High-frequency cylindrical magnetic cloaks with thin layer structure

Highlights: • Broadband cloaks for harmonic magnetic fields can be made using isotropic materials. • Analytical formula for permeability required to achieve cloaking effect is presented. • Bandwidth of the cloak may be changed by adjusting the conductivity of an inner layer. • It is possible to find this bandwidth using introduced concept of cloaking frequency. The article presents a design methodology of a thin layer, high-frequency cylindrical invisibility cloak intended for quasi-static magnetic fields. The proposed bilayer structure of a cloaking shell is synthesized using isotropic ferromagnetic and conductive materials. An analytical formula is derived to find the properties of a ferromagnetic layer in the wide frequency spectrum of an external time-harmonic magnetic field. The cloaking properties of the shell can be adjusted by the selection of conductive materials and estimated using an introduced formula. Analytical calculations are accompanied with numerical results of exemplary cloaking structures. The thin layer structures are able to maintain cloaking effect from a specific frequency, up to high frequencies of the magnetic field as long as displacement currents can be neglected. Additionally, the results of an experimental verification of the broadband magnetic cloak efficiency are reported. The cloaking shell composed of ferromagnetic polymer and conductive materials is subjected to the time-harmonic magnetic field. Three samples are presented and the magnetic field distortion, measured in the background area, is discussed. In order to obtain a desired effective electrical conductivity, a method of fabricating conductive layer, utilizing copper film on polytetrafluoroethylene base, is used. The experimental results are compared with both 3D and 2D numerical model of the cloak.