This study denotes a procedure of a small phase noise (SPN) oscillator employing three different substrate integrated waveguide (SIW) resonators as a frequency stabilization‐component. The SIW resonator as modifying phase noise (PN) is investigated and branchline coupler in the feedback loop is presented. The design technique to attain SPN usage for optimized efficiency is proposed. Then, three different X‐band SIW oscillators using a packaged GaAs FET transistor are planned. Due to the PN and compacting, the miniaturized conventional SIW (MCSIW) oscillator is selected. This MCSIW oscillator operates at 8.15 GHz, mentions a PN of −169 dBc/Hz at 1‐MHz frequency‐offset.