Correlations and mutual couplings in antenna arrays lead to performance deterioration of multiple-input-multiple-output (MIMO) systems. The relation between mutual coupling and correlation is complex, especially in non-isotropic multipath environments. Reducing mutual coupling does not guarantee an eventual reduction of the overall correlation and vice versa. In general, it is not a trivial task to reduce mutual coupling and correlation simultaneously in arbitrary multipath environments. In this correspondence, a planar metamaterial structure (PMS) and an array-antenna decoupling surface (ADS) are applied to the antenna array for simultaneous reductions of correlations and mutual couplings. The PMS comprises regularly arranged metamaterial elements holding certain transmission features. The ADS placed between the PMS and array is a substrate with printed rectangular metal strips. A 1 × 4 patch array is taken as an example to demonstrate the effectiveness of the proposed scheme. According to the simulated and measured results, the combination of PMS and ADS can work together without interfering or affecting the matching condition of the antennas. As a result, the mutual coupling and overall correlation between the antenna elements are reduced in a general multipath environment.

A frequency-diverse bunching metasurface antenna (FDBMA) that can be used for microwave computational imaging (MCI) systems is proposed in this paper. The proposed FDBMA can generate low-correlated radiation patterns with a reduced frequency interval of 20 MHz and a bunching angle of 45° from 32 to 36 GHz. The frequency interval is reduced by combining a disordered cavity and an optimized frequency-diverse random metasurface. The directivity of the radiation patterns is improved by leveraging the joint-bunching method that combines the metal baffle, the Fresnel dielectric lens, the quasi-gradient random metasurface, and the random-coherent superposition comprehensively. The performance of the proposed FDBMA is evaluated in terms of the reflection coefficient, singular values and correlation coefficients of measurement modes. The reduced frequency interval and the bunching characteristic are also demonstrated. Finally, MCI experiments are implemented using the proposed FDBMA. Comparative experiments are also carried out to validate the advantage of reducing the frequency interval and improving the directivity.

A frequency-diverse bunching metamaterial antenna for coincidence imaging in the Ka band is proposed in this paper. The bunching metamaterial antenna includes a broadband circular array and a frequency-diverse bunching metalens. Firstly, in order to enhance the bunching characteristic, the broadband circular array is designed based on the 60-degree beamwidth design to generate radiation patterns from 32 GHz to 36 GHz. Then, types of metamaterial elements with different transmission phases are selected to form the frequency-diverse bunching metalens based on a random distribution design and gradient zoom coefficient design. Moreover, the bunching metamaterial antenna is constituted by loading the frequency-diverse bunching metalens to the broadband circular array, which can generate frequency-diverse bunching random radiation patterns with beamwidth less than 100 degrees from 32 GHz to 36 GHz. Furthermore, the performances of the bunching metamaterial antenna, including the reflection coefficient, the radiation efficiency, and the correlation coefficients of radiation patterns at different frequencies are evaluated. Finally, the coincidence imaging experiment is implemented using the bunching metamaterial antenna and the image of the target is reconstructed successfully. The design is verified by simulations and measurements.

In this letter, a frequency-diverse transmission metamaterial aperture (FDTMA) with bunching random beam (BRB) is proposed for coincidence imaging. The FDTMA consists of a random modulation metamaterial lens and a broadband microstrip patch antenna array which is used as a feed source. Firstly, an equivalent medium containing both dielectric layer and metamaterial structure is modeled and analyzed using the electromagnetic theory. The zoom coefficients of the metamaterial elements are analyzed to simplify the design of the broadband aperture. Then the basic metamaterial elements are designed based on the proposed model. Moreover, the FDTMA is designed using the basic metamaterial elements considering the requirements of the coincidence imaging system. Finally, the waved metal plate (WMP) leveraged in the FDTMA with BRB is optimized using the High-Frequency Structure Simulator (HFSS) software. The radiation patterns of the proposed FDTMA with BRB are randomly distributed in a limited angle range in the X band. The proposed equivalent model and FDTMA with BRB are validated through simulations and experiments.

In order to solve the low signal-to-noise ratio (SNR) problem of the differential coincidence imaging (DCI) system, a frequency-diverse metacavity Cassegrain antenna (FDMCA) that is able to generate low-correlated bunching radiation patterns is proposed in this communication. The FDMCA is designed according to the Cassegrain antenna form, which consists of a frequency-diverse half-spherical metacavity etched with backprojecting slot arrays and a parabolic reflector. In total, 81 useful measurement modes with a bunching angle of 40° are obtained from 32 to 36 GHz. First, frequency-diverse field distributions in the metacavity are obtained utilizing a high-dispersion metasurface. Backprojecting slot arrays etched on the metacavity would couple the energy from the metacavity and backradiate to the reflector. When placing the phase center of the feed source at the reflector focal point, the reflected patterns would be focused. Then, the performance of the proposed FDMCA is evaluated. In total, 81 radiation patterns with correlation coefficients (CCs) under 0.3 are generated. Finally, imaging experiments using the proposed FDMCA are carried out and the target image is reconstructed successfully using the DCI method. Comparative experiments are also implemented under the same conditions using a nonbunching frequency-diverse metasurface antenna (FDMA) to verify the bunching advantage of the FDMCA. The design is validated by simulations and measurements.

In this paper, metasurfaces with both cross-polarization conversion and vortex beam-generating are proposed. The proposed finite metasurface designs are able to change the polarization of incident electromagnetic (EM) waves to its cross-polarization. In addition, they also can modulate the incidences into beams carrying orbital angular momentum (OAM) with different orders ( $l=+1,l=+2,l=-1$ and $l=-2$ ) by applying corresponding transmission phase distribution schemes on the metasurface aperture. The generated vortex beams are at 5.14 GHz. The transmission loss is lower than 0.5 dB while the co-polarization level is −10 dB compared to the cross-polarization level. The measurement results confirmed the simulation results and verified the properties of the proposed designs.