Gain Enhancement of Wideband Metamaterial-Loaded Loop Antenna With Tightly Coupled Arc-Shaped Directors

A wideband and unidirectional loop antenna with enhanced and stable gain is presented. To reduce the size of the antenna, the loop is loaded with three mu-negative (MNG) metamaterial unit-cells. The proposed unit-cells consist of a series-gap capacitor and a shunt meandered 'boating inductor. The series capacitive loading is used to excite the zeroth order resonance (ZOR) of the MNG unit-cell, whereas the 'boating inductor is used to improve impedance matching. To enhance the front-to-back-ratio (FBR) of the antenna and widen its operating bandwidth, the loop is loaded with a tuning slot in the vicinity of the loaded unit-cells. To overcome the low gain of the antenna at frequencies above the ZOR and thus obtain a stable gain, two tightly coupled dumbbell and arc-shaped directors are proposed. The shape and operation of these directors are inspired by the cylindrical capacitor, which achieves higher coupling compared to a planar capacitor with the same area. The antenna achieves a compact size of 0.25λ0× 0.29λ0with a wide fractional bandwidth of 34% at 0.76-1.07 GHz, and peak measured gain and FBR values of 4.5 dBi and 16 dB, respectively.