Based on the proposed miniaturized polarization conversion metasurface (PCM), a checkerboard metasurface is designed, which can realize radar cross section (RCS) reduction in the ultra-wide range of 4.3~13.5 GHz (103%) by using the principle of reflection phase cancellation. The characteristic mode analysis (CMA) method is introduced to establish the checkerboard PCM as the radiation patch, and a wideband low-profile antenna for the C-band is achieved, with a bandwidth of 4.8~6 GHz (22.2%). The highlight and advantages are that the size of the checkerboard PCM is significantly reduced, and its side length is close to the radiation wavelength of the antenna. The antenna element based on this checkerboard PCM has the characteristics of small size and wide frequency range of RCS reduction, and the low-RCS antenna array can be realized by paving them seamlessly. Moreover, the checkerboard PCM not only suppresses scattering but also improves antenna performance, plays an important role in achieving wideband and enhancing radiation directivity.

In this article, a compact and low-profile beam steering circularly polarized folded reflectarray antenna (CPFRA) with low scan loss for wide-angle coverage is proposed and designed. In order to obtain lower scan loss over a wider beam coverage range (BCR), a multibeam optimization method considering the illumination weighting factor is introduced to obtain minimum phase errors for the designed beam direction. To improve antenna compactness and alleviate the mutual couplings among multiple feeding sources, small-size substrate integrated waveguide (SIW) cavity-backed slot antennas are employed as the feeding sources, which are integrated with the single-layer reflector board. An incident-angle-insensitive circular polarizer is designed and integrated over the antenna aperture to achieve circularly polarized (CP) radiation over a wide BCR. Finally, a 15-beam CPFRA working at the Ka-band with a size of 140 mm $\times140$ mm is designed, manufactured, and measured to validate the proposed design method. Measurement results show that the designed CPFRA can achieve a rather flat gain, with only a 2.1 dB scan loss, across the wide 3 dB BCR of ±29°. The good CP beams are formed over the entire coverage range without any blind zone. With the advantages of wide-angle coverage, low scan loss, and high integration, the proposed beam steering antenna is promising in applications such as 5G communication, radar imaging, and target recognition.

A new design method for ultrabroadband radar cross section (RCS) reduction by exploiting characteristic complementary polarization conversion metasurfaces (PCMs) is proposed and validated. The proposed method lifts the conventional bandwidth limitation enforced by the performance of a single PCM and expands the RCS reduction bandwidth. A systematic strategy for ultrabroadband RCS reduction design is developed and an effective reflection coefficient amplitude of the composite surface ( $\Gamma _{ {eff}}$ ) is derived as a new RCS reduction indicator. Based on this indicator, complete polarization conversion (CPC) frequency points of PCM are identified as important characteristics, and PCM pairs with interleaved CPC points, termed as initial PCM (I-PCM) and complementary PCM (C-PCM), are designed to compensate each other for ultrabroadband RCS reduction. A scaling-and-tuning two-step method for designing the C-PCM of a specific I-PCM is developed and validated. To further improve RCS reduction performance, taking the ${\Gamma _{ {eff}}}$ as an indicator, the particle swarm optimization (PSO) algorithm is used to select the optimal I-PCMs, C-PCMs, and their area ratios. Finally, an ultrabroadband RCS reduction surface is designed, fabricated, measured, and validated. It realizes 10-dB monostatic RCS reduction ranging from 7.6 to 26.2 GHz (110.7% relative bandwidth) which exceeds the polarization conversion bandwidth of each individual PCM. Besides, good polarization insensitivity and bistatic RCS reduction performance are also obtained. The proposed method provides a new route for designing broadband RCS reduction surface based on PCMs with unsatisfactory characteristics and greatly alleviates the performance requirement for PCMs.

The whip antennas used for HF/VHF band communication on the vehicles or ships are generally larger in length and diameter. It would be of great significance if it could be easily hidden from microwave detection radar searches. Aiming at the lower RCS of whip antenna in X-band and maintaining the original impedance and radiation characteristics under high power input, we propose a whip antenna integrating a novel curved artificial magnetic conductor (AMC) to achieve the RCS reduction in X-band. The backscattering of the cylinder can be dispersed by using the 180° phase difference cancellation principle of different AMC structures, and the mechanism is demonstrated by simulation and experiment. The measurements are in good agreement with the simulations, and the measurement results indicate that in the band of 8 ~ 12 GHz, the backward RCS is reduced by an average of 11.2 dBsm in VV polarization and by 10.2 dBsm in HH polarization. Moreover, the proposed whip antenna can be applied to high-power HF/VHF transmitting systems, and the withstand voltage can reach 15kV (continuous output above 10KW). This method has important theoretical significance and practical values for the whip antenna in practical engineering applications.

A super diffuse broadband radar cross section (RCS) reduction surface design method based on rotated phase coding polarization conversion metasurfaces (PCMs) is proposed. Multiple PCMs with coding phases are employed for wideband RCS reduction, and the spatial locations of the PCMs are utilized for bistatic RCS reduction since it arouses spatial phase. Further, it is derived that the Pancharatnam-Berry (P-B) phase provides extra design freedom for bistatic RCS reduction. These three types of phases are combined for optimal wideband bistatic RCS reduction. To facilitate bistatic RCS reduction design. the variance of the scattered field (δ _E ) is derived as an indicator for the evenness of the whole space electric field distribution, and an optimal design method is developed thereafter. For comparison, two RCS reduction surfaces, termed as MS1 and MS2 respectively, composed of phase coding PCMs are designed, fabricated, and measured to demonstrate the effectiveness of the proposed method. The MS1 is designed by only using spatial phase, and multiple coding phases, while MS2 is designed by further exploiting the P-B phase. The MS2 achieves 10-dB monostatic RCS reduction ranging from 8.8 to 25.4 GHz (97.1% relative bandwidth, which is 2.2% better than that of the MS1). More importantly, the MS2 realizes super diffuse characteristic under different incidences. It achieves 10-dB bistatic RCS reduction ranging from 9 to 25 GHz (94.2% relative bandwidth), and the maximum bistatic RCS reduction can reach upto 20 dB. The proposed broadband super diffuse RCS reduction design method is particularly desirable for both monostatic and bistatic RCS reduction of various complex coupling structures.

In this letter, a model of wide-area distribution multistatic $\text{T}^{M}$ - $\text{R}^{N}$ ( $M$ independent transmitters and $N$ independent receivers) sky-wave over-the-horizon radar (WDMS-OTHR) is investigated. A localization algorithm that utilizes the hill-climbing algorithm based on the weighted least squares (HC-WLS) method for this model is also proposed. The localization algorithm mainly includes three stages. First, the geodetic coordinate of station position is transformed into the spatial rectangular coordinate. Second, the initial position is estimated by WLS that forms a pseudolinear equation from the measurements of the time of arrival (TOA) and introduces nuisance variables as additional unknowns. Third, the spatial rectangular coordinate of the initial position is transformed into the geodetic coordinate, and the initial position is optimized by HC that performs eight neighborhood searches around the initial position and adopts multiple iteration searches until the optimal position is obtained. Numerical simulations demonstrate that the positioning accuracy of the proposed algorithm is better than WLS.