| Currently, domestic and overseas researchers focus on issue of measuring shield effectiveness of physically small enclosures in the field of measuring shield effectiveness of enclosures.It is a main stream to measure shield effectiveness of overmoded state enclosures by using reverberation chamber. Because of statistical uniform field in reverberation chamber and physically small cavity, probability distribution of electromagnetic field influence test.In this thesis, repetitive problem of measurement by frequency-stirred and combined-stirred is analyzed.then, confidence interval and uncertainty are ascertained.When 100 MHz is set as stirring bandwidth,the number of sampling must exceed 3 to ensure confidence interval is less than 3 dB by combined stirring method.Besides,when measuring shield effectiveness,the receive antenna in physically small enclosures is generally monopole antenna which is the high impedance compared with 50 ohm system. Broadband microstrip antenna which replace monopole antenna are used in shielding effectiveness test. The result of shied effectiveness by using broadband microstrip antenna coincide with the result of shied effectiveness by using monopole.it indicate that high impedance of monopole doesn't influence measurement of shield effectiveness of physically small cavity.At the same time, in order to demonstration accuracy of method of nested reverberation chamber, experiment adopts time domain method to measure shield effectiveness of physically small cavities.Moreover,in order to analyze wall effect of measuring shield effectiveness of physically small enclosures, this thesis deduces received power of monopole antenna which is close to metal wall,and compared theoretical calculation value with measured shield effectiveness of physically small enclosures by experiment. When the distance between monopole and wall of the cavity is about 0.15?,shied effectiveness of cavity measured by nested reverberation chamber are 3 dB more than shield effectiveness in ideal situation.Finnaly, this thesis analyze effect of measuring shield effectiveness of physically small enclosures when enclosures include load object.lt is elaborated that load object has influence on field distribution in cavity, when Q-factor of physically small cavity including load object is five times than limited Q-factor of physically small cavity,the distribution of electromagnetic field incavity including load object is still rayleigh distribution,and independent sampling points of measurement are also analyzed. |
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