Overview of Planar Antenna Loading Metamaterials for Gain Performance Enhancement: The Two Decades of Progress

Metamaterials (MTMs) are artificially engineered materials with unique electromagnetic properties not occurring in natural materials. MTMs have gained considerable attention owing to their exotic electromagnetic characteristics such as negative permittivity and permeability, thereby a negative refraction index. These extraordinary properties enable many practical applications such as super-lenses, and cloaking technology, and are used to design different electromagnetic devices like filters, polarization converters, sensors, and absorbers. Advances in MTMs have made new application fields to emerge in communication subsystems, especially in the field of antennas. MTMs are usually arranged in front or above the radiating element, or incorporated in the same substrate to improve the performance of planar antennas, in terms of improving directivity, gain, bandwidth, and efficiency, reducing the size and mutual coupling, and deflecting the radiation characteristics. High gain antennas are demanded in modern wireless communication systems. Their importance is in improving the signal strength by reducing the interference and alleviating the free space path loss. This review paper provides a brief introduction to MTMs, with the focus on their operating principles. Furthermore, a detailed study of antenna gain enhancement based on the various properties of MTMs is carried out. MTMs with low values of constitutive parameters; zero-index material (ZIM), low-index material (LIM), epsilon-near-zero (ENZ), and mu-near-zero (MNZ) are discussed in detail in the context of their capability to enhance the gain of a broad class of planar antennas. The low impedance property and lensing property, which is achieved by three different characteristics: high refractive index (HRI), gradient refractive index (GRIN), and negative refractive index (NRI) materials, are loaded to planar antennas for gain enhancement. The scope of this review has been limited to antennas that were experimentally validated in the respective source papers.