Bandwidth and directivity enhancement of metamaterial-loaded loop antennas for microwave imaging applications

Compact, wideband and unidirectional loop antennas are presented, that are based on a conventional planar loop antenna that is loaded in a mu-negative (MNG) fashion with series capacitors, enabling a mu-zero (MZR) resonance to be excited. This, together with the inherent loop resonance and a third resonance introduced by a director element, result in a wide impedance bandwidth and enhanced directivity. Two variants of the MNG-loaded loop antenna are presented; one with a uniform distribution of the MNG-unit cells, and one with a non-uniform distribution. It is shown that both antennas, which have a compact size, achieve a wide operating bandwidth in excess of 50% in the range of 0.6 to 1.1 GHz in the lower UHF band. The uniformly loaded antenna has a measured gain of 3.2 dBi, while the non-uniformly loaded antenna has a higher measured gain of 4.8 dBi. Both antennas have high front-to-back ratios in the range of 10 dB throughout the operating band.