Limitation of Q_(ext) calculation method based on reflectance phase
The principle of the method based on reflectance phase for calculating the Q_(ext) 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 Q_(ext) 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 phase is sure to fail in calculating the Q_(ext) of the cavity.Finally,by analyzing all the possible values of the parameters of the practical cavities,a conclusion is got that the primary cause of the failure of the method is that the transmission line couples with the cavity by a too small part of inductance in the cavity.