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Plasma conductivity is of general interest for both fundamental research and specific applications. For this purpose, plasma equivalent impedance and complex conductivity are measured at 2.2 GHz, at pressures between 1 and 103 mbar, as a function of microwave power in a slot-type resonator, predominantly capacitively coupled to plasma. The plasma impedance is self-adjusting, maintaining a quasi-constant microwave amplitude. The sign of the imaginary part of the impedance (or conductivity) depends on pressure and, consequently, on electron density. The reactive part becomes significant if the Debye length is comparable with the size of the resonator and the plasma frequency is close to the microwave driving frequency.