Design Considerations on the Minimum Size of Broadband Antennas for UWB Applications

The practical realization of Chu's minimum-sized antenna is investigated using self-complementary antennas. Rigorous bandwidth formulas for broadband antennas are derived for broadband antennas based on Chu's fundamental mode equivalent circuit. By comparing input impedances and radiation patterns, the fabricated self-complementary antenna used for Japan's ultra-wideband higher band (7.25-10.25 GHz) is found approximated by the minimum-sized antenna. Using the rigorous bandwidth formulas above, the fabricated antenna is shown to be smaller than the minimum-sized antenna. This breakthrough is achieved by using a more complicated matching circuit for broadband antennas instead of the single-stage inductor used in bandwidth estimation with the Q factor.