This paper describes part of the experimental results from an optimization work done on a coaxial vircator. The microwave source is driven by a Marx generator and excludes any type of pulse forming device between the two subsystems. There are two main reasons for not including any pulse forming device. The first is the final goal to build an extremely compact and robust high power microwave system. The second reason is to get as high energy efficiency as possible from the system, calculated as the ratio between the output microwave energy and the stored energy in the Marx generator. The vircator operates with an applied maximum voltage somewhere between 400–500 kV and the impedance is between 25–35 Ohms during the process when the main part of the microwave radiation is generated. The microwave pulse is for most of the cases as long as 400 ns. For the experiments described in this paper focus has been on variations in the axial geometry and on different emitter materials. Results that are given include typical current and voltage signals, RF-signals and corresponding spectral contents and the normalized radiated RF-energy.