In recent years, Microwave Discharge Thrusters have been successfully deployed in various space missions, most notably Hayabusa and Hayabusa II, and continue to be the subject of ongoing research aimed at their improvement. Previous studies have demonstrated that increasing the frequency from 2.45 GHz to 4.25 GHz yields significant performance gains. Building upon these findings, this work experimentally examines the effects of further increasing the microwave frequency from 4.25 GHz to 5.8 GHz. The experiments were conducted at the Electric Propulsion Laboratory of ISAS, JAXA, Japan, where preliminary tests on the ion source had already indicated that operation at 5.8 GHz produces a higher ion beam current and, consequently, a higher specific impulse under specific configurations.