Limitation of Qext calculation method based on reflectance phase

The principle of the method based on reflectance phase for calculating the Qext of cavities is analyzed and the differences between the ideal coupling circuit used in the method and the practical equivalent circuit are pointed out in this paper. Firstly, the transmission line couples with a cavity by only partial inductance in the practical circuit. Secondly, there are coupling slots in π mode double-gap cavity. These two differences will bring on failure of the method in calculating the Qext of cavities. Quantitatively, the calculation results show that, if the ratio of the coupling inductance to the non-coupling inductance is smaller than 0.1, or the resonant frequency of the coupling slots is smaller than the 2π mode frequency of a double-gap cavity, the method based on reflectance ph