In this paper, we first demonstrate a metasurface that offers spectral filter performances not yet reported for a single-layer metasurface because of the limited number of geometric parameters. The configuration of the metasurface is determined by a conformational space annealing algorithm, which is a key toward being able to design for a desired frequency, bandwidth, and response type for various wireless communication devices, including the fifth-generation (5G) millimeter-wave communication devices. As a proof-of-concept, we fabricate a very thin (50 μm) single-layer metasurface combined with a 5G antenna array to experimentally demonstrate high frequency-selectivity with stable frequency responses with oblique incidences and polarizations. The gain of the 4 × 4 patch antenna array with the metasurface bandpass filter was reduced by only 0.48 dB compared with that of the 4 × 4 patch antenna array itself at 27.5 GHz, whereas the gain of the 4 × 4 patch antenna array with the metasurface bandstop was reduced by 20 dB compared with that of the 4 × 4 patch antenna array itself at 28.5 GHz.

In this letter, we propose a multifunctional frequency selective surface (FSS) designed for wide incident angles, optimized for X-band reflection and K-/Ka-band transmission. To verify its applicability, we analyzed the performance variations of the FSS with respect to polarizations and incidence angles. The proposed FSS facilitates space communication in the K/Ka-band, while simultaneously minimizing electromagnetic interference for X-band defense applications. The pixelated structures of the FSS are optimized to realize fine frequency selectivity in the target frequency bands. The proposed FSS is implemented in a double-layer to provide higher-order characteristics. To validate its performance, measurements are conducted in a free-space measurement system with spot-focusing lens horn antennas. The proposed structure demonstrates a -1.5 dB bandwidth from 17.41 to 20.97 GHz in the pass band of the K-band and 30.09 to 33.24 GHz in the pass band of the Ka-band, with fractional bandwidths of 18.6% and 9.9%, respectively. Additionally, it satisfies -15 dB blocking performance across the entire X-band.

We propose dual-band filters made from a single-layer metasurface for operating on the 28 and 39 GHz bands in mm-wave fifth generation (5G) communication systems. We use the conformational space annealing method, a global optimization technique, and test the usefulness of dual-band metasurfaces through design, fabrication, and measurement with 2916 square-shaped pixels per unit cell defined on a single layer. Each square-shaped pixel has only two states: either filled with metal or free of metal. We were able to obtain transmissive metasurfaces capable of performing three different functions, bandpass/bandpass, bandpass/band-stop, and band-stop/bandpass, by setting three different objective functions applicable to 5G communications. The designed filters have insertion loss of less than 3 dB in the passband region and greater than 20 dB in the stop band region. As a proof-of-concept, we fabricate very thin (50 μm) single-layer metasurfaces etched on only one side of a single dielectric substrate combined with a 5G antenna array to demonstrate high-frequency-selectivity with dual-band operation experimentally.

Synthetic aperture radar automatic target recognition (SAR ATR) is a powerful tool for recognizing targets. SAR ATR needs a massive database of inverse synthetic aperture radar (ISAR) images from computer aided design (CAD) models for multiple aspect angles. To rapidly build the multi-view database, we present a fast method to simulate the ISAR images over multiple aspect angles using the shooting and bouncing ray (SBR) technique. Compared to the previous fast method where a ray tracing process must be performed at each aspect angle of the ISAR images, the proposed method generates the ISAR images over multiple aspect angles through only a one-time ray tracing process at an arbitrary angle. Therefore, the computational time is decreased as much as the number of ray tracing processes is decreased. The ISAR images of a tank CAD model are simulated over multiple aspect angles to verify the performance of the proposed method.

In this letter, we propose multifunctional radome-based frequency selective surface (FSS) composites for stealth applications. The proposed composite simultaneously implements band stop characteristics in the X-band and band pass characteristics in the K- and Ka-bands. The composite consists of a sandwich structure, where two FSS layers are separated by a constant spacing. Thick dielectric materials with higher permittivity and loss tangent than air degrade the filtering performance. We thus performed conformational space annealing (CSA)-based optimization on both the FSS shape and dimension, resulting in a miniaturized pixelated FSS. This approach effectively addresses the downward shift in the resonance frequency by employing an FSS that is 22% smaller than a standalone FSS. We validated the proposed composite through simulations and measurements under varying polarizations and incident angles. The proposed composite achieved insertion losses greater than 20 dB across the entire X-band at a 45° oblique incidence in transverse electric (TE) mode. Under transverse magnetic (TM) mode at a 30° oblique incidence, the composite allowed transmission in the dual K-band and Ka-band while maintaining insertion losses below -18 dB within the stop-band.