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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 letter focuses on whether accurate over-the-air (OTA) performance of large-form-factor (LFF) Internet-of-Things (IoT) devices (especially for vehicles) can be obtained from medium-sized anechoic chambers (ACs). For LFF IoT devices under test (DUTs), antenna locations are usually deviated significantly from the center of the OTA testing system due to mechanical positioner or chamber constraints in medium-sized ACs, which will lead to distortion for assessed OTA performance. This work evaluates the effect of offsetting the antenna under test integrated in LFF IoT DUTs from the center of the quiet zone on OTA integrated-type quantities. A new figure of merit ROFF has been defined to characterize the off-center extent, while the influence of which on OTA performance as well as associated simulations is presented based on statistical analysis. It can be found that the performance deviation is determined by ROFF rather than absolute off-center distance. The relationship between ROFF and performance deviation can be derived by using the least-squares fitting method. Additionally, it is acquired that for off-center scenario, performance deviation cannot be eliminated by increasing measurement grid density, yet can be reduced significantly by introducing off-center correction. [ABSTRACT FROM AUTHOR]

In this communication, a novel metasurface with multiple unit cells of different thicknesses based on optimized multielement phase cancellation (OMEPC) is proposed for extremely wideband, omnidirectional, and polarization-independent radar cross section (RCS) reduction under wide-angle oblique incidence. Omnidirectional characteristic refers to the capability of RCS reduction under all azimuth angles ($\varphi ^{i}$) at certain elevation angles ($\theta ^{i}$). The equivalent circuit model of the square patch unit cell is analyzed under both normal and oblique incidences. Unit cells of multiple thicknesses are adopted to solve the in-phase reflection at certain frequencies. The scattered fields of the proposed multiple-element metasurface under different frequencies, polarizations, and incident angles are jointly optimized and simultaneously suppressed by OMEPC. When the range of incident angles is increased from 0° to 40°, the proposed multiple-element metasurface can achieve the 10 dB specular RCS reduction from 7.34 to 64.85 GHz with a ratio bandwidth of 8.84:1 for both TE and TM polarizations, and the 8 dB RCS reduction bandwidth is approximately 6.76–83.70 GHz. The measurement results are in agreement with the theoretical analysis and simulation results. [ABSTRACT FROM AUTHOR]

In this article, two wideband choke flanges (Flange 1 and Flange 2) are designed to reduce the passive intermodulation (PIM) level of the orthmode transducer (OMT). Since flange connections are important sources of PIM, circular Flange 1 introduces a choke groove and achieves a power suppression level of 41.95 dB. To meet the requirement of miniaturization and lightweight, Flange 2 with rectangular structure and rectangular choke groove is proposed for reducing height waveguide. Compared with conventional circular choke flange for reduced height waveguide, it has a smaller cross area and better power suppression level (39.92 dB). To evaluate the PIM performance of the proposed OMT with two flanges, temperature circulation test is executed. It is proved that the third-order PIM level is about −135 dBm from measurement results. Therefore, the proposed flanges can be utilized in large-scale multibeam array for low PIM satellite communication applications. [ABSTRACT FROM AUTHOR]

This paper considers the hybrid precoder design in millimeter wave (mmWave) multi-input multi-output (MIMO) systems with low-resolution analog phase shifters. Aiming at reducing the complexities of the near-optimal algorithms, we propose a low-complexity multi-user hybrid precoding scheme based on tree-coding-aided adaptive-cross-entroy (TC-ACE) algorithm. By defining some discrete variables for the analog precoders and combiners, the problem of hybrid precoding is transformed into a cross-entroy (CE) optimization problem, which can be solved by iteratively updating the probability distributions of the predefined discrete variables. In order to derive the closed-form expression of the probability distributions, tree-coding is used to encode each entry of the analog precoders and combiners with a binary number. Through iterations, optimal analog precoders and combiners will be obtained when its probabilities are sufficiently high. Simulation results show that when the number of users exceeds a certain value, the proposed scheme outperforms the alternating minimization algorithm and coordinate descent method in terms of both the sum-rate and complexity. [ABSTRACT FROM AUTHOR]

Over-the-air (OTA) measurement is mandatory for 5G mm-wave wireless devices. The compact antenna test range (CATR) is the baseline method in the current version of TR 38.810 in 3rd generation partnership project (3GPP) and cellular telecommunications industry association (CTIA) millimeter-wave (mm-Wave) test plan. For CATR, both amplitude and phase variations inside the quiet zone (QZ) could affect the final OTA results significantly and hence need to be characterized carefully. Especially in the phase validation of QZ, even slight misalignments between sampling plane and incident plane wavefront due to the limited precision of the positioner or tolerance in antenna fixture could result in large phase variations, which is not the true phase property of QZ. This letter presents a method to remove the effect of the angular error due to misalignment when measuring the phase variation inside QZ. By adopting the proposed method to the measured phase data, any angular error due to misalignment with respect to the incident plane wavefront will be eliminated mathematically, leaving the true phase variations of QZ only. In the meanwhile, the proposed method can alleviate the requirements of fixture manufacture and alignment precision and reduce the test cost. [ABSTRACT FROM AUTHOR]

In this letter, a single-layer bandpass frequency selective surface (FSS) working at 2.6 GHz is proposed to decrease the interference to communication systems and the cost of processing based on a multimode resonator. A metasurface unit cell is adopted to suppress the grating lobe and achieve miniaturization. Therefore, the performance of the proposed FSS can remain stable under a large oblique incidence. The surface current distribution and equivalent circuit model at each design step are extracted to explain its working principle. Good agreement between the experimental results and the simulated response verifies the feasibility of the proposed FSS. [ABSTRACT FROM AUTHOR]

This article proposes a novel wideband single-layer coplanar waveguide (CPW)-fed antenna element and multiple-input, multiple-output (MIMO) antenna based on a multimode resonator. Equivalent circuits of the multimode resonator are analyzed by adopting the odd- and even-mode analysis approach. To prove the effectiveness of the equivalent circuit, scattering (S)-parameters of the equivalent circuit are simulated in Advanced Design System (ADS). Additionally, transmission coefficients are studied under the weakly coupling condition. The first and second resonant points are introduced by even and odd modes, respectively, whereas the third resonance is caused by the embedded stepped impedance resonator (ESIR). The CPW-fed antenna element is designed based on the proposed resonator. CPW fed not only excites the multimode resonator but also generates an extra resonance between the odd mode and ESIR resonant frequencies. Furthermore, the odd-mode resonance creates the lower cutoff frequency, and the resonance caused by ESIR forms the higher cutoff frequency. Hence, a filteringlike operating frequency band is achieved without introducing an extra filtering circuit. The filtering characteristic is favorable for the system to realize high sensitivity and reduce outof-band interferences simultaneously. [ABSTRACT FROM AUTHOR]