Broadband electromagnetic cloaking

Pekka Alitalo, Olli Luukkonen, Constantin Simovski, Sergei Tretyakov

Introduction

Electromagnetic cloak is a device that minimizes total scattering of illuminating field. Electromagnetic cloaks could be used for instance to minimize effect of supporting structure of antenna to the radiation. One the main challenges is to make such a device reasonably broadband.

Simulated E-field inside the measurement waveguide at 3.2 GHz with cloak (upper) and without cloak (lower).

Transmission-line cloak

Instead of guiding the fields around the cloaked object, the transmission-line technique enables fields to travel through the object. This minimizes the problems related to the finite propagation time of wave. Transmission line network has to be placed inside the object which sets limits to cloaked object.

Cloak for cylindrical objects

Thin parallel metal sheets do not disturb propagating electromagnetic wave if the polarization is correct. The idea of cloak design is to gradually decrease the distance between plates when approaching the cloaked object. This forces the amplitudes of electric and magnetic fields to become higher close to the cloaked volume. Also the structure remains impedance-matched to free space. Wave travelling towards the cloaked volume can not continue inside the central opening, but it is forced to propagate around the cloaked object.

The dimension of cloaked objects are approximately one third of wavelength at microwave frequencies. The same design has been optimized also for visible range, but fabrication of such structure is not easy.

References

1. Physorg.com: Broadband invisibility in the microwave range
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5. Pekka Alitalo, "Microwave Transmission-Line Networks for Backward-Wave Media and Reduction of Scattering", Dissertation for the degree of Doctor of Science in Technology.
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