Theoretical study on the radiation efficiency of electrically small and thin spherical shell antennas

Electrically small antennas have always attracted a lot of interest because these antennas have been utilized for the mobile communication system. However, these antennas have been suffered from low performance such as the low gain, the narrow bandwidth (high Q factor) and the low radiation efficiency. The radiation efficiency is especially important since this parameter affects the energy efficiency of the whole communication system. In recent years, electrically small folded spherical helix antennas have begun to draw attention since these antennas are employed to obtain low Q. A lot of antenna designers have challenged to design high efficiency antennas, while only a few researchers published results on the theoretical limitation of the radiation efficiency. Theoretical research on the property and the condition of extremely small antennas may enable us to propose a global strategy to design high efficient antennas.