ELF/VLF waves are generated via amplitude-modulated heating of the lower ionosphere. Based on the modulated heating theory, the directional radiation models of dual beam amplitude modulation (DAM) and circle geometric modulation (CGM) are established by introducing the phased array ideas, which are validated by comparison with the experimental data. According to the models, differences between the above two directional radiation models and the normal amplitude modulation (AM) are analyzed with parameters of HARRP phased-array HF facility, and the influences of the modulation frequency (fELF/VLF) and elevation of the heating waves on mode are investigated as well. Compared with the AM mode, DAM and CGM can realize the directional radiation, and enhance the radiant intensity by properly setting the initial phase, the modulation frequency (fELF/VLF) and angle of inclination of the heat wave to the vertical direction. The ELF/VLF radiant intensity may be increased by 11.3 dB when replacing AM with CGM.