At and below the Chu limit: Passive and active broad bandwidth metamaterial-based electrically small antennas

The solution to the canonical problem of a radiating infinitesimal electric dipole antenna that is centred in a multilayered, concentric metamaterial-based spherical shell system is presented. It is demonstrated that when this system is electrically small, a specifically designed homogenous and isotropic epsilon-negative (ENG) layer can function as a distributed matching element to the antenna enabling a resonant radiation behaviour. A finite element model of the corresponding centre-fed cylindrical dipole antenna-based resonant system confirms that such designed ENG-based spherical layers can act as a distributed matching element, which can be optimised to produce a reactance free, resistively matched and, hence, efficient radiating system. Several limits on the dispersion properties of the homogenous and isotropic ENG media used in these matching layers are considered and their impact on the bandwidth of these resonant systems is established. Although the dispersionless resonant antenna-ENG system has a bandwidth substantially below the Chu limit, the bandwidths of the corresponding dispersive systems are shown to be at or just slightly below the Chu limit. An analytical model of an idealised gaseous plasma-based ENG layer sandwiched between two glass layers, a potential realisation of these metamaterial-based ENG spherical shell systems, is introduced and its solution is used to study these efficiency and bandwidth issues further. Resonant systems based on active ENG metamaterial layers realised with two types of idealised gain medium models are shown to have bandwidths that approach the idealised dispersionless medium values and, consequently, are substantially below the Chu limit.