An RKA(relativistic klystron amplifier) input cavity with dual coupling holes is analyzed by means of equivalent circuit method, and the scattering matrix (S^matrix) of the coupling system is obtained. Based on the derivative of phase of Szi with respect to its angular frequency, the expression of loaded quality(Q) factor of the system is derived. The value of loaded Q factor can be obtained upon the resonant frequency and group delay, given by high frequency field analysis software. The result is validated against that derived by field decaying method, with good agreement. Moreover, the study method is fast and can realize structure optimization with parameter scanning.%针对一种具有两个注入耦合口的相对论速调管放大器输入腔系统,利用等效电路方法,分析得到了系统的散射矩阵.在对散射矩阵元S21的相位对角频率求导的基础上,推导出了系统有载品质因数表达式.利用高频场分析软件给出的系统的群时延曲线,可得到计算有载品质因数所需的谐振频率和群时延时间,进而得到系统的有载晶质因数.与衰减法得到的有载品质因数进行比较,结果符合较好,但计算速度更快,且可进行参数扫描,以优化结构.
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