Abstract: A device for propagating microwave energy to a plasma discharge tube from an antenna coupled to a microwave generator includes a hollow rectangular conduit and a strip-line resonator or strip-line conductor disposed therein. The discharge tube is mounted perpendicular to and through a central opening in the strip-line resonator and is disposed at a distance, from one end of the strip-line where the antenna is located, of one-half the wavelength of the microwave energy and one-quarter wavelength from the opposite end. The strip-line is attached to one end of the rectangular conduit and is supported by a dielectric member at a point one-quarter wavelength from the antenna. Microwave energy is propagated from the antenna by the strip-line dimensioned to the resonant frequency of the microwave the energy. A gas, such as argon or helium, is introduced into the discharge tube and is ionized by microwave energy to form a plasma.