain of reflection and transmission modes is 19.3 and 19.5 dBi, corresponding to an aperture efficiency of 19.6% and 20.5%.

Directivity enhancement for a conducting cylinder coated with a uniaxial dielectric layer and excited with an electric current source is investigated. A detailed investigation is conducted for all source polarizations. A distinct contribution of the current work relative to the literature is the realization of extremely high directivity corresponding to space- and leaky-wave radiations for cylindrical structures. The computed space- and leaky-wave radiation patterns are compared with simulations using two simulation tools. The creeping-wave characteristics for the configuration are also investigated.

In this communication, a 1-bit reconfigurable reflectarray antenna (RRA) providing high aperture efficiency is presented for beam scanning applications. Each element is configured as an aperture-coupled patch antenna composed of a radiating patch, a slotted ground for feed of the patch, a microstrip delay line, and a p-i-n diode switch. Unlike conventional methods, the p-i-n diode switch is not placed on the microstrip line but instead is inserted strategically at the center of the coupling slot on the ground layer. In this novel configuration, the elements of the RRA do not endure the loss through the p-i-n diode switches. The loss associated with the p-i-n diode in the proposed configuration is very small since only a small part of the RF currents flows through the diode in the ON state. Two phase states with a phase difference of 180°±20° are realized from 4.9 to 5.7 GHz. The reflection loss of each element is limited to less than 0.1 dB in both the ON and OFF states. To demonstrate the feasibility and performances of the proposed approach, a $14\times14$ -elements RRA is fabricated and measured. The measured aperture efficiency is above 18.8% in the entire band of 4.9–5.7 GHz, with a peak value of 36.5%. Furthermore, the efficiency stays almost unchanged, even when the equivalent resistor of the p-i-n diode varies from 0.9 to $9 \Omega $ . This is used to lower the bias forward currents of the p-i-n diodes to lower the power consumption to merely 1.9 mW for the 196-elements RRA.

In this article, a dual-polarized 2-bit electronically reconfigurable reflectarray (RRA) is proposed for 5G beam scanning applications. By introducing two parallel transmission lines and a T-junction, only two p-i-n diodes are used to generate 2-bit phase quantization is analyzed and verified. Based on this concept, a novel 2-bit dual-polarized RRA element is presented that makes use of an aperture-coupled patch and microstrip delay lines behind its ground plane. Only two p-i-n diodes are loaded on each delay line to realize the 2-bit phase quantization. Two orthogonal linear polarizations are independently controlled by deploying two perpendicular coupling slots and two delay lines. A $16\times16$ RRA prototype is fabricated and tested at 3.6 GHz. The measurement and simulation results show good agreement. Both polarizations can provide a ±60° 2-D beam scanning range with a maximum scanning gain drop of 2.75 dB. The maximum measured gain is 23.11 dBi, which corresponds to an aperture efficiency of 40.56%. A 1-dB measured gain bandwidth of 8.06% is also achieved.

Satellite and airborne radars currently monitor agricultural regions on Earth. Corn is a globally important crop that may benefit from radar observations for estimating soil moisture (SM) and other quantities. Estimates of SM could be used to enhance crop yield and aid in weather prediction. A scattering model is needed, however, to accurately estimate these quantities. Historically, corn is a difficult crop to model at microwave frequencies, and only approximate models for it exist. Novel models based on full-wave electromagnetic solvers can be more accurate by accounting for multiple scattering among plant constituents, other adjacent plants, and the underlying soil surface. Such a model is computationally expensive, but the increased availability of computing resources may make it more feasible. This article presents a model for corn at L-band based on finite element method (FEM) simulations in conjunction with Monte Carlo methods to estimate polarimetric backscattering coefficients. The FEM simulation uses periodic boundary conditions to limit its size. The physical representation of the corn plants comes from data-based 3-D plant models. The results of simulations are validated with synthetic aperture radar (SAR) data obtained during the SM active passive validation experiment of 2012 (SMAPVEX12) experimental campaign. The estimated backscattering coefficients of the SAR data are within ±2 dB for all polarization channels. Validation is performed for two days within the experimental campaign. Good agreement is observed between the simulated and measured values. This result indicates that the model can give novel insights into the scattering characteristics of corn. Future work remains to build an invertible model for estimating SM from backscatter measurements.

This article presents a model for the statistics of field fluctuations, phase aberration, and field spatial covariance of electromagnetic waves propagating in the rain at millimeter-wave frequencies as a function of range and rain rate. This statistical model is derived based on a full-wave S-matrix approach and using the fact that the fluctuating component of the field is a Gaussian process. The model is useful in the performance evaluation of coherent communication systems and imaging radars, such as multiple-input multiple-output (MIMO) and synthetic aperture (SAR) systems. The mean field component is represented by a field propagating in a medium with an effective refractive index characterized by the S-matrix approach. It is shown that the statistical behavior of the magnitude and phase of the fluctuating field at any point on the wavefront with respect to a reference point on the wavefront can be described by only one parameter. The statistics of the magnitude of the fluctuation field at the reference point can also be described by one more parameter. In this article, the effective refractive index of the medium for estimating the mean field and the two required parameters for describing the fluctuating field at a given point and its covariance with that of an adjacent point are given for different rain rates in the range 2–50 mm/h and at 77 GHz. Simple algebraic expressions for the parameters for describing the fluctuating field and its covariance are provided that are valid for propagation ranges of up to 500 m and the separation distance (between any point on the wavefront and the reference point) of up to eight wavelengths.

In this work, the Green’s function for a dipole in a stratified uniaxial anisotropic spherical configuration is evaluated, with the dipole in the anisotropic layer. The particular part of the Green’s function is computed by matching the electromagnetic fields at the source in the anisotropic layer. The homogeneous part of the Green’s functions is found by imposing boundary conditions at the anisotropic and isotropic layer interfaces. The source-free resonant frequencies of the transverse electric (TE) and transverse magnetic (TM) modes for the antenna configuration are also evaluated. The input impedance and fields radiated by the dipole in the multilayer anisotropic sphere are computed. The technique is also used for the evaluation of the Green’s function, input impedance, and radiated fields for a dipole located in the vicinity of the stratified uniaxial anisotropic sphere. The variation of input impedance, reflection coefficient, and excited modes with change in anisotropy is thoroughly investigated for both the antenna configurations. The excited modes in both the configurations are also characterized in terms of their quality factor, bandwidth, gain, and radiation efficiency. The radiation characteristics with change in anisotropy are also evaluated.

The problem of detecting concealed weapons with a low-cost 3-D imaging radar system is addressed. The concept of affine phase center translations to generate a synthetic array is proposed as an alternative to conventional synthetic aperture and multistatic imaging radars. This enables the generation of a synthetic 2-D array with a dynamic dual reflector antenna system and a single transceiver. The dual reflector antenna is composed of a 3-D printed ellipsoidal sub-reflector and a conic main reflector. The phase front of the horn feed is shown to be translated to different locations by controlling the relative positions of the two reflectors. The reflectors are fabricated using a simple protocol that keeps the cost very low but introduces significant phase aberrations. These phase aberrations are corrected by developing a novel geometrical optics based optimized calibration routine. The imaging system is tested using a 76.5–84.5 GHz stepped frequency transceiver in different concealed weapons detection scenarios. As a demonstration, a metallic handgun and a nonmetallic pipe-bomb are concealed on a mannequin which serves as a human surrogate with similar skin reflectivity. At a target distance of 2.8 m the achieved depth resolution and lateral resolution are 2 and 1.5 cm, respectively. The 3-D images constructed using back-projection are of high enough quality to successfully identify the weapons while maintaining the privacy of the individual.

In this article, the design and fabrication of a 230-GHz planar and low-mass reflectionless lens antenna are presented. Phase compensation for the spherical phase front of the feed to a planar one emerging from the other side of the lens is accomplished locally using a three-layer spatially varying miniaturized-element frequency-selective surface (MEFSS) structure. The MEFSS structure is designed to present two-pole bandpass filters with different center frequencies across the surface of the lens. These filters make use of only shunt equivalent circuit elements and their substrates' thickness as impedance inverters. The very small feature size of the metallic components that constitute the unit elements of the MEFSS structures necessitates the development of a special and accurate (within $\pm 1\; \mu$m tolerance across the extent of the lens) multilayer microfabrication process. This article presents the fabrication process and performance assessment of a 10-cm-diameter lens with $f/D=1.4$ operating at 230 GHz. This lens is fabricated using two thin ($\text{193}\; \mu$m) low-loss prime fused silica wafers on which fine MEFSS structures are patterned. The provision of shallow groves for the metallic patterns on one face of one of the wafers allows for the ease of wafer-to-wafer bonding. The fabricated lens shows a measured beamwidth of $\text{0.88}^{\circ }$ and a high gain of 44 dBi, while at the same time providing a very low surface reflection coefficient of $-$31 dB (at its center frequency) measured at the feed's terminal. The average reflection coefficient over the desired band is 10 dB lower than that of a conventional dielectric lens of the same $f/D$ and diameter. This lens that weighs only 7 g and has a total thickness of $\text{387}\; \mu$m is a key component of a dual-polarized common aperture radar designed for the phenomenological study of traffic scenes.

This communication describes the design of a planar spatial power divider structure, which splits the energy of a wave incident at 45° to its surface into a transmitted component and an orthogonally directed reflected component. The reflection and transmission coefficients for both TE and TM incident waves are designed to be approximately equal, hence the power divider (combiner) is polarization-independent. The power divider is designed using a combination of all-dielectric metamaterial techniques and concepts of miniaturized-element frequency selective surfaces (MEFSS). The design is facilitated by a circuit modeling approach supplemented with full wave electromagnetic simulations in HFSS. It is fabricated on a silicon wafer using standard microfabrication technology. The measured device exhibits at most 0.9 dB of difference between reflection and transmission for both polarizations over a band from 76 to 84 GHz. This spatial power divider has been integrated into the design of a common-aperture transceiver antenna system in a fully polarimetric E-Band radar.

Robust, secure, low-power wireless communications and networking are needed to support emerging applications in emergency and disaster response, robotics, autonomous vehicles, and the Internet of Things (IoT) in physically complex environments, such as indoor/outdoor, dense urban, and other challenging scenarios. These applications require a variety of wireless communications modalities, including high and very-high frequency (HF and VHF) bands. To effectively deploy compact wireless communications systems at these low frequencies, high-performance electrically small antennas (ESAs) are needed. In this article, we present a survey of state-of-the-art low-frequency ESAs, including passive resonant, tunable, multiband, and actively matched designs. While passive designs are subject to classic tradeoffs in size and efficiency, active and platform-integrated techniques offer potential performance-enhancing alternatives. And, though single ESAs tend to omnidirectional beam patterns, multiantenna arrays can achieve directionality, and designs include parasitic, biomimetic, and distributed approaches.

This article proposes a device for real-time measurement of the complex dielectric constant of liquids at $X$ -band. The device consists of a narrow waveguide channel operating at the cut-off frequency to achieve a large field enhancement based on epsilon-near-zero (ENZ) supercoupling theory. A micro-3-D-printed sample holder is placed inside the channel and connected to feeding tubes that are designed to prevent energy leakage from the channel. The field enhancement and confinement inside the channel create a strong interaction with the liquids under test, thereby providing very sensitive measurements over a wide range of permittivities (approximately 1–65). Two variations of the device are fabricated. One that provides a higher accuracy and another that provides a higher sensitivity. A full-wave simulation-based forward model is developed and optimized using a constrained Nelder-Mead simplex. An inversion is performed based on the optimized forward model to find the complex permittivity from the measured scattering parameters. Two sets of experiments are performed, one of the static liquid mixtures and another of time-varying liquid mixtures. The static measurements are done with binary mixtures of ethanol-water and methanol-water. The time-varying measurements are done with ethanol-water-sugar mixtures and ethanol-water mixtures. The measured results are consistent between the two devices and are in very good agreement with the published literature over the entire range of dielectric constants tested.

A mechanically scanned high-resolution 2-D imaging radar system with full 360° field of view (FoV) is presented. The system is composed of a multifunction 222–228-GHz frequency-modulated continuous-wave (FMCW) radar and a moment of inertia optimized 3-D-printed offset reflector antenna. The reflector geometry is made from an elliptic toroidal surface which is shown to produce a high-gain fan beam with an 11:1 aspect ratio. The reflector is rotated at speeds up to 10 Hz for high-throughput imaging. The wobble generated by the high-speed rotation is suppressed by optimizing the mass distribution of the reflector body. It is fabricated in-house using a low-cost table-top fused deposition modeling (FDM) 3-D printer. The reflecting surface is metallized with a silver-coated copper paint and processed using simple steps to ensure smoothness and accuracy. The reflector achieves beamwidths of 4.6° in elevation and 0.42° in azimuth, with a peak gain of 38.7 dBi. The 3-dB azimuthal resolution of the radar two-way beam is measured to be 0.3°. The system is tested in real-world scenarios, and the resulting images and videos are of unprecedented quality due to the large area of coverage, high resolution, high speed, and excellent dynamic range. To the best of our knowledge, this is the first demonstration of a compact radar system that is comparable in resolution, speed, and angular coverage to lidars, while offering higher dynamic range and immunity to severe weather and lighting conditions.

In this article, the merits of impedance matching a small antenna to the receiver’s impedance are investigated. The goal is to demonstrate that a small antenna with low radiation resistance and high input reactance, when directly connected to a high-impedance (high-impd) receiver, can provide significantly better signal-to-noise ratio (SNR) than when it is matched to a standard 50 $\Omega $ input-impedance receiver. Theoretical analysis is first carried out to compare the noise performance of the two receivers by deriving analytical expressions for the SNRs of the receivers. It is shown that the SNR of a high-impd receiver is superior to that of an impedance-matched receiver. Subsequently, two test receivers are constructed and characterized to verify the theoretical calculations. The measured SNR of the high-impd receiver is 11.4 dB higher than that of the matched receiver, even though the first-stage amplifier used in the high-impd receiver has a much worse noise performance compared to that used in the matched receiver. This study demonstrates that matching a receiving antenna to the load impedance is not always the optimal solution as far as the SNR is concerned. Furthermore, unlike a small antenna with a narrow-band matching circuit, a small antenna connected to a high-impd receiver without any matching network provides a very wide bandwidth and overcomes the gain-bandwidth limitation of small antennas. This discovery breaks new ground in the realization of compact wideband systems for low-frequency communications.

Detection and localization of buried pipelines is an important problem for oil and water distribution industries as well as for excavation and construction companies. A novel 3-D forward scattering model and dynamic grid search-based inversion algorithm of multiple buried pipelines using multistatic synthetic aperture radar (SAR) configuration is demonstrated in this article. Traditional imaging algorithms that focus the SAR signal by compensating only for the phase work well when the targets are far away from each other. By considering both the phase and amplitude of the scattered field and casting the inversion problem as an optimization problem, the detection and localization resolutions are improved for extended targets. A forward scattering model for multiple adjacent pipes is developed to track the scattering phase centers (SPCs) of each pipeline as the receiver location is changed. The model is used to calculate the resulting propagation phase and amplitude attenuation due to propagation in lossy media. The misfit between the measured data and the forward model (FM) is used as the cost function. The cost function is minimized by the dynamic grid search algorithm that starts with an initial focus and then performs a more careful search to detect closely spaced pipelines. The FM and inversion algorithm are validated and compared with a basic focusing algorithm using full-wave simulations as well as experimental data for both metallic as well as dielectric pipes. The results show improvement in range and cross-range detection and localization resolutions over the basic SAR focusing algorithm.

The angular resolution of an imaging radar system is limited by the aperture size and its associated cost and complexity. Relaxing the $\lambda /2$ element spacing condition for large arrays introduces sidelobe and grating lobe effects that severely degrade the image quality. In certain imaging applications, such as automotive radars, the targets are sparsely located and the imaging domain can be approximated as a collection of point scatterers. In such cases, it is possible to “deconvolve” these sidelobe effects to recover a clean image. In this article, a double-recursive deconvolution algorithm titled Sparse Array Target-Segregation Using Recursive Nulling (SATURN) is proposed. It differs from existing CLEAN deconvolution algorithms in two steps: target response estimation and multiple target decorrelation. The target response is estimated using a Sweep and Extinguish step that removes the reliance on the complex image phase. Correlation between targets is suppressed using a Recursive Nulling step that prevents the breakup of point targets. The algorithm is applied to synthetic aperture radars (SARs) and multiple-input multiple-output (MIMO) radars in sparse 3-D imaging scenarios with canonical targets and real-world targets. Dynamic range improvement of 25 dB and thinning factors of over 200 are experimentally demonstrated using a 31-element circular array of X-band U-slot patch antennas with an array diameter of $30\lambda $ .

Driven by the perpetual demand for higher data rates, extensive research efforts have been devoted to millimeter-wave (mm-wave) 5G technologies in recent years. Highly integrated mm-wave antennas for 5G user equipment are considered one of the key components in the realization of mm-wave-based mobile networks. An accurate characterization of antennas is as important as antenna design. Despite great advances in measurement capabilities over the years, we are still facing many difficulties in probe-based mm-wave antenna measurements. This article presents a fast, simple, and accurate near-field (NF)-to-far-field (FF) technique for gain and radiation pattern measurements of probe-fed mm-wave antennas. A setup based on an electro-optical (EO) NF measurement system (NeoScan) is utilized to measure the NF of the antenna under test (AUT). The system uses a high-spatial-resolution nonintrusive probe to scan the NF in very close proximity to the antenna’s surface, which significantly alleviates multipath effects and reduces the required scan area. The radiation pattern of the AUT is computed using the NF-to-FF transformation. To obtain the gain of the AUT, a ground-backed dipole antenna is created and used as a gain standard. The absolute gain of the ground-backed dipole is characterized using the image method. Measurement accuracy is checked against the simulation. With the help of the standard gain antenna, the NF-to-FF method can be used to measure both the gain and radiation pattern of any type of probe-fed mm-wave antenna. For demonstration purposes, two test antennas, a horizontally polarized tapered slot antenna, and a dual-band vertically polarized hexagonal bridge antenna operating at 28 GHz are fabricated and characterized. Excellent agreement against full-wave simulation is achieved.

All existing state-of-the-art high-resolution millimeter-wave imaging systems experience a tradeoff between image acquisition time and transceiver array complexity. The proposed dual-reflector antenna breaks this tradeoff by drastically reducing the array formation time while maintaining the relative simplicity that comes with using a single transceiver element. It consists of a dual-mode horn feed, a rotating ellipsoidal subreflector, and a conic main reflector. The rotating subreflector creates a virtual phase center that rotates about an axis to produce a synthetic circular array with a diameter of $120\lambda $ . The main reflector redirects the beams from each of these virtual phase centers to overlap and illuminate the scene over a wide field of view (FOV). Two cases are simulated, one with a 14° FOV and another with 25° FOV in both azimuth and elevation. The former case is experimentally verified and achieves a measured half-power beamwidth of 0.4° over the entire scan range. The proposed system can reduce the image acquisition time to the order of milliseconds/seconds, which makes real-time SAR imaging a practical alternative to multiple-input–multiple-output (MIMO) and phased arrays at millimeter-wave and sub-millimeter-wave frequencies.

Electromagnetic (EM) radiation from Staphylococcus aureus (S. aureus) biofilms has been recently detected in the 3.16 – 3.2 GHz frequency range. In this work, a regenerative RF sensing system is designed and implemented to enhance the detection of signals generated by biofilms. The system utilizes the concept of regeneration (also known as positive feedback) for the amplification of emitted signals at a desired frequency (3.18 GHz). Comparative experiments were conducted, where the power measured from peptone-NaCl-glucose (PNG) media with biofilms (biofilm samples) was compared with that measured from fresh PNG media void of biofilms (PNG samples). The measured power level differences between the biofilm samples and the PNG samples are many orders of magnitude higher than what was previously reported using the wideband near-zone radiative system. Furthermore, illumination experiments were conducted to examine biofilms’ response to external EM stimuli. A sinusoidal signal at the emission frequency of 3.18 GHz was used to illuminate the biofilms. It is shown that biofilm samples exhibited stronger radiation behavior after being exposed to the external 3.18 GHz signal. For comparison, a similar setup was constructed at a different frequency (6.3 GHz), and the same experiments were carried out. No radiation is identified in the other band for both illuminated and unilluminated biofilm samples, which is consistent with the results reported in previous studies. This work not only confirms the existence of EM radiation generated by biofilms, but also demonstrates that cells actively respond to external EM signals at the same frequency as the signal they generate. This discovery is a major step toward understanding cell communications.

This paper reports the first successful detection of electromagnetic (EM) radiation from Staphylococcus aureus biofilms in the gigahertz (GHz) frequency range. Two novel sensing systems are deployed for the measurement. A very sensitive wideband near-zone radiative system specifically designed for this application is first used to search for signals in the 1–50 GHz frequency region. Notable radiation is observed in the 3–4 GHz band. Exposure to lethal doses of Zinc oxide nanopyramids (ZnO-NPY) is used to verify that the signals are indeed produced by living cells rather than material thermal emission. Afterwards, a spiral antenna system is exploited to further examine the band of interest in the near-field region. Radiation from 3 identical biofilm samples is monitored and recorded over 70 days. Two distinct frequency bands, namely the 3.18 GHz and the 3.45 GHz bands, are identified as potential “communication bands”. Furthermore, long-term and short-term cycles of the total radiation intensity within the band are observed over the course of the experiment. This work confirms the presence of EM radiation within bacterial communities, which is a key requirement to demonstrate EM signaling among bacterial cells. The insight could lead to breakthroughs in demystifying how cells communicate as well as advancement of important technologies in biology and communication systems.

Millimeter-wave (MMW) radar sensors are considered key components of autonomous vehicles. Because of the performance degeneration of cameras and lidars under inclement weather conditions, robust autonomy must rely on radar sensors to perform target detection and classification. Unlike basic target classifier methods in literature that make use of target velocity, the proposed approach is far more comprehensive and can be applied to targets with zero-Doppler. Depending on the radar type and target range, this paper presents four target classification models based on four different types of radar data: statistical RCS, distributed (time-domain) RCS, range-azimuth angle radar images and 3D radar images. The classification models are implemented by machine learning approaches artificial neural network (ANN) and convolutional neural network (CNN) with a comprehensive simulated dataset. Good classification accuracies are demonstrated, and the proposed model is validated with measured data. Different radar target classification approaches are compared, which clearly reveals the trade-off between classification performance and system complexity. The proposed radar target classification methods can be applied effectively to both static and dynamic targets, at near or far ranges, using traditional or imaging radars, resulting in improved safety for autonomous vehicles in a wide variety of complex environments.

An antenna system is proposed that uses a novel polarization-independent spatial power divider to align the beams from two orthogonally oriented dual-polarized feeds. This antenna system uses a common aperture for transmitting and receiving and provides high port isolation while maintaining compatibility with fully polarimetric radar. Unlike conventional two antenna radar systems, a radar using this antenna does not suffer from the parallax problem. The spatial power divider is designed using an all-dielectric metasurface approach and is fabricated on a silicon wafer using standard micromachining technology. The measured device exhibits at most 0.9 dB of difference between the reflection and transmission for both transverse electric and transverse magnetic incident waves at 45° incidence, over a band from 221 to 250 GHz. Conical horns are fabricated as feeds and together with the spatial power divider, a focusing lens, and microwave absorbers, are assembled in a three-dimensional-printed housing to complete the antenna system. Its measured gain is approximately 39 dB for each port and polarization, and isolation between ports is at least 40 dB for all polarization combinations over the band of interest.

This paper describes design and development of a novel frequency beam-scanning array antenna operating at Y-band (220-325 GHz). The proposed antenna is a traveling-wave antenna, with a meandered rectangular waveguide structure as the propagating medium. The array generates a very narrow beamwidth of about 2.5° with ±25° beam steering capability as the frequency sweeps from 230 to 245 GHz. The antenna beam in elevation is about 10°, which is generated with a slot-coupled cavity-backed patch array antenna along the elevation direction. The proposed antenna provides an overall gain of over 29 dBi and exhibits a radiation efficiency of more than 55% over the entire frequency range of operation. The antenna topology is designed to present a very low-profile structure to meet a very low volume and low mass requirements. The antenna consists of over 600 patch elements and is confined within a volume of 45 mm × 8.5 mm × 1.25 mm and weighs less than 4.5 g. A prototype of the antenna is fabricated using silicon micromachining with a high level of accuracy and is characterized using a special near-field measurement setup. The antenna exhibits a measured scanning range of over 48° with a gain of over 28.5 dBi, all of which are in close agreements with simulation results.

An analog phase shifter operating at 240 GHz and implemented within a rectangular waveguide is presented in this paper. The phase shifter relies on mechanically actuating a tunable perfect magnetic conductor (PMC) inside an upright rectangular waveguide supporting a modified TE10 mode. The PMC boundary condition is realized using a metallic strip suspended slightly above the short wall of the rectangular waveguide. It is shown analytically and numerically that vertically actuating the metallic strip changes the dispersion relation of the modified TE10 mode leading to a controllable phase shift. The proposed structure is designed using full-wave simulations and optimized for minimal losses. It is then fabricated using standard silicon microfabrication techniques including photolithography, deep reactive ion etching, and metal sputtering. The actuation is done through an external piezoelectric actuator that is assembled with the microfabricated structure to form a single unit. The phase shifter shows a measured phase shift of 380° with worst case insertion loss of 3 dB leading to an excellent figure of merit (FoM) of 127°/dB. To the best of our knowledge, this is the best reported FoM for all passive phase shifters operating beyond 60 GHz.

A numerical integration of the Sommerfeld integral is performed using the Schelkunoff formulation for cylindrical media. The Schelkunoff kernel for cylindrical media involves higher order modified Bessel functions with azimuthal summation over higher order modes. As such, the convergence characteristics of the cylindrical integral kernel are strongly dependent on complex linear combinations of higher order Bessel/Hankel/modified Bessel functions, compared to the case of the planar media where only a single Bessel/modified Bessel function of zeroth order is present. Two cylindrical configurations are analyzed using the new formulation, viz. a conducting cylinder and a dielectric‐coated conducting cylinder. The branch‐point singularity in the first configuration is removed using the angular transformation for the Sommerfeld/Schelkunoff formulations. A path deformation technique is used for the second configuration to address the problem of poles and branch‐point singularities on the real axis of integration. The in‐depth analysis of the cylindrical kernels and the integrals with variation in the location of the observation point clearly bring out the relative merits of both formulations for the cylindrical configurations, with the TE/TM coupling for the coated cylinder considered. The Schelkunoff kernel for cylindrical media involves higher order modified Bessel functions with higher order azimuthal modal summationThe Sommerfeld integral converges faster than Schelkunoff integral for large radial separation between source and field pointsThe Schelkunoff integral converges faster compared to Sommerfeld integral for large axial separation between source and field points The Schelkunoff kernel for cylindrical media involves higher order modified Bessel functions with higher order azimuthal modal summation The Sommerfeld integral converges faster than Schelkunoff integral for large radial separation between source and field points The Schelkunoff integral converges faster compared to Sommerfeld integral for large axial separation between source and field points

This paper reports on complex permittivity values of different natural and synthetic materials at J-band (220-325 GHz). The materials considered in this study include different types of fabrics (wool, polyester, acrylic, jeans, vinyl, and cow leather), in addition to human skin. For the wool and jeans materials, we consider both dry and wet conditions. The knowledge of dielectric constant of these materials is needed for evaluation of radar based concealed object detection methods. The measurement setup used consists mainly of a vector network analyzer (VNA) combined with two -band frequency extenders. A free space transmission only measurement technique is used to obtain both the complex dielectric constant and the effective thickness of all the fabric materials. Such approach is adequate for fabric materials as they do not have a well-defined thickness. The major factors affecting the uncertainty in the measurements are shown to be related to the sample thickness inhomogeneity, and to the unrepeatability of the network analyzer measurement itself. For the human skin case, a reflection measurement method from a terminated waveguide is adapted.

This paper presents a non-contact on-wafer $S$- parameter measurement method for submillimeter-wave and terahertz frequency range. The proposed method is based on using open-ended waveguide probes along with on-wafer waveguide transitions to measure the $S$ -parameters of waveguide based components and devices. To enable non-contact measurements, an RF choke is designed and machined on the metallic cross section of the probes using electric discharge machining. Additionally, to enhance the accuracy and repeatability of the measurements, a probe aligner is micromachined over the on-wafer transition. In order to validate the measurement concept, a full-band transition operating at $J$-band (220–325 GHz) is designed and tested. To achieve high accuracy, the fabrication of the on-wafer waveguide and the transition is performed using silicon micromachining. It is shown that the designed back-to-back transition has a return loss of better than 15 dB and an insertion loss of less than 0.2 dB over the entire frequency band. The measurement results of the fabricated transition also show a good agreement with the simulated results.

This paper presents a novel non-contact broadband on-wafer S-parameter measurement technique for submillimeter-wave and terahertz applications. The proposed method enables S-parameters measurement of multi-port micromachined devices using a two-port measurement setup. In this method, a small fraction of the signal at each waveguide port is weakly coupled to free space using a small array of reflection-canceling slots which are then measured by an open-ended waveguide probe. The S-parameters of the device-under-test are calculated from the measured signals obtained from each port and from that of a reference match waveguide. A broadband waveguide slot array antenna with high return loss is designed as a matched load to terminate all ports of the device except the input port. To assess the reliability and accuracy of the proposed measurement method, three different micromachined waveguide directional couplers are designed, fabricated, and tested. Multiple apertures on the common wall between the adjacent waveguides are designed and optimized to achieve high directivity couplers over a broad frequency range. The measured S-parameters of the couplers are shown to be in good agreement with the simulations, which indicates the accuracy of the proposed measurement method.

This paper presents a broadband fully micromachined transition from rectangular waveguide to cavity-backed CPW line for submillimeter-wave (sub-MMW) and terahertz applications. A cavity-backed CPW line (CBCBW) is a planar transmission line that is designed and optimized for minimum loss while providing 50-$\Omega$ characteristic impedance. This line is shown to provide less than 0.12 dB/mm loss over the entire $J$-band (220–325 GHz). The transition from CBCPW to waveguide is realized in three steps to achieve a broadband response with a topology amenable to silicon micromachining. The first step is a tapered transition from the CBCPW line to a 50-$\Omega$ reduced-height waveguide. The next two steps utilize a novel in-plane impedance tapering technique to transition from the reduced-height waveguide to the on-wafer regular height waveguide. The full transition has less than 0.9 dB of insertion loss and more than 13 dB of return loss over the entire $J$-band (39% bandwidth). Silicon micromachining technology is used to fabricate prototypes of back-to-back CBCPW line-to-waveguide transitions. A novel waveguide-probe measurement setup is introduced and utilized to evaluate the performance of the transitions and the CBCPW lines.

A novel high-impedance nanoantenna with an embedded matching network is implemented to realize a highly sensitive infrared detector. A bowtie antenna is operated at its antiparallel resonance and loaded with a small low-bandgap (E_g=0.52  eV) indium gallium arsenide antimonide (InGaAsSb) p-n junction. The structure is optimized for maximum power transfer and significant field enhancement at its terminals for a desired frequency band where the maximum quantum efficiency of InGaAsSb is observed. The sensitivity improvement of the proposed detector is evaluated against the traditional bulk detector and it is shown that the detectivity is improved by the field enhancement factor, which is approximately 20 for the case considered here.

A periodic, corrugated, dielectric layer is simulated by an anisotropic dielectric layer of equal thickness. The tensor elements of the equivalent dielectric layer are given in terms of the permittivity of the dielectric material, the period of the surface corrugation, and the width of the corrugations. The validity of this technique is verified by comparing the reflection coefficient of the equivalent layer with that of the corresponding corrugated layer using the moment method. Employing a multiple layer approach, the technique is extended to handle periodic surfaces with arbitrary cross sections which can be used to design millimeter wave dielectric plate polarizers and absorbers.

The Michigan Microwave Canopy Scattering model (MIMICS) is based on a first-order solution of the radiative-transfer equation for a tree canopy comprising a crown layer, a trunk layer and a rough-surface ground boundary. The crown layer is modelled in terms of distributions of dielectric cylinders (representing needles and/or branches) and discs (representing leaves), and the trunks are treated as dielectric cylinders of uniform diameter. This report describes MIMICS I, which pertains to tree canopies with horizontally continuous (closed) crowns. The model, which is intended for use in the 0·5-10GHz region at angles greater than 10° from normal incidence, is formulated in terms of a 4 × 4 Stokes-like transformation matrix from which the backscattering coefficient can be computed for any transmit/receive polarization configuration.

Ad hoc communication among small robotic platforms in complex indoor environment is further challenged by three limiting factors: 1) limited power, 2) small size antennas, and 3) near-ground operation. In complex environments such as indoor scenarios often times the line-of-sight communication cannot be established and the wireless connectivity must rely on multi-path propagation. As a result, the propagation path-loss is much higher than free-space, and more power will be needed to obtain the need coverage. Near ground operation also leads to increased path-loss. To maintain the network connectivity without increasing the required power a novel high gain miniaturized radio repeater is presented. Unlike existing repeater systems, this system utilizes two closely spaced low profile miniaturized planar antennas capable of producing omnidirectional and vertical radiation patterns as well as a channel isolator layer that serves to decouple the adjacent antennas. The meta-material based channel isolator serves as an electromagnetic shield, thus enabling it to be built in a sub-wavelength size of 0.070 2× 0/70, the smallest repeater ever built. Also wave propagation simulations have been conducted to determine the required gain of such repeaters so to ensure the signal from the repeater is the dominant component. A prototype of the small radio repeater is fabricated to verify the design performance through a standard free-space measurement setup.

This paper presents the development of a static estimator for obtaining state information from optic flow and radar measurements. It is shown that estimates of translational and rotational speed can be extracted using a least squares inversion. The approach is demonstrated in a simulated three dimensional urban environment on an autonomous quadrotor micro-air-vehicle (MAV). The resulting methodology has the advantages of computation speed and simplicity, both of which are imperative for implementation on MAVs due to stringent size, weight, and power requirements.