In this article, an orbital angular momentum (OAM) vortex wave antenna with low radar cross section (RCS) and reduced divergence angle is proposed. The antenna is composed of four slot-coupled elements working from 7.5 to 8.0 GHz and fed by a phase shift network. OAM vortex wave with l = 1 is generated. In order to reduce the RCS and divergence angle simultaneously, complementary split-ring resonators (CSRRs) are designed and integrated to the antenna. The CSRRs can stop the transmission of surface wave and reflect the incident energy with phase canceling scheme. As a result, the RCS and divergence angle is reduced simultaneously.

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In order to improve the research of electromagnetic scattering characteristics and mechanism of the foam in the marine environment, considering the complexity and sensitivity of the electromagnetic scattering characteristics of the sea surface, the electromagnetic scattering characteristics of the foam sea surface are studied by the improved small slope approximation (SSA) method. The double Debye dielectric constant model is adopted to calculate the dielectric constant of seawater and foam. Based on the Elfouhaily sea spectrum model, the expression of scattering coefficient of the foam sea surface under SSA is derived, and then the theoretical calculation results of the SSA method are compared with the finite‐difference time‐domain method and the measured values in the literature, which verifies the effectiveness of the proposed algorithm. The model of the relationship between the thickness of the foam layer and the wind speed is established, and the composite backscattering characteristics of the ocean below the wind speed of 20 m/s are simulated. It is found that as the wind speed increases, the foam layer becomes thicker and the scattering intensity increases. The foam layer will weaken the ocean specular reflection and increase the diffuse reflection, which is more obvious when viewed at grazing and large incidence angle, and when the incident angle is greater than 83 degrees, the inflexion point caused by the foam will be clearly observed in the backscattering curve. The results of this study can provide a reference for civil and military activities and optimise the detection capability in the marine environment.