Experimental demonstration of superdirectivity for coupled dimers of meta-atoms

Radkovskaya, A.; Vallecchi, A.; Li, L.; Faulkner, Grahame; Stevens, Christopher J.; Shamonina, E.

doi:10.1109/metamaterials.2016.7746388

Cited by 6 publications

(6 citation statements)

References 11 publications

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“…The above results of the analytical model, taking advantage of incorporating a realistic tapering of the current density on the resonant SRRs both to predict the inter-element coupling and their combined radiation properties, are consistent with FW simulations and experiments. These results all confirm that only the "CC" configuration for dimers made of coupled SRRs leads to superdirectivity, as previously briefly reported in 27 and illustrated in detail in the next section. It is noteworthy that from a physical standpoint this finding appears justified based on the observation that the "CC" meta-dimer is the only one providing simultaneous, approximately in anti-phase, excitation of both the in-plane electric and out-of-plane magnetic dipole responses of each of the SRRs forming the dimer, in full consistency with superdirectivity requirements.…”

Section: Resultssupporting

confidence: 90%

“…These waves provide a high spatial frequency current distribution as is required for superdirectivity. More recently, a superdirective antenna design using coupled SRR dimers was reported 27 , which in both simulations and experimental results exhibited superdirective emission only a fraction of a decibel below the theoretical maximum for a two element dipole array. In order for a subwavelength device to achieve superdirectivity, it is required that the constituent resonators have high quality factors Q and strongly couple to each other with a high coupling constant κ.…”

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confidence: 98%

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Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

Vallecchi

Radkovskaya

et al. 2020

Sci Rep

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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. Directional and smart antennas are one of the approaches that could be used to increase the capacity and flexibility of wireless systems in future integrated gigabit communications (5G and beyond). The advantages of a steerable antenna system include the possibility of tracking a terminal signal and mitigating interference between network nodes, thus improving network capacity. Directional antennas can also improve power efficiency 1. The most common technique to realize a directive antenna consists in assembling a multiple-antenna array in which the radiation pattern can be reinforced in a particular direction and suppressed in undesired directions. By individually controlling each array element in phase and amplitude, the direction of radiation can be electronically steered (phased arrays) 2. Multi-element arrays are by now widely used in based stations of cellular networks, but simple implementation for mobile devices is severely constrained by the need for small antennas, the conventional phased array techniques for enhancing the directivity leading to a significant increase of the antenna size and requiring a separate RF chain for each antenna element. On the other hand, antenna miniaturization is usually achieved at the expense of efficiency, bandwidth, directivity and gain, but especially miniaturized antennas tends to present a practically omnidirectional radiation pattern. However, future regulations might require the use of adaptive directional antennas also for mobile terminals, due to the invaluable role...

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Section: Resultssupporting

confidence: 90%

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confidence: 98%

Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

Vallecchi

Radkovskaya

et al. 2020

Sci Rep

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“…Superdirective antennas are used in cases when high directivity (narrow beams in order to avoid electromagnetic pollution) is preferred over efficiency. By generalising the method developed in the present paper for use at higher frequencies will enable us to achieve reasonable radiation efficiency [25]. The method developed in the present paper assumed that the elements can be treated as simple magnetic dipoles with real coupling constant.…”

Section: Discussionmentioning

confidence: 99%

“…Directivity of antennas can be increased by using metamaterial-inspired structures [21], [22], [23]. [24], [25], [26], [27] studied metamaterial superdirective arrays both analytically and experimentally. Ludwig et al [28] considered superdirectivity in studying near-field subwavelength focusing.…”

mentioning

confidence: 99%

“…In dimers comprising resonant meta-atoms only one element is driven and the others are passive [19]. Recent studies [24], [26], [25], [27] provided an explanation that the desired rapid spatial variation of currents required for superdirectivity is imposed via to magnetoinductive waves [32] propagating by virtue of magnetic coupling along chains of coupled metaatoms. Radkovskaya et al [27] postulated two 'superdirective conditions' that have to be satisfied for two coupled resonators to become superdirective.…”

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A Method for Optimising Superdirectivity of Coupled Meta-Atoms via Planar Directivity Evaluation

Yan

Radkovskaya

Khromova

et al. 2020

IEEE Open J. Antennas Propag.

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We propose a simple and rapid way of optimising directivity in metamaterial-inspired endfire antenna arrays with strong interelement coupling. We introduce, in addition to the traditional 3D directivity, also its planar equivalents defined as the ratio of the power density in the desired endfire direction to the average power density calculated either in the horizontal (azimuthal) or vertical (elevation) plane. Using dimers of magnetically coupled split-ring resonators with only one element driven by an external source, we derive conditions that must be satisfied in order to realise superdirective current distributions. The superdirective conditions link the quality factor of the resonators and their coupling constant to the array size and the operating frequency. We demonstrate that a rapid measurement in the azimuthal plane can be used as a reliable indicator for whether the superdirective conditions for the 3D directivity are satisfied. Analytical calculations are verified by CST simulations in the MHz frequency range for meta-atoms of circular and square shape. Our method can be extended to arrays comprising larger number of meta-atoms of various shape and would enable rapid prototyping of 3D-printed meta-atoms with desired radiation properties.

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Wideband and polarisation‐independent antireflection coating using metamaterials

Sepehripour

Karimi

Khavasi

2020

IET Optoelectronics

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In this paper, we propose a new approach for realizing antireflection coating using metamaterials. In this approach, a subwavelength array of metallic pillars (with square cross-section) is used for implementing antireflection coating. The effective impedance of the array can be duly adjusted by the size and distance of pillars. Therefore, we design the effective impedance of the antireflection coating to be the geometrical mean of the upper and lower mediums impedance and we choose its height to be a quarter of operating wavelength. Consequently, the reflection vanishes at the desired frequency and fractional bandwidth of 56% is achieved with a criterion of 10% reflectance (the refractive index of the substrate is assumed to be 4). The proposed structure is symmetric in both directions. So, it is not sensitive to the polarization of the incident wave at normal incidence. Furthermore, we show that using the multilayer Chebyshev matching transformer of transmission line theory increases the bandwidth of the antireflection up to 107% at the expense of pass-band ripples. This structure can be used from very low frequencies up to infrared regime by appropriate scaling.

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Experimental demonstration of superdirectivity for coupled dimers of meta-atoms

Cited by 6 publications

References 11 publications

Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

A Method for Optimising Superdirectivity of Coupled Meta-Atoms via Planar Directivity Evaluation

Wideband and polarisation‐independent antireflection coating using metamaterials

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