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Microwave remote sensing offers a powerful tool for monitoring the growth of short, dense vegetation such as soybeans. As the plants mature, changes in their biomass and 3-D structure impact the electromagnetic (EM) backscatter signal. This backscatter information holds valuable insights into crop health and yield, prompting the need for a comprehensive understanding of how structural and biophysical properties of soybeans as well as soil characteristics contribute to the overall backscatter signature. In this study, a full-wave model is developed for simulating L-band backscatter from soybean fields. Leveraging the ANSYS high-frequency structure simulator (HFSS) framework, the model solves for the scattering of EM waves from realistic 3-D structural models of soybean, explicitly incorporating the interplant scattering effects. The model estimates of backscatter match well with the field observations from the SMAPVEX16-MicroWEX and SMAPVEX12, with average differences of 1–2 dB for co-pol and less than 4 dB for cross-pol. Furthermore, the model effectively replicates the temporal dynamics of crop backscatter throughout the growing season. The HFSS analysis revealed that the stems, pods, and soil are the primary contributors to HH-pol backscatter, while the branches contribute to VV-pol, and leaves impact the cross-pol signatures. In addition, a sensitivity study with a 3-D bare soil surface resulted in an average variation of 8 dB in co- and cross-pol, even when the root-mean-square height and correlation length were held constant. These capabilities underscore the model's potential to provide insights into the underlying dynamics of the backscatter for growing vegetation.

Precipitation and in particular rain present a random medium with constituent particles having dimensions comparable to the wavelength at millimeter-wave frequencies. Due to considerable scattering of waves from such particles, significant signal attenuation and phase front aberration can take place which are important factors that must be considered in modern communication system or radar design. This paper presents a fast and accurate numerical method for phase coherent modeling of wave propagation and scattering in random media, like rain. In this approach, the random medium is divided into sufficiently large finite slabs and each slab is modeled as a network with multiple input/output ports where ports represent rays with different polarizations propagating in different directions entering and leaving the slabs. The cascaded networks of slabs can be used to find the response of the random medium to any arbitrary source. The proposed method considers multiple scattering among all scatterers and thus the entire physics of wave-particle interactions is accounted for. In addition, the method can model both the mean and the fluctuating parts of waves in the random medium. To demonstrate the applicability of the proposed method, it is used for the calculation of the specific attenuation in rain media as a function of rain rates and frequency within the millimeter-wave band. The results show a good match with the available rain attenuation data in the literature.

Design procedure and experimental validation for a millimeter-wave (MMW) vehicular antenna with simultaneous multi-beam, for 5G V2X application, is presented. The design overcomes the constraint for requiring both wide angular coverage and high gain through a compact multi-array design. Each array is designed to have a fixed beam direction and sufficient beamwidth so that the overall pattern guarantees simultaneous coverage in the elevation plane. The multi-array idea is a cost-effective technique compared to employing phase shifters and beam steering of a single array and avoids latency associated with beam steering. The proposed design is composed of three compact dual polarized patch arrays. Each array covers different elevation angles, and their beam maxima are chosen to be at 10°, −20° and −40° with respect to the normal of the windshield. The antenna covers the entire 28 GHz 5G band which ranges from 26.5 GHz to 29.5 GHz. Each feeding network is designed to minimize beam scanning with frequency. Dual-polarized antenna arrays are implemented to provide polarization diversity. Antenna mounting behind the laminated windshield have been investigated and proven to have a negligible effect on the antenna matching and a minor effect on the radiation pattern.

This paper presents a very low-profile ( $0.105\lambda _{0}$ at 28 GHz) dual-band dual-polarized millimeter-wave phased array for 5G smartphones. The array element is based on two distinct dual-band antennas to realize two orthogonal linear polarizations. A planar folded dipole with a parasitic strip is used to produce horizontally polarized radiation, and a novel hexagonal bridge antenna is devised to generate vertically polarized radiation while maintaining a very low profile. A prototype of a $1\times 4$ phased array is designed and implemented on a multilayer laminate using standard printed circuit board process. The real estate of the array is kept within $6.5\,\, \mathrm {mm}\times 23\,\, \mathrm {mm}\times 1.12\,\, \mathrm {mm}$ . For both polarizations, the −10 dB bandwidth covers 25.4–30.84 GHz and 38–40 GHz with peak gains of 11.31 and 11.93 dBi at two center frequencies, respectively. The overlapped 3 dB scan ranges between both polarizations are ±45° at 28 GHz and ±39° at 39 GHz. This work considers a number of important design challenges of implementing low-cost mmWave antennas in smartphones, providing a practical solution to mmWave 5G mobile devices.

This paper presents a model that describes a possible mechanism for electromagnetic (EM) signal transmission and reception by bacterial cells within their biofilm communities. Bacterial cells in biofilms are embedded into a complex extracellular matrix containing, among other components, charged helical nanofibrils from amyloid-forming peptides. Based on the current knowledge about the nanoscale structure and dynamics of the amyloids, we explore a hypothetical model that the mechanical vibration of these nanofibrils allows the cells to transmit EM signals to their neighboring cells and the surrounding environment. For the reception, the induced electric field can either exert force on the charges of adjacent nanofibrils associated with the neighboring cells or affect the placement/conformation of a certain charged messenger protein within the cell. The proposed model is based on a coupled system of electrical and mechanical nanoscale structures, which predicts signal transmission and reception within kHz-GHz frequency ranges. Different mechanisms for generating EM signals at various frequency bands related to the structure of the cell and their biofilm constituents are discussed.

Abstract Four‐dimensional (4D) relativistic scattering of electromagnetic waves from an arbitrary collection of uniformly translational moving lossy dielectric spheres is discussed. Two reference frames, four 4D coordinate systems and Lorentz transformation are used to obtain the scattered electromagnetic fields. The direct scattering of the spheres and their interactions are considered with a novel approach. The introduced method is straightforward and the analytical relations for the fields are achieved. To check the validity of the proposed method, different examples for both stationary and moving scatterers are investigated. The effects of key parameters such as the size, material, velocity, number, position of the spheres and also the frequency of the incident wave are discussed. The derived scattered fields are valid for low, medium and high velocities but according to practical applications low and moderate velocities are highlighted in numerical results.

Abstract The dielectric‐coated impedance cylinder configuration is investigated thoroughly with respect to the space‐wave, leaky‐wave and creeping‐wave excitations for all source polarizations. A very high directivity is achieved for the space‐ and leaky‐wave radiation for the antenna structure compared to that from a dielectric cylindrical rod investigated earlier. It is also observed that such high directivity for the dominant‐mode leaky‐wave is accompanied by higher‐order azimuthal harmonics with low attenuation constant contributing to the leaky‐wave pole, instead of the n = 0 harmonic for the previous configuration. A large enhancement in the space‐wave directivity is also achieved, contributed by higher space‐wave harmonics compared to lower‐order azimuthal harmonics for the cylindrical dielectric rod antenna. The space‐wave and leaky‐wave directivities are examined with variation in the design parameters and azimuthal harmonics. The phase and attenuation characteristics of the leaky‐wave are used to evaluate the optimized leaky‐wave structure parameters and appropriate azimuthal harmonics that result in high directivity for the configuration. The creeping‐wave characteristics for the configuration are also investigated, with a relatively stronger creeping‐wave excitation observed for an inductive surface‐impedance.

A new time-domain technique for an accurate and fast measurement of the resonant frequency and $Q$ -factor of resonators is presented. In this technique, the phase slope with respect to frequency of a resonator is characterized and used to specify its $Q$ -factor and resonant frequency. To measure the phase slope, a slow-rate linear chirp signal is generated and passed through the resonator under test. The output signal is amplified and subsequently divided into two equal parts which are delayed by different amounts. Due to the linear time-frequency relationship of the chirp signal, the instantaneous frequencies of the two delayed signals are slightly different at any given time, and their phase difference can determine the resonator’s phase slope. Detailed analysis of the proposed approach is presented and its accuracy is verified through simulations and measurements. Based on the measured results, the proposed approach characterizes the resonant frequency and $Q$ -factor by less than 0.6% and 4% error, respectively.

In this paper, a dual-band high-gain planar corrugated antenna is presented for applications in space and satellite communications. An analytic method based on the unit-cell analysis of a corrugated structure is presented to estimate the resonant frequency of the corrugated antenna where the maximum broadside gain happens. Therefore, the design of periodic corrugated antenna structures, which is very time-consuming due to the large size of the structure, is reduced to the design of a unit-cell of the structure which is very fast. A 2D corrugated (bull's-eye) antenna structure is then parametrically studied to find the parameters which affect the resonant frequency of the corrugated structures. A dual-band bull's-eye antenna with an integrated feeder structure at center is designed for the center frequencies of 9.7 GHz and 13.85 GHz. The dual-band operation is enabled using two different corrugations realized on two laminate boards. A prototype is fabricated and measured maximum broadside realized gains of 15. 8 and 17.5 dBi are achieved for the first and second bands, respectively.

We propose a novel cross bowtie antenna using phase-shift lines to realize circular polarization (CP) in the infrared (IR) spectrum. The CP antenna consists of a vertical, a horizontal antenna, and two elliptical loops connecting adjacent tips of the antennas to create a 90° phase shift. The phase-shift lines are conceived to realize a single terminal of the antenna where a detecting material can be mounted and enable the detecting material to fully utilize the field enhancement from the antenna. In full-wave simulations, a nanometer-scale low-bandgap semiconductor, i.e., indium gallium arsenide antimonide (InGaAsSb) capable of detecting IR wave is loaded at the antenna terminal and the antenna structure is designed to achieve CP absorption at 180 THz. The antenna includes a transmission line-based impedance matching structure to compensate for the reactance of the InGaAsSb semiconductor load. To verify the CP detection of the antenna near 180 THz, we numerically show a higher field enhancement value and absorption rate in the antenna terminal in a specific CP case when the antenna is illuminated with incident waves with various polarization states. Also, we design 2 × 1 and 2 × 2 antenna arrays using the CP antennas connected with metallic traces and show that the CP absorption at 180 THz is maintained. Finally, an illustration of a focal plane array based on the proposed CP cross bowtie antennas coupled with the InGaAsSb for a full Stokes polarimeter is presented.

This paper presents a Synthetic Aperture Radar (SAR) image classification algorithm based on multi-feature using Fruit Fly Optimization Algorithm (FOA) and Least Square Support Vector Machine (LS-SVM). First, pixel-based information derived from three elements of coherency matrix, six parameters obtained by $H/\alpha /A$ decomposition and Freeman decomposition techniques, and three polarimetric parameters including the total receive power (SPAN), pedestal height, and Radar Vegetation Index (RVI), as well as region-based information derived from eight texture parameters obtained by Grey Level Co-occurrence Matrix (GLCM) are combined to use as the features of land cover. Second, Kernel Principal Component Analysis (KPCA) is used to reduce the dimensionality of the multi-feature data derived from the integration of the pixel-based and region-based information. Third, LS-SVM is used as the classifier in this study due to its fast solving speed and desirable classification capability. Since the input parameters of LS-SVM significantly affect the classification performance, we employ FOA to obtain the optimized input parameters. Finally, the experiments on two fully polarimetric SAR images of various crops with a limited number of samples are implemented by the proposed method and other commonly used methods, respectively. The results show that the proposed method can attain better classification performances compared with other methods.

This paper presents a novel approach to achieve a very high level of isolation between the two ports of a compact antenna system for communication and radar applications. Orthogonal polarization and a special symmetric arrangement of two-element arrays of transmitter and receiver antennas are used to achieve high isolation levels. Identical rectangular patch antennas are designed for operation at 6 GHz, and are arranged in a square grid configuration with 90° rotational symmetry in this design. A pair of transmitters and a pair of receivers are fed perfectly out of phase with 180° couplers. The entire system is simulated and fabricated. We measure the prototype antenna's isolation, and use the absolute gain measurement technique and standard far field antenna pattern measurement technique to characterize it's realized gain and radiation pattern. It is found that exploiting symmetry in this manner leads to isolation of 63.7 dB. This indicates that carefully designed and constructed systems could reasonably exhibit isolation on the order of 70 dB or more using this method.

A low-power, low-frequency, ad hoc networking paradigm is considered for robust communications among mobile agents in complex non-line-of-sight (NLOS) indoor and urban-type scenarios. Compared with higher frequency, the lower portion of the very high frequency (VHF) band offers improved penetration and reduced multipath in such scenarios. Low VHF is underutilized for mobile ad hoc networking due to the lack of compact low-power systems and efficient miniature antennas. We investigate the proposed approach through experiments in realistic scenarios. In order to carry out the experiments, we leverage a compact, low-power ZigBee radio operating seamlessly in the low-VHF band by introducing a bi-directional frequency converter, which translates ZigBee signals into low-VHF carriers, along with a recently developed highly miniaturized efficient antenna. The experimental low-VHF radio system and a conventional ZigBee operating at 2.4 GHz are both integrated on a compact ground robotic platform for autonomous experimentation and comparison in NLOS indoor and outdoor settings. Measurements quantify the significant advantages of the low-VHF radio system in terms of packet error rate, fading, radio signal strength, and extended spatial coverage, in a number of complex communication environments.