Abstract: The article gives the preliminary experimental results of a foilless low-impedance transition-radiation oscillator. In the experiments, copper dielectric and polymer velvet are respectively adopted as explosive emission cathodes and stainless steel and graphite are respectively adopted as collectors of exiting electrons. The experimental results from various states are compared and some possible causes affecting device operation are analyzed. As indicated by the experimental results, the combination of velvet cathode and graphite collector is more advantageous to microwave generation. Dissipating the energy of an intense relativistic electron beam and choosing a proper manner to collect electrons are important to realizing the stable operation of the oscillator.