The coupling modes and efficiency in terms of transmitted and absorbed powers were studied using an open-ended microwave applicator (MWA) provided with a magnet. This configuration allowed the acquirement and investigation of plasmas over four decades in pressure, from 10−4 to a few torrs, for powers up to 200 W. Plasma impedance measurements revealed different coupling modes and transitions from the capacitive to the inductive mode. These also permitted the unraveling of the conditions for which maximum power and heating efficiencies can be obtained, which were found to be primarily linked with the resistive coupling. Moreover, the MWA was seen to be effective in extending the domains of pressures (1–50 mTorr) and powers (higher than 60 W), maintaining at least 80% of the transmitted/absorbed power within the working range. However, by knowing the values of impedance for different pressures, the MWA design can be adjusted for other domains as well.