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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.

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 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.

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.