Compact Wideband Loop Antenna Partially Loaded With Mu-Negative Metamaterial Unit Cells for Directivity Enhancement

A wideband and unidirectional loop antenna partially loaded with mu-negative (MNG) metamaterial unit cells is presented. To reduce the electrical size of the antenna, its first resonance is formed by capacitively loading a conventional one-wavelength loop antenna to excite the mu-zero resonance that is independent of the resonator's size. As a result of partial loop loading using properly designed and distributed slots, the amplitude of the surface current can be engineered to be higher in the feeding arm while achieving a zero phase shift along each arm of the loop structure. Consequently, unidirectional radiation with a moderate front-to-back ratio (FBR) is achieved without using any reflectors. Moreover, as a result of the capacitive loading, the loop antenna is divided into an array of dipoles that are excited with a 90° phase difference, enabling unidirectional radiation at the loop-mode resonance. To both enhance the impedance matching of the antenna at lower frequencies and excite an additional resonance, a strip patch is added in the vicinity of the loop. The proposed structure is smaller by 80% than similar MNG loaded loops and achieves a wide fractional bandwidth of 52% (0.64-1.1 GHz) with a peak FBR and gain values of 12 dB and 3.2 dBi, respectively.