The purpose of this study is to understand the ICRF wave heating and the sideband coupling characteristics in the HANBIT mirror plasmas. The effects of the magnetic field strength on the RF field fluctuations have also been studied. Amplitudes and frequencies of the sideband field are found to depend on the applied magnetic field. When the magnetic field strength is over 0.229 T~0.233 T (/ci ~ 1), it is shown that the resonance of ICRF wave enhances the sideband coupling on the lower frequency side. The coupling of two split RF frequencies result in a beat wave signal. This phenomenon is shown to be particularly noticeable at a magnetic field of 0.229 T.