To better understand the working principles of the helical antenna, the ESPRIT algorithm is introduced to extract the different current modes on the antenna. The strengths and phase velocities of the current modes are calculated and their effects on the polarization and gain of the antenna are investigated. We found that both the reflected current and their image currents contaminate the circular polarization purity of the antenna. Also it was found that the dominant radiation mechanism on the helical antenna switches from the T0 mode to the T1 mode as the frequency is increased. These physical interpretations are facilitated by an accurate decomposition of the current. Such an approach may be particular useful in gaining insights for designing more complicated helical structures.