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The face-centered cube (FCC) grid has more sampling points than the traditional Yee cell (as it contains a total of 14 grid points, of which six are center points and eight are vertices, and each grid point has three field components). Thus, FCC is better than the former in dealing with the dispersion error of anisotropic media. On the basis of this feature, this article first derived the FCC-finite-difference time-domain (FDTD) iterative formula for nonuniform plasma on the basis of FCC grid and verifies the accuracy of the algorithm. This algorithm is combined with the nonuniform plasma flow field model of hypersonic vehicles, and the backward single-station radar cross sections (RCSs) of vertical polarization (VV) and horizontal polarization (HH) are calculated under different flight altitudes, flight velocities, and external conditions. Results showed that as flying height increased or flying speed decreased, it results in an increase in RCS. Different external magnetic fields and incident angles also had an obvious impact on the electromagnetic scattering characteristics. From the perspective of electromagnetic wave polarization, both VV polarization and HH polarization have a great impact on RCS. These results provide a theoretical basis for studying the electromagnetic scattering characteristics of nonuniform plasma targets. [ABSTRACT FROM AUTHOR]

The electromagnetic (EM) scattering properties of the chaff cloud in airflow are rarely addressed in previous works. To deal with this issue, two types of the chaff clouds in airflow are first established based on aerodynamics theories. Then, the K-means algorithm is utilized to divide the chaff cloud into several parts according to the elements’ number density. Meanwhile, the EM transmission property of each part is extracted with the help of the method-of-moments (MoM). Based on this, the vector radiative transfer (VRT) method is introduced to estimate the bistatic scattering coefficients of the established chaff clouds with 40 million elements, which takes into account the radiation pattern of the transmitting antenna and the EM coupling between chaff elements. To speed up the VRT program, parallel computing technologies are adopted. Besides, the verifications of the VRT method are proposed by performing simulations with a numerical method and conducting experiments. Comparative investigations indicate that the results predicted by the proposed VRT method have a good agreement with experimental ones with an error of less than 10%. [ABSTRACT FROM AUTHOR]

In the past, most of the transient electromagnetic (EM) analyses focused on the scattering from the target. The transient scattering from rough surface was rarely studied, especially for the layered rough surface. When the target is above the rough surface, it needs to solve the composite transient scattering from the rough surface and the target above it. In this article, we mainly focus on the time-domain (TD) analysis for transient scattering from the target and layered rough surface based on time-domain ray tracing (TDRT) modeling. Applying geometrical optics (GO), the multiple bounces of the transient EM waves between layered rough surfaces and target are traced to obtain the incident field of the irradiated surfaces. Then considering the particularity of the layered rough surface, we modify TD Gordon integral and reduce the TD physical optics (TDPO) integral to closed-form expressions to improve the computing efficiency. To demonstrate the efficiency and accuracy of the proposed algorithm, the transient scattering simulations of the composite model involving target and layered rough surface are compared with frequency-domain ray tracing (FDRT) with inverse fast Fourier transform (IFFT), as well as the results obtained by the multilevel fast multipole algorithm (MLFMA) of FEKO in conjunction with IFFT (IFFT-MLFMA). The characteristics of transient scattering from the composite model are further studied to show the physical phenomenon of scattering mechanisms. Specifically, the transient scattering from the composite model for different parameters is simulated for sensitivity investigation of the transient scattering to the variations in the related parameters. [ABSTRACT FROM AUTHOR]

In this article, we propose a hybrid approach that combines machine learning and experimental design to efficiently and accurately predict the monostatic radar cross section (RCS) of a conducting target versus the incident angle. The approach is called physical optics-inspired support vector regression (POI-SVR). The design of its kernel function is inspired by PO. Uniform design (UD) and uniform design sampling (UDS) are introduced to obtain highly representative training samples. Numerical experiments dealing with simple and complex targets are carried out to evaluate the accuracy and efficiency of the proposed method. The results show that our method can reduce the predictive root-mean-square error (RMSE) by 29.38%–64.78% compared with the alternative methods of combining a Gaussian SVR with the centrically located sampling (CLS), the Latin hypercube sampling (LHS), or the simple random sampling (SRS). Under the same sampling strategies (i.e., UD and UDS), POI-SVR can reduce the predictive RMSE by 11.30%–53.56% compared with the Gaussian SVR. The well-trained POI-SVR can predict the monostatic RCS of the target in any direction within 0.1 s, and in 20 000 directions within 10 s. [ABSTRACT FROM AUTHOR]

A new electromagnetic (EM) calculation model, the inhomogeneity zonal medium model (IZMM) (based on the physical optics (PO) method), is proposed to calculate and analyze the EM scattering characteristics of hypersonic vehicle covered with a plasma sheath. First, the flow field of the plasma sheath is divided into nonuniform calculation subregions with different spacing sizes. The shape of the external flow field of the aircraft is modeled by fitting with a paraboloid and circular platform with different radii. Then, the electron density, temperature, and pressure distribution are obtained by collecting data from different points in the flow field of the sheath. Finally, a piecewise PO method is used to calculate the backscattering radar cross section (BRCS) of the hypersonic vehicle covered with plasma sheath from $L$ -band to $X$ -band. The numerical results show that the EM scattering characteristics of a target spacecraft covered with a plasma sheath can be described more accurately by using zoned medium model simulation. Under different flying conditions, the BRCS of hypersonic vehicle covered with a plasma sheath appears to be reduced to different degrees in the $S$ -band and the band range of 7–11 GHz, and a large attenuation of BRCS occurs in the head area of the aircraft. The research results enrich the EM modeling method of hypersonic vehicles, overcome some shortcomings of the existing PO method, and provide technical support and innovative ideas for the accurate analysis of the EM scattering characteristics of hypersonic targets. [ABSTRACT FROM AUTHOR]

Time-domain physical optics (TDPO) contour integral representations are developed for backscattered near fields from a perfectly conducting surface illuminated by a dipole source. The proposed representations are derived from the TDPO surface integral representations by introducing two theorems: surface gradient and surface divergence theorems. The proposed representations are free from singularities for all the source/observer positions in which the dipole antenna is located. In addition, the time-domain physical theory of diffraction is introduced to modify the physical optical field considering the effect of the edge diffraction. Numerical examples are presented to demonstrate the efficiency and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This communication presents a new reconfigurable filtering monopole antenna design with three switchable states for UWB/WLAN applications. The antenna has three independent ports for ultra-wideband state, 2.4 GHz WLAN narrowband state, and 5.8 GHz WLAN narrowband state, respectively. The narrowband state at 2.4 GHz is produced by a first-order microstrip filter using an open-loop resonator, and the narrowband state around 5.8 GHz is obtained by involving a third-order hairpin bandpass filter in the RF path. Frequency reconfiguration is achieved by RF path selection using dc controlled p-i-n diodes. As a result, narrowband filtering responses at desired frequency bands can be achieved. One antenna prototype is simulated, fabricated, and measured. Measured reflection coefficients and radiation patterns demonstrate that the proposed reconfigurable filtering antenna is an eligible candidate for future multifunctional systems incorporating both UWB and WLAN systems. [ABSTRACT FROM AUTHOR]

This paper presents a feasible and efficient scattering model of the time-varying complex sea surface process to the field of sea spike modeling. To describe the phenomena of sea spikes as well as the “super events” from realistic sea clutter, wedges are posited on the two-scale sea surface to provide an approximate simulation of the breaking waves. The scattering of the wedge and multiple reflections that occur from the sea and wedges are considered in ray tracing for a large HH/VV ratio. With this technique, we obtained the time sequence of the time-varying sea surface and identified the spiking events in the background. Sea clutter properties with breaking waves are compared with that of the sea. Results indicate that spiking events are more likely to occur in the sea with breaking waves. The probability distribution function (PDF) curve of the sea surface with breaking waves has a heavy trail in the large amplitude region. The decorrelation time of the sea with breaking waves is significantly longer compared with the sea without breaking waves, particularly, for HH polarization. The spike properties of sea clutter in different wind speeds and resolutions are also investigated. Results show that spike events are more likely to happen under a high sea state and radar resolution. The Doppler spectrum of the sea with breaking waves showed remarkable improvement for both polarizations. All simulation results indicate that the proposed scattering model effectively stimulates sea clutter with a low grazing incident angle and explains the physical mechanism of the sea spikes. [ABSTRACT FROM PUBLISHER]