反射系数相位法计算谐振腔外观品质因数的局限性

Limitation of Q_(ext) calculation method based on reflectance phase

分析了反射系数相位法原理中所采用的完全耦合电路和实际等效电路之间的差异,指出采用反射系数相位法计算谐振腔外观品质因数失效的原因是:传输线与谐振腔只通过部分电感耦合;π模双间隙腔中存在耦合槽。定量计算表明:当谐振腔中耦合电感与非耦合电感之比小于0.1,或者缝模频率小于2π模频率时,反射系数相位法就会失效。结合实际谐振腔的参数值进行分析得出:导致反射系数相位法失效的最主要原因是传输线与谐振腔只通过部分电感耦合,并且耦合电感太小。

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 phase is sure to fail in calculating the Qext 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.