[en] A new microwave plasma source concept, particularly for use in the atmospheric pressure region, is presented where impedance matching is realized by a quarter-wavelength waveguide resonator structure. The waveguide is formed by a slot of 100 μm width machined into a copper sheet of 50 mm x 10 mm with a thickness of 200 μm. The slot length for a resonance frequency of 2 GHz is approximately 37.5 mm. This allows generating voltages high enough for ignition of an atmospheric plasma by this very small, simple and easy-to-fabricate structure. After ignition, the plasma extends over approximately 12 mm length of the slot, so a high electrode lifetime can be expected. Furthermore, the planar geometry of this source facilitates up-scaling to array configurations, which allows realization of a distributed source for treating large areas. Basic ignition parameters are investigated. Statistics for multiple ignitions are presented and the resulting Paschen characteristics are shown in the pressure region from 100 Pa to atmospheric pressure.