[en] A novel foilless S-band low-impedance transit-time oscillator was designed. The impedance of this oscillator is about 20 Ω. Such an oscillator with coaxial structure has the advantages of low guide magnetic field and quick saturation time, expectantly operating repetitively and in a long-pulse state. As indicated by PIC simulations, the average power of output microwave is about 4.0 GW at the main frequency of 3.175 GHz with the input-diode voltage of 550 kV, the input-diode current of 27.6 kA, and the extra magnetic field of 0.8 T. Based on the simulation results, the power efficiency of microwave is about 26.4%, much better than that of a high-impedance transit-time radiation oscillator at the same input voltage. (authors)