Abstract: A method and apparatus for the generation of an ionized gas or plasma for measurement of ultra-high frequency electromagnetic radiation utilizing a casing containing an ionizable gas, an oscillating circuit comprising an inductor and a capacitor in the casing, the oscillating circuit being adapted to resonate at a predetermined resonance frequency in the ultra-high frequency range and to have a Q factor in a vacuum of greater than 1 in order to render the circuit capable of producing a firing voltage across the capacitor for ionizing the gas when the inductor receives electromagnetic radiation near the resonance frequency, the casing further including optical conducting means having one end attached to the casing near the capacitor and means attached to the other end of the optical conducting means for analyzing the light transmitted from the conductor as a function of the power of the electromagnetic radiation.