Compactness and miniaturization have become increasingly important in the development of high-power microwave devices.Based on this rising demand,a novel C-band coaxial transit-time oscillator (TTO) with a low external guiding magnetic field is proposed and analyzed.The proposed device has the following advantages: simple structure,short axial length,high power conversion efficiency,and low external guiding magnetic field,which are of great significance for developing the compact and miniaturized high-power microwave devices.The application of a shorter axial length is made possible by the use of a transit radiation mechanism.Also,loading the opening foil symmetrically to both ends of the buncher helps reduce the external magnetic field of the proposed device.Unlike traditional foils,the proposed opening foil has a circular-hole;therefore,the electron beam will not bombard the conductive foil to generate plasma.This makes it possible to realize long pulse and high repetition rate operation of the device in future experiments.Through numerical calculation and PIC particle simulation,the stability of the intense relativistic electron beam (IREB) and the saturation time of the device are improved by using the conductive foil.The voltage and current of the diode are 548 kV and 11.4 kA,respectively.Under a 0.4-T external guiding magnetic field,a C-band output microwave with a frequency of 4.27 GHz and power of 1.88 GW can be generated.The power conversion efficiency of the proposed device is about 30％.