A new mechanism-"Pertubation Augmentation" is proposed to raise the sensitivity of the sensor for microwave moisture measurements. In the light of it

a high sensitive sensor is designed. The sensor is the open type of rectangular cavity

which resonates at two frequencies and has a eva-mode cavity mounted on it. Theoretical analyses for this kind of sensor are given. The results of experiments made on this sensor agree well with the theoretical analyses. A sheet of capacitor paper

which has a thickness of 8μm and a moisture of 8%

can cause the measuring mode a frequency shift of about 3MHz at the temperature of about 20℃.