The relationship between the monitoring pulse waveform and the waveform of equivalent radiation source is analyzed theoretically for the directional coupler.The impact of variation of the through port impedance on monitoring power of coupled port is studied experimentally with constant input power of the directional coupler.Experimental result is consistent with theoretical analysis.The effect of phase difference between the coupled port and the monitoring on the monitoring waveform is simulateded.The results indicate that: ① the intensity of radiation field shows a linear relationship with the injection voltage for non-linear interconnection system with invariable response;② the relationship between the monitoring voltage and the equivalent radiation supply voltage is linear when the system’s ports are matched;③ the phase difference between coupled port and monitoring port will result in distortion of the monitoring waveform

so only antisymmetric or asymmetric directional coupler be chosen as monitoring device.