Abstract
Superdirective antennas developed over the last century have received renewed interest in recent years from the development of metamaterials. These arrays of electromagnetic resonators (or meta-atoms) carrying short wavelength electro- and/or magneto-inductive waves support current distributions with very high spatial frequency as required by the classical conditions for superdirectivity. As meta-atoms can have both electric and magnetic dipole characteristics (and hence radiation properties), developing antennas exploiting these distributions can challenge conventional intuitions regarding the optimal configurations required. In this work we are reporting the development of a genuinely superdirective array using split ring resonators (SRRs). We provide a comprehensive analytical model characterizing the radiation from SRR dimers in which excitation of only one split ring leads to superdirective radiation via mutually coupled modes. Our model exploits simple circuit descriptions of coupled resonant circuits, combined with standard radiation formulae for curvilinear current distributions. Using this simple model we are able to map directivity against possible SRR locations and orientations in two dimensions and identify the unique optimal configuration which meets the requirements for superdirective emission. We validate the theoretical findings by comparison to both full wave simulations and experiments showing that our SRR dimer achieves endfire directivity very close to the maximum theoretical value.
Highlights
Superdirective antennas developed over the last century have received renewed interest in recent years from the development of metamaterials
I0 is the current provided by the source
The analytical model we have developed allows the rapid calculation of the directivity of our dimer, regardless of the orientation of its two split ring resonators (SRRs), and can be used to create maps of the maximum possible directivity for all dimer configurations generated as the SRR orientations are varied
Summary
Superdirective antennas developed over the last century have received renewed interest in recent years from the development of metamaterials. Due to the small inter-element spacing and the large magnitudes of the excitation coefficients, ohmic losses increase and the antenna efficiency decreases very sharply While these factors hampered the achievement of significant superdirectivity, interest in superdirective antennas and arrays resurfaced intermittently through the years (for details of these advances see, for example, reviews in3,9), and has continued up to recently[10,11,12]. In20,22 it was shown theoretically that far-field superdirectivity could be achieved using arrays formed by two or three split ring resonators (SRRs) These structures relied on the physics of slow electro- and magneto-inductive wave propagation[25,26]. The overall radiation properties of dimers formed by SRRs depend strongly on how these radiation contributions combine in the far-field, which in turn is sensitive to the relative orientation of the SRRs
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