The Arecibo HF heater is normally composed of two separate sets of antenna array, transmitting waves vertically at the same frequency and polarization. However, when these two sets of antenna array radiate at slightly different frequencies, the intensities of the HF heater-induced plasma lines (HFPLs) can be drastically modulated. In recent Duncan et al/ s (1989) experiments the 100% intensity modulation of HFPLs was seen to persist even when the secondary set of antenna array radiatied at a few perdents of the power transmitted by the primary set of antenna array. We offer an explanation and show that there exists a minimum power Pmin, which can be roughly estimated from Pth[(E0/Eth)-1>2 where Eth and Pth are the threshold field and threshold power of the parametric decay instability, respectively, E0 is the field intensity of heater wave. If the secondary set of antenna array radiates at a power lower than Pmin, the 100% intensity modulation of HFPLs will not be observed. The functional dependence of Pmin on the difference frequency of the two sets of antenna array is also predicted for future experiments to corroborate.