We report the design, fabrication and characterization of two microwave-excited microplasma sources based on coaxial transmission line resonators (CTLR). The sources are capable of generating electric fields of ∼10 6 V m −1 at 900 MHz and 2.45 GHz. These devices can self-ignite helium or argon discharges in a wide pressure range including atmospheric pressure. The gas temperature in an argon discharge open to atmospheric air is ∼400 K. Using air as a dielectric, the working gases can be passed through the CTLR, resulting in the formation of plasma jets suitable for surface treatments. The device efficiency on transferring the input power into the plasma is 50-85% depending on the gas used. No thermal damage or electrode erosion has been observed in the devices.