1. Institute for Pulsed Power and Microwave Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany
2. Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
3. Institute for Pulsed Power and Microwave Technology, and the Institute of Radio Frequency Engineering and Electronics, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany