Cyclotron-resonance maser (CRM) oscillator experiments in a nondispersive (TEM-mode) waveguide are reported in this paper. The table-top CRM oscillator constructed in our laboratory operates with a low-energy (< 5 keV), low-current (< 1 A) electron beam. The electron beam is rotating in the cyclotron frequency due to an axial magnetic field produced by an external solenoid. The large electron transverse velocity, needed to obtain amplification in a TEM-CRM, is achieved by a strong kicker coil. The coplanar waveguide used in this experiment supports odd and even TEM-modes, and enables cyclotron interactions with both first and second harmonics. Microwave output power at the first cycoltron harmonic is observed in the range of 3-6 GHz, where the frequency is tuned by the axial magnetic field in this range. A considerable second harmonic emission is observed around 7 GHz frequency. This experiment may lead to the developement of a new compact high-power microwave source.