Abstract: During the debugging process of an airborne radar, there is an abnormal modulation signal with steady frequency in the electrical signal. Based on the fact that the failure only occurs in vibration environment, a preliminary conclusion is made that the abnormal modulation is caused by vibration. By analyzing the primary components of the radar, it is found that the natural frequency of the antenna fits the modulation frequency with highest robability. By finite element analysis and experimental verification for the antenna, it is finally concluded that the abnormal modulation is caused by the fact that the natural frequency of the antenna is in the scope of the first high-level narrow-band of the random vibration of the airplane. Design improvement is conducted for the antenna to make its natural frequency far from the high-level narrow-band. Finally the electrical signal becomes normal.