Bi-State Frequency Selective Surfaces Made of Intertwined Slot Arrays

Novel arrangements of active frequency selective surfaces (AFSSs) with integrated voltage control wiring are proposed for bi-state (transparency/reflectance) operation at specified frequencies. The AFSSs are comprised of passive arrays of intertwined patterns of slots in a conductor screen and an active dipole array with pin diodes placed either on the same or opposite sides of a thin dielectric substrate. Simulation and measurement results show that such AFSSs exhibit good isolation (~ 15 dB) between the translucency and reflection states at normal incidence that slightly decreases at oblique incidence. The proposed AFSSs maintain the high angular and polarisation stability over broad fractional bandwidths (FBWs) inherent to the constituent periodic arrays of intertwined conductor patterns with substantially subwavelength unit cells. The merits of these AFSS arrangements also include resilience to parasitic effects of real switches, whose insertion loss in the on-state only enhances the AFSS on/off isolation. Such AFSSs are essential elements for reconfiguring and controlling the electromagnetic architecture of buildings

Superdirectivity from arrays of strongly coupled meta-atoms

This is the final version of the article. Available from AIP Publishing via the DOI in this record.We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-atoms of various shapes. The calculated bandwidth of 5 MHz for a dimer operating at 150 MHz indicates that, contrary to popular belief, superdirective antennas exist not only in theory but may have practical applications.Financial support by the John Fell Fund (University of Oxford) and by the EPSRC UK (SYMETA, EP/N010493/1) is gratefully acknowledged

Meta-molecular devices

We report the development of a new method to both describe and design metamaterials, meta-surfaces and meta-structures in which discrete passive structures are combined into ensembles. We call these structures, meta-atoms and the structures that result are meta-molecules. By using this concept new structures can be designed with a wide range of desirable properties

3-D printed bandpass filters with coupled vertically extruded split ring resonators

The additive manufacturing process of multimaterial extrusion offers performance advantages using functional materials including conductors while making accessible the third dimension in the design of electronics. In this work we show that the additional geometrical freedom offered by this technique can be exploited for the design and realisation of filters made of three- dimensional (3D) resonators that exhibit enhanced characteristics. The coupling properties of 3D grounded square split ring resonators (SRRs) are initially explored. We demonstrate by simulations and experiments that SRRs with finite height display significantly stronger coupling compared to equivalent thin printed circuit structures. The observed trend can be exploited for designing filters with wider operational bandwidths for a given footprint, or miniaturized layouts and enhanced compatibility with fabrication limits for minimum feature size and spacing without performance degradation. This concept is demonstrated by presenting results of full-wave simulations for sample bandpass filters with identical footprint but formed by coupled 3D square SRRs of different heights, showing that filters with taller resonators exhibit increasingly wider bandwidths. Two filter prototypes with centre frequencies at 1.6 GHz and 2.45 GHz are manufactured by multimaterial 3D printing. The measured characteristics of these prototypes are found to be in good agreement with numerical simulations taking into account the effect of the lossier metallic and dielectric materials used in 3D printing and confirm the predicted larger bandwidth of the filters made of 3D SRRs with marginally higher insertion losses