A method of improving the diversity and capacity of multiple-input-multiple-output (MIMO) systems in nonisotropic multipath environments by loading a scatterer array is proposed. The difference between array diversity and super-resolution imaging is discussed first. The phase differences caused by the “effective space” between adjacent antenna elements are the critical factors of diversity enhancement. The scatterer array, which is made of cross-structure periodically, is placed on top of the antenna array to adjust the phase center of the radiation pattern of each antenna port. By increasing the “equivalent size” of the antenna array, the correlation of part of the adjacent ports in the antenna array is decreased; thus, the array diversity is improved. Through a set of comparative examples, the effectiveness of the scatterer array is clearly demonstrated.

In this paper, a tangential network transmission theory is established for the reflective metasurface composed of periodic subwavelength metallic elements and a perfect electrical conductor (PEC)-backed substrate. The theory is divided into two parts. The first part shows that the oblique incidence can be handled the same way as a normal incidence for a PEC-backed substrate in terms of tangential components of electric fields and a normal wave vector. Therefore, the network transmission theory is extended from the PEC-backed substrate to the reflective metasurface, whose total tangential reflection matrix depends on the induction matrix of the metallic elements in the air-substrate interface. The second part deals with the conversion of the induction matrices of different metallic elements into equivalent circuits, whose electrical parameters can be theoretically calculated or extracted from numerical simulations. Based on the obtained electrical parameters, the theory can be used to analyze or design reflective metasurfaces. Finally, the theory is verified by a metasurface, which is composed of periodic double-L shaped elements. The analytical results agree with the simulated ones for arbitrary substrate and incidence angle. The measured results further validate the proposed theory.

A spherical sparse sampling near-field measurements method based on the compressed sensing (CS) approach is proposed in this letter. Since the energy of the spherical wave expansion coefficients (SWECs) of the antenna is concentrated in a limited set of coefficients, sparse optimization sampling becomes a viable choice. To minimize the testing time, non-uniform or adaptive sampling has been extensively explored. However, in practical testing scenarios, these sampling methods fail to guarantee a reduction in the mechanical motion time of the sampling path, making the CS approaches a huge challenge. The proposed uniform undersampling method based on the two-step iterative shrinkage/thresholding (TwIST) algorithm ensures a reduction in sampling time and is suitable for both single-probe and multi-probe sampling systems. A comparison of the near-field to far-field transformation (NFFFT) accuracy between the classical CS methods and the proposed scheme validates its effectiveness. The proposed method achieves a truncation mode number of only one-third of that required by traditional FFT methods at the same error level. Simulation and measurement results verify the feasibility of this method.

A new approach for integrating unequal power dividers into a series-fed resonant slot antenna subarray is proposed, to reduce the sidelobe level (SLL) while avoiding bandwidth degradation in an extended linear slot antenna array. Subsequently, a compact dual-polarization antenna array is designed to utilize gap waveguide technology, which features high gain, low sidelobe, and high efficiency. By coordinating power-weighted feeding networks, a two-dimensional low SLL array is accomplished. Furthermore, monopulse comparator networks are incorporated to expand the potential application scenarios. A prototype array antenna was fabricated and measured to validate the design performance. The measured and simulated results are compared, indicating -10 dB impedance matching of 10% and 12% for vertical and horizontal polarization, respectively, while maintaining consistent performance across both sum and difference ports. For both polarizations, the measured peak gains of sum ports exceed 30.6 dBi over the operating frequency band, with antenna efficiency generally exceeding 50%. At 23.5-24.5 GHz, the first sidelobe levels in azimuth and elevation planes are lower than -19 dB and -22 dB, respectively. Meanwhile, the difference patterns exhibit a null depth of approximately -37 dB relative to the sum pattern peak within the operating band. The proposed antenna array can provide individual amplitude for each element, demonstrating exceptional performance.

It has been shown that the angular correlation of the antenna can be obtained by rotating it in a reverberation chamber (RC) and the radiation pattern of the antenna under test (AUT) can be reconstructed from the measured correlation coefficients using spherical wave decomposition. However, the correlation coefficient needs to be obtained by measuring the complex-valued S-parameters in an RC, which is no longer applicable to active antenna tests where the phase information is unavailable. To solve this problem, we propose an antenna radiation pattern reconstruction method based on phaseless measurement in an RC. In this method, the power correlation coefficient of AUT is calculated using the signal amplitude received by the antenna at different angles. According to the relationship between complex correlation coefficient and power correlation coefficient of the AUT, the nonlinear least squares method is used to reverse the spherical wave coefficients and reconstruct the antenna radiation pattern. Simulations and measurements are carried out in RC and anechoic chamber, which demonstrated the feasibility and applicability of the proposed method.

In this paper, we propose a theory by extending the Babinet principle to periodic subwavelength anisotropic metallic crevice elements Ae in the interface of different substrates. The significance of the proposed theory is that it can readily be used to transfer the solution of Ae to that of its complementary metallic elements Ac. First, the Babinet principle is established for Ae and its complementary magnetic elements Am according to the uniqueness theorem of boundary conditions, and Am is replaced with the corresponding metallic elements Ac based on the dual principle. Then, the waves associated with Ae and Ac are considered as plane waves since their period is smaller than the operation wavelength. Thereby, the Babinet principle is reexpressed as mathematical relations between tangential transmission matrices of Ae and Ac. Next, the tangential transmission matrices of Ac are solved using the corresponding circuit model, where the effective relative permeability is introduced to describe the inductive influence of the dual substrate. Finally, the tangential transmission matrices of Ae are derived, and the proposed theory is verified by two examples. The theoretical results agree well with the simulated ones at normal incidence (for various substrates) and over a large range of incidence angles (up to 75° and 60° for the two examples, respectively). The measured results support the simulation and the proposed theory.