Miniaturization of Planar Yagi Antennas Using Mu-Negative Metamaterial-Loaded Reflector

A technique to reduce the size and enhance the operating bandwidth (BW) of planar Yagi antennas, while maintaining their unidirectional radiation characteristics, is proposed in this paper. In this technique, the reflector element is loaded with mu-negative (MNG) metamaterial unit cells to excite a mu-zero resonance (MZR), which is independent of the size of the antenna. Thus, the antenna can operate at lower frequencies without the need to increase its physical size. To implement the proposed idea, the reflector, which acts as the host transmission line, is positioned at a close distance to the driven element, which is typically a dipole antenna. To validate the proposed technique, two design concepts based on coplanar and broadside-coupled strip configurations are presented, and a wideband Yagi antenna is presented. The antenna achieves a wide operating BW of 70% (0.72-1.48 GHz) with a peak front-to-back ratio (FBR) of 14 dB and peak gain of 4.1 dBi with a compact size of 0.24λ× 0.24λ, where λ is the wavelength at the lowest operating frequency. The proposed antenna achieves a 45% wider operating BW compared to similar structures, while being miniaturized by more than 60% compared to available structures that have over 50% fractional BWs.