A method of plasma diagnostics comprising the steps of: - application of an alternating voltage of frequency f to a reference electrode of measurable area A AP located in a plasma discharge chamber; - generation of a first set of values of the electric current and voltage waveforms on the reference electrode; - termination of applying the alternating voltage to the reference electrode; - generation of a second set of values of the of the electric current and voltage waveforms on the reference electrode by subtracting the parasitic values of the current and voltage of the reference electrode from the first set of values; - and determination of the differential conductivity Y AP of the space charge sheath around the surface of the reference electrode and the differential capacity C AP of the reference electrode, wherein these quantities are calculated from the second set of values for the values close to the floating potential of the plasma excited in the discharge chamber, wherein the method further comprises the steps: - application of an alternating voltage of frequency f to a dielectric-covered main electrode placed in the plasma discharge chamber, wherein the area of the main electrode A HP is unknown; - wherein the value of the frequency f is the same value as the frequency f of the alternating voltage applied to the reference electrode and is in the range 0.01-100 MHz. - generation of a third set of values of the electric current and voltage waveforms on the main electrode; - generation of a fourth set of values of the electric current and voltage waveforms on the main electrode by subtracting the parasitic values of the current and voltage of the main electrode from the third set of values; - determination of the complex impedance Z HP of the main electrode from the fourth set of values - and calculation of the area of the main electrode A HP using the relation k = Y AP Re Z HP Y AP 2 + ω 2 C AP 2, where the coefficient k corresponds to the ratio of the areas of the main electrode and the reference electrode and ω is the angular frequency of the voltage. - application of an alternating voltage of frequency f to a reference electrode of measurable area A AP located in a plasma discharge chamber; - generation of a first set of values of the electric current and voltage waveforms on the reference electrode; - termination of applying the alternating voltage to the reference electrode; - generation of a second set of values of the of the electric current and voltage waveforms on the reference electrode by subtracting the parasitic values of the current and voltage of the reference electrode from the first set of values; - and determination of the differential conductivity Y AP of the space charge sheath around the surface of the reference electrode and the differential capacity C AP of the reference electrode, wherein these quantities are calculated from the second set of values for the values close to the floating potential of the plasma excited in the discharge chamber, - application of an alternating voltage of frequency f to a dielectric-covered main electrode placed in the plasma discharge chamber, wherein the area of the main electrode A HP is unknown; - wherein the value of the frequency f is the same value as the frequency f of the alternating voltage applied to the reference electrode and is in the range 0.01-100 MHz. - generation of a third set of values of the electric current and voltage waveforms on the main electrode; - generation of a fourth set of values of the electric current and voltage waveforms on the main electrode by subtracting the parasitic values of the current and voltage of the main electrode from the third set of values; - determination of the complex impedance Z HP of the main electrode from the fourth set of values - and calculation of the area of the main electrode A HP using the relation k = Y AP Re Z HP Y AP 2 + ω 2 C AP 2,