This paper proposes a compact Q-band relativistic backward wave oscillator (RBWO) operating at low magnetic fields, aiming to advance the miniaturization of high-power microwave (HPM) sources. The device consists of a resonant cavity reflector and two sections of periodic slow-wave structures (SWS), designed in a coaxial configuration. By exploiting the inherent characteristics of the coaxial structure, an optimal inner diameter was selected to enhance power handling capacity while mitigating space charge effects induced by the reduced device size. Through comprehensive simulation and optimization, the device achieved a microwave output power of 470 MW with an efficiency of 39.1% under operational conditions of a guiding magnetic field (0.9 T), a diode voltage of 400 kV, and a beam current of 3 kA. The generated microwave signal exhibited a central frequency of 45 GHz.