Q factors of 2 types of self-injection locked NRD guide Gunn oscillators;reflection and band-stop types,were calculated based on the Ohira's theory for the purpose of reducing the phase noise at millimeter wave frequencies.The calculated results were similar to the measured results,and hence the validity of the calculation.According to these results,it was obvious that the band-stop type had the narrow injection range,but the good noise reduction performance compared with those of the reflection one.Oscillation spectrums of the free-running and self-injection locked Gunn oscillators were calculated by using the calculated Q factors,and it was confirmed that agreement between the measured and calculated phase noise characteristics was quite satisfactory.