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Closed form expressions for the modal dispersion equations and for the characteristic impedance of a metamaterial based gap waveguide
No full textIn a recent sequence of papers, a parallel-plate ridge gap waveguide has been introduced, that consists of a metal ridge in a metamaterial magnetic conductor surface, covered by a metallic plate at a small height above it. The gap waveguide is relatively simple to manufacture, especially at millimetre and sub-millimetre wave frequencies when compared with other solutions. The metamaterial surface is designed to provide a frequency band where parallel-plate modes are in cut-off, thereby allowing for a confined gap wave to propagate along the ridge. In a previous work, the authors have presented an approximate analytical solution for the confined quasi-TEM dominant mode of the ridge gap waveguide, when the surrounding metamaterial surface is in the form of a bed of nails. In this study, the authors investigation continues by providing an analytical expression of the modal dispersion equation of the first higher order ridge mode and of the characteristic impedance of the dominant mode. As in the previous paper, the field problem is divided in three regions, the central region above the ridge and the two surrounding side regions above the nails. Transverse mode-matching applied to a few modes representation in each region, results in a closed form expression of the dispersion equation of the first higher order mode. After summarising the formulation for the dominant quasi-TEM mode, the dispersion equation of the first higher order mode is derived, in order to give a criterion to maximise the unimodal bandwidth. Furthermore, three different closed form expressions of the dominant mode characteristic impedance are derived and compared with approximate expressions already used in literature
Recent developments in diffraction theory for impedance structures
No full textA brief overview of the most recent results relevant to the impedance wedge canonical problem has been presented in this paper. It is worth noting that there are still open problems to be faced with, as well as a continuous interest in the above canonical problems due to the need of efficient high frequency techniques in effective engineering numerical codes. The authors are presently working on revisiting some known exact solutions for both isotropic and anisotropic impedance wedges on the basis of the results recently obtained by Prof. Tiberio. Work is also in progress to derive heuristic approximate diffraction coefficients for edges in penetrable screens, starting from the exact solution for the scattering at the edge of a semi-infinite planar strip gratin
A leaky-wave groove antenna at optical frequencies
No full textIn the framework of nanoantennas functioning at optical frequencies, we present here a new kind of leaky-wave antenna realized as a groove in an aluminum superstrate, supported by a silver substrate. The antenna works in the optical frequency range where the silver acts as a dielectric with equivalent refractive index between zero and one. Under these conditions, the dominant mode launched in the structure exhibits a phase velocity larger than the speed of light in free-space, thus producing a leaky-wave radiation in free-space. We propose a simplified analytical form of the dispersion characteristic of the fundamental mode supported by the structure, which allows for identification of the radiative leaky-wave condition. We also propose approximate formulas for calculating the antenna gain and loss efficiency. The results obtained through these formulas are successfully compared with full-wave simulations. The final parametric study shows how the radiation characteristic is affected by the variation of geometric features. © 2012 American Institute of Physics
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