Abstract In the Ion Cyclotron Range of Frequency (ICRF) heating experiment, impedancematching is of great practical significance, because wide variations in antenna loading are observedwithin the discharge, in tokamaks operating in H-mode. A sudden decrease in antenna loadingaccompanying the L-mode to H-mode transition typically occurs on a timescale of a few millisec-onds, as does the increase in loading at the H- to L-mode transition. Therefore, it is necessary tomatch dynamically in the transmission line between the generator output and the antenna inputconnections. A new type of stub tuner being developed utilizes the difference in radio-frequencywavelengths between gas and liquid due to different relative dielectric constants. The impedancematching can be adjusted in realtime in an attempt to track the variations in the antenna loading.Since there are no mechanically moving parts in the short ends of stub, the change can be moreconvenient and safe, moreover, it can withstand higher voltage without Abstract In the Ion Cyclotron Range of Frequency (ICRF) heating experiment, impedancematching is of great practical significance, because wide variations in antenna loading are observedwithin the discharge, in tokamaks operating in H-mode. A sudden decrease in antenna loadingaccompanying the L-mode to H-mode transitionmost occurs on a timescale of a few millisec-onds, as does the increase in loading at the H-to-L-mode transition. Thus, it is necessary tomatch dynamically in the transmission line between the generator output and the antenna inputconnections. A new type of stub tuner being developed utilizes the difference in radio-frequency wave lengths between gas and liquid due to different relative dielectric constants. The impedancematching can be adjusted in realtime in an attempt to track the variations in the antenna loading.Since there are no mechanically moving parts in the short ends of stub, the change can be moreconvenient and safe, moreover, it can withstand highe r voltage without