Within the design of devices for aerospace applications, we focus on absorbers, impedance-matching lenses, and beamforming networks for satellite applications. The properties of glide-symmetric absorbers for guided waves have been studied and compared with non-glide symmetric metasurfaces and homogeneous materials. We are currently working on the design of metasurfaces capable to convert impinging plane waves on them into guided waves travelling along the surface (either with the excitation of leaky waves, or through a phase matching on a surface having a reflection coefficient with a linear phase gradient). The study is currently focused on the evaluation of the quality of this conversion, and on methods to absorb the guided wave with concentrated devices along the edges of the structure or with a suitable distribution of losses along the surface. In both cases the properties of glide-symmetric absorbers, studied in the first step of the work, are relevant for the future progress of the activity.
Impedance-matching lenses and beamforming with glide-symmetric metamaterials is performed by developing an analytical derivation to characterize composite materials in which a periodic arrangement of metallic pieces are embedded in a host medium. This characterization allows to obtain an equivalent refractive index and the wave impedance of modes propagating inside this artificial medium. The advantage of the formulation is that it is based on the processing of results of simulations with common commercial software and does not require writing an entire full-wave code with periodic boundary conditions. Knowledge of those parameters is expected to drastically change the design rules for lens based on artificial structures.
For automotive applications, the design of different glide-symmetric metamaterials based on different technologies and working in different ranges of frequencies are currently explored. In one project, the development of a novel gap-waveguide GS technology in PCB has been started, for the design of a slot array antenna for automotive applications. Frequency bands of interest are the W-band (76−81 GHz) and G-band (122.25−148.5 GHz), as initially planned. A gap waveguide based on a 1-D substrate-integrated-hole unit cells was designed. Manufacture limitations lead to the choice of a 1D glide symmetric, reaching a periodicity down to 500 microns to operate in the G band. Initial results show that the slot antenna based on this unit cell achieves a 14 GHz bandwidth with a gain compatible with automotive applications. In a second project, a new GS unit cell capable to efficiently confine the field in integrated waveguides in a multilayered configuration has been designed. The frequency-dispersive properties of this unit cell have been simulated with the multimodal transmission-matrix method (thus obtaining also the attenuation constant of the modes in stopband, not available in commercial software). Also a method based on an analytic formulation of a multiconductor transmission line has been developed and gives a very good agreement with full-wave simulations. The unit cell has been used to realize a waveguide which has been realized and measured, thus validating the theoretical tools and the capability of the unit cell to efficiently confine fields in the waveguide. Finally, a metamaterial unit cell working in the H band (220-330 GHz) consisting in a multi-layered all metal technology has been chosen. A groove waveguide with horizontal feeding has been designed, by suitably adding slots in the space between the metamaterial and the groove in order to prevent undesired leakage. Good results have been obtained in this configuration, so a more realistic scenario is now under analysis: a groove waveguide with a vertical transition.
