On False Radiation Enhancement of Small Antennas Coated with Materials and Metamaterials

摘要

Incorrectly claimed radiation enhancement of antennas using material and metamaterial coating is revisited. The antenna is represented by a coated spherical dipole, and its exact electromagnetic problem is solved analytically, in terms of spherical wave functions. Then, it is excited by a practical voltage source and the resulting antenna aperture voltage is determined using the equivalent circuit model of the source and the antenna, in terms of the source internal and antenna input impedances. It is shown that the coating parameters affect the antenna input impedance significantly, which causes dramatic changes on the antenna excitation voltage. In particular, at resonance of the modes inside the coating the antenna input impedance approaches zero, and the antenna aperture excitation diminishes accordingly. This dramatic variation of the antenna excitation voltage compensates for the perceived enhancement of the antenna radiation power due to the coating resonances. Consequently, the antenna radiation power remains finite and well behaved.