This work presents the utilization of square ring slots as resonators on active and passive metasurfaces operating within the K a-band. Thereby, fully functional prototypes of reflectors and lenses based on a two-layer printed circuit board were fabricated and verified. The analytical model of the square ring slot structure is depicted extensively, which allows finding simulation optimized designs that can be applied to the passive prototypes. Additionally, the gained information on self-resonances is used for combinations with layers, activated through lumped varactor diodes. In the end, the reflection and transmission parameters of single active and passive designs as well as beam patterns of the lenses are measured and interpreted, resulting in a switching dynamic range of 15 dB at a bandwidth of 1 GHz for active reflectors and a maximum gain of 22 dBi at 26 GHz for the passive lens.

This letter presents a concept for a surface exhibiting tunable reflectivity that operates within the K-band using varactor diodes. Thereby, the impedance of a two-layer metasurface is controlled through lumped components at its back side and not from the front, where discrete elements would scatter the incident wave. A reflector's design, verification, and modelling through an equivalent circuit as well as full-wave simulations are presented in detail. Finally, the tunable reflection parameters of the surface as well as its reflection beam pattern under different biasing conditions are shown.

The presented method describes reflection and transmission of electromagnetic waves at multiple closely stacked metal grids using multi-modal S-parameter propagation. For that, the surface admittance of every metal layer was determined, through a novel semi-analytic approach that uses simple Fourier transformations instead of solving an integral equation. The modal components of this surface admittance were used to express the generalized scattering matrices of the individual grids. By applying multi-modal propagation techniques to the resulting scattering parameters, it was possible to model the electromagnetic interactions within a multilayer stack of periodic impedance sheets. Resulting reflection and transmission parameters perfectly matched the corresponding full-wave simulations even above the grating lobe regime. In the end, the universality of the proposed method was proven on a layer stack, connected to lumped components.