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A symmetry relation of the integral operators in a layered medium proved by the reciprocal theory can be applied to improve the accuracy of the magnetic-field integral equation (MFIE). Based on the symmetric layered medium Green’s function, the normal vector can be transferred into the inner integral to keep the field integral as the inner integral of the elements in the impedance matrix. When discretized by the divergence-conforming function, this field-extracted scheme shows better performance than the classic expression in accuracy. Meanwhile, this form of expression can be applied in the combined-field integral equation (CFIE). Several numerical results are provided to validate the formulation of the symmetry relations from the layered medium Green’s function to the impedance matrices and to reveal its advantages in enhancing the accuracy of the field-extracted form of MFIE. [ABSTRACT FROM AUTHOR]

In this article, an ultrawideband radar scattering cross-sectional (RCS) reduction metasurface is proposed combining with absorption and phase cancellation mechanisms. A two-lossy-layer absorber is designed first, which is responsible for low-frequency RCS reduction from 1.3 to 13 GHz (163.6%). The equivalent circuit of the proposed absorber is extracted, proving that the wideband characteristic is due to the occurrence of multiple resonant points. To further improve the RCS reduction bandwidth, the absorber unit cells are arranged with three different heights. The proposed metasurface consists of 3 $\times $ 3 lattice and each lattice has $5\times $ 5 absorber unit cells. The reflection phase cancellation can be achieved owing to the wave path difference by introducing a certain height difference among subregions. Resultantly, high-frequency RCS reduction from 13 to 40 GHz can be realized since the backward scattering can be canceled effectively. Finally, an extraordinary ultrawideband RCS reduction from 1.5 to 40 GHz (185.5%) can be acquired successfully. A prototype with the thickness of 0.104 $\lambda _{L}$ (the wavelength of the lowest working frequency) is fabricated and measured for demonstration. The measured results prove that the proposed design has numerous advantages including minimization, ultrawideband RCS reduction, wide angle stability, and polarization insensitivity. [ABSTRACT FROM AUTHOR]

This article investigates the bandwidth limit of radar cross-section (RCS) reduction (RCSR) and proposes a new strategy for realizing multiband and ultrawideband RCSR. The high-frequency bound is broken while guaranteeing the stability of the low frequency and is extended to the grating-lobe appearance region. The short circuit blind zone of RCSR was first discovered in structures, such as the metasurface, frequency-selective surface (FSS), artificial magnetic conductor (AMC), and electromagnetic (EM) bandgap (EBG), through analyzing the equivalent circuit model and the reflection phase of the unit cell. [ABSTRACT FROM AUTHOR]

Spoof surface plasmons (SSP) on the corrugated metal surfaces have been widely studied over the past decade in both microwave and Terahertz (THz) band. Previous studies show that SSP mode on the metal grating suffers from high dissipative losses in high THz band especially near asymptotic frequency which may limit its real applications. The issue can be addressed by utilizing long-range SSP mode (LRSSP). LRSSP is a special low-loss symmetric propagating surface wave mode along double-layer corrugated metallic waveguides. In this paper, the asymmetric double metal gratings are proposed to support LRSSP which can significantly reduce the damping losses of the other short-range SSP mode (SRSSP) on the conventional doubly-corrugated metallic surfaces, thus enabling new possibilities to utilizing this SRSSP mode within long-range manner. According to our dispersion theory and the numerical calculations, the propagation loss of LRSSP is still lower than that of SRSSP within asymmetric regime, which demonstrates good tolerance to the asymmetry of the double-layer corrugated structure. However, its long-range propagation superiority of LRSSP vanishes gradually as the degree of asymmetry is enlarged. This work provides an alternative approach to increase the propagation length of SRSSP mode on the doubly-corrugated metallic surfaces and can open up new avenues to develop some novel LRSSP-based plasmonic devices such as waveguides, filters, sensors and active powerful radiation sources induced by energy beam below optical spectrum. [ABSTRACT FROM AUTHOR]

In this article, the working mechanism of a wideband absorber designed with an adjustable and highly selective notch band is studied, in which the narrow notch band is independently controlled by the lower lossless layer of the absorber, whereas the upper lossy layer loaded with lumped resistors realizes absorption. We present two instances with geometrically controlled and electrically controlled notch bands, respectively. Without decreasing absorption performance, the notch position can be flexibly adjusted throughout the entire frequency band by simply modifying the dimension of the lossless frequency-selective surface (FSS) or changing the capacitance of the varactor, i.e., using geometric control or electrical control. The narrow notch band allows two wide absorption bands to be retained on both sides; therefore, good stealth performance is still guaranteed. Equivalent circuit models (ECMs) are proposed to further explain the principle. The frequency-domain simulation, ECM, time-domain simulation, and experimental results are in good agreement and validate the adjustability and high selectivity of the notched absorbers. At the end of this article, an FSA-backed monopole antenna is simulated and measured, which clearly illustrates that these FSAs can serve as the ground plane for antennas and realize out-of-band RCS reduction. [ABSTRACT FROM AUTHOR]

In this article, the famous Sommerfeld integrals (SIs) have been revisited by assuming that the approximation of $\gamma _{0}\approx {\mathrm{ i}}\lambda $ is valid for the near-field propagation of low frequencies over a plane conducting ground. A deduction from that is proposed and adopted in the present study, which is defined by $|\gamma _{0}\gamma _{1}|\gg k_{0}^{2}$ , extending to cover a magnetic dipole source in air-to-undersea communications. The focus is on deriving complete and effective solutions in air and seawater regions under the cylindrical coordinates for the electromagnetic near-field, which is generated by a vertical magnetic dipole above sea surface. The resulting formulas can be given by a few summands in closed form as the well-known Fourier–Bessel integrals. It is believed to go a step further by relaxing the reliance on various distance conditions and replacing the integrands by simple, exact integrated terms which are usable at the frequencies of interest for any distance from the source. Computations and discussions are carried out by comparing with related works and late experiments, including the field components and Poynting vectors. It is verified that the derived formulas can be degenerated into some simplified forms which are in accord with those obtained by a “quasi-static” treatment. [ABSTRACT FROM AUTHOR]

In this article, an automatic and forward method is realized to establish attributed scattering center models directly from the computer-aided design (CAD) model of the complex target. The main steps include the preprocessing of the CAD model, the separation of scattering sources, the selection of strong scattering sources, and the automatic determination of model parameters. With the proposed method, the scattering sources, scattering mechanisms, and model parameters of the scattering centers can be identified and derived, such that the complicated manual intervention is completely avoided. Moreover, the method establishes the distributed scattering center models formed by curved surfaces with large curvature radii. Therefore, the formation mechanism of the distributed scattering center is extended from typical scattering structures to a general case. Thus, the model of the attributed scattering center is extended and can be applied to describe the scattering from the real structures of the complex target. The geometric shape of the scattering source is distinguished based on the principal curvature radii, which are calculated by differential geometry theory. Thus, the frequency dependence parameter is obtained according to its corresponding relationship with the geometric shape. In addition, based on the automatic method, a technology is studied to diagnose and correct the scattering center models of a target whose CAD model is unknown or partially known. Finally, parametric models of several targets in the Moving and Stationary Target Acquisition Recognition (MSTAR) program are established, and then compared with the measured data. The results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]