Experimental Investigation of Radiation Properties of Waveguides Filled with Uniaxial Single Negative Metamaterials

Basic radiation properties of a rectangular waveguide filled with anisotropic, either mu-negative (MNG) filling or epsilon-negative (ENG) filling are investigated experimentally. Two antennas based on waveguides filled with double-ring resonators (an MNG filling) and thin wires (an ENG filling) were designed, manufactured and measured in 8 GHz band. It was found that although these waveguides may in principle operate at frequencies arbitrarily lower than cut-off frequency they do radiate into free space. However, the radiation pattern and directivity are similar to those of equivalent apertures with uniform current distribution. Thus, the anisotropic single-negative (SNG) metamaterial filling cannot beat out the fundamental relations between antenna size and directivity. The resonant MNG filling offers narrowband operation and high degree of miniaturization, but (at the present state of the art) inherent losses cause poor gain (lower than 0 dB). On contrary, the non-resonant wire-based ENG filling offers low loss and broad bandwidth, but technological constraints limit lowest operating frequency (i.e. a degree of miniaturization)