Creation of a magnetic boundary condition in a radiating ground plane to excite antenna modes

The creation of a magnetic boundary condition into the plane of symmetry in a radiating ground plane of a portable device is proposed to enhance its radiation efficiency and bandwidth. This magnetic boundary condition is achieved by exciting the antenna through symmetrical feeding. A novel antenna consisting of a symmetrical folded dipole is presented in order to exemplify how a broad bandwidth and very good radiation properties can be obtained with the proposed technique. The Theory of Characteristic Modes is used to analyze and compare a folded dipole with the symmetrical folded dipole. This study shows how the magnetic boundary condition favors excitation of antenna modes in a broad bandwidth while avoiding excitation of transmission line modes. Two different prototype antennas, a monopole and a symmetrical folded dipole with an LC-balun, were implemented and measured. The measured -6 dB impedance bandwidth of the monopole extends from 0.95 GHz to 2.15 GHz. In turn, the bandwidth of the symmetrical folded dipole extends from 0.90 GHz to 1.96 GHz. The average of the measured total efficiencies at the aforementioned bandwidth is -1.6 dB for both prototypes. The measured maximum total gain at 0.95 GHz is 2.6 dBi for the monopole and 1.9 dBi for the symmetrical folded dipole. © 2011 IEEE.