The design of electromagnetic invisibility cloaks is based on singular mappings prescribing zero or infinite values for material parameters on the inner surface of the cloak. Since this is only approximately feasible, an asymptotic analysis is necessary for a sound description of cloaks. We adopt a simple and effective approach for analyzing electromagnetic cloaks - instead of the originally proposed singular mapping, nonsingular mappings asymptotically approaching the ideal one are considered. Scattering and radiation from this type of imperfect cylindrical cloaks is solved analytically and the results are confirmed by full-wave finite element simulations. Our analysis sheds more light on the influence of this kind of imperfection on the cloaking performance and further explores the physics of cloaking devices.
A frequency tunable half‐wave resonator at 3 GHz is presented with a microelectromechanical systems (MEMS) variable capacitor as the tuning element. The capacitor is fabricated using the multi‐user MEMS process (MUMPs) technology provided by JDS/Cronos, and transferred to an alumina substrate by an in‐house developed flip‐chip process. This capacitor is electrostatically actuated. The resulting C‐V response is linear with a slope of 0.05 pF/V for a wide range of actuation voltages. The MEMS device has a capacitance ratio of 3:1 for 0‐70 V bias, with a Q‐factor of 140 measured at 1 GHz. A half‐wave tunable microstrip resonator with bias lines is designed to include this MEMS device, which exhibits linear tuning over 180 MHz (6 percent) centered around 3 GHz with a constant 3 dB bandwidth of 160 MHz over the entire tuning range. The power consumption of the MEMS device was measured to be negligible.
The effective constitutive parameters of a hightemperature superconducting (HTS) metamaterial are extracted from measured scattering parameters at liquid nitrogen temperature (≈75.68 K at the National Institute of Standards and Technology, Boulder, CO, USA). The YBa 2 Cu 3 O 7 split-ring resonator (SRR) arrays are placed inside a WR-90 waveguide with two different orientations and are excited with the dominant TE 10 mode in both cases. By treating the SRR array as a homogeneous medium described by the biaxial permittivity and permeability diagonal tensors, the two sets of S parameters allow for the extraction of three out of the six tensor components. The extracted parameters are then compared to full-wave simulations of SRR arrays in free space. The samples are measured from 8.2 to 12 GHz and show a frequency band between 9.25 and 10.45 GHz with a negative effective permeability, as expected. Finally, numerous high-quality factor resonances that accompany the main resonances are observed in both the low-temperature measurement and the low-loss simulation.Index Terms-High-temperature superconducting (HTS), magnesium oxide (MgO), split-ring resonators (SRRs), yttrium barium copper oxide (YBCO).
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