In this study, we use about 6.5 years of observation of Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite to study the spectral broadening of NWC ground transmitter signals and examine key parameters that control the width and intensity of the broadening power. First, we analyze a typical spectral broadening event, with enhanced wave intensity between lower hybrid resonance frequency and NWC signal frequency (19.8 kHz). The width and intensity of broadening power are positively proportional to the NWC wave amplitude. A following statistical analysis reveals a similar dependence on the NWC wave amplitude. The statistical analysis also indicates a significantly negative correlation of broadening spectral intensity and width with the background plasma density. The observations are consistent with existing theories predicting that lower plasma density drives a lower threshold for spectral broadening.