The paper discussed the problem of multi-mode purification method used in TE0mn cylindrical cavity for measuring the complex permittivity of microwave dielectric material. The traditional method is applied, which is rejecting the degenerate modes by the non-contact between the end plate and cylindrical wall of the cavity and putting absorbing material at the end plate. The length of the cavity is fixed that is different from the traditional method in which the length of the cavity should be changed and the complex permittivity can be obtained by the changes of resonant frequency and unloaded quality factor after loading the material under test. The wanted modes are TE01n and TE03n and the position of the coupling holes should be able to couple TE01n and TE03n and to reject TE02n which is unwanted. Through the method mentioned above, the modes needed can be purified and the resonant frequency and unloaded quality factor can be measured more accurately, therefore, the measurement accuracy can be improved.
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