In this report, the phenomena of low-frequency spoof surface plasmon polaritons (spoof SPPs) in a new metallic cylindrical antenna at terahertz (THz) frequencies are demonstrated. The antenna is designed as a periodic structure whose unit cell includes an open hollow ring (OHR) as compared with the conventional periodic subwavelength corrugated metallic wire (PSCMW) with cylindrical structure. Near- and far-field distributions, dispersion properties, and radiation efficiency are numerically evaluated and then compared for both PSCMW and PSCMW-OHR. It is indicated that when the dispersion line passes through the light line into a radiation zone, the field becomes a leaky mode. We find that the far-field radiation of the leaky mode can generate frequency scanning effects with a scanning angle of 23°, and the radiation efficiency can be increased to 90% as the number of periodic unit cells is large enough.