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The majority of the designs of electromagnetic cloaks presented in the current literature require a metamaterial, the relative permittivity and/or permeability of which are smaller than one. Due to basic dispersion constraints, such permittivity (or permeability) can exist only within some restricted frequency band. This fact causes a passive electromagnetic cloak to be an inherently narrowband device. Here, it is shown both numerically and experimentally that the basic dispersion constraints of any passive medium might be overcome by using an active transmission line-based metamaterial. This type of metamaterial appears promising for the design of broadband cloaking devices.

Summary form only given. All known passive materials (or metamaterials) that have either negative (ENG, MNG) or less-than-unity (ENZ, MNZ) real parts of permittivity or permeability suffer from inherent narrow operating bandwidth caused by basic energy-dispersion constraints. Recently, it has been shown possible to go around these basic constrains by the use of so-called non-Foster elements (negative capacitors and negative inductors). Typical examples include both 1D and 2D ENZ metamaterials with extremely broad bandwidth varying from four to six octaves (S. Hrabar, I. Krois, et al., Negative capacitor paves the way to ultra-broadband metamaterials', Applied physics letters. Vol. 99, No. 25, 25403-1-25403-4, December 2011, S. Hrabar, Active non-Foster Metamaterials: From Intriguing Background Physics to Real-world Application, Proceedings on Metamaterials 2012, Sankt Petersburg, 2012, pp. 601-603). All these implementations contain electronic circuits that generate fixed ENZ values of effective permittivity.

An active dispersionless ENZ metamaterial, based on periodic loading of a transmission line with non-Foster negative capacitors, has been introduced recently. Direct experimental determination of the equivalent permittivity of homogenized active transmission line might be difficult due to inherent instability of negative capacitors. It is also difficult to develop a realistic model of active negative capacitor, which is a prerequisite for numerical homogenization. As a first step, one can model the active metamaterial as a transmission line periodically loaded with one-port ‘black boxes’, properties of which are derived from measured scattering parameters of a single negative capacitor. This method is demonstrated in homogenization of the experimental active ENZ metamaterial operating in the lower RF band (20MHz-120 MHz).