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It was recently demonstrated that a horizontal dipole radiates with a vertical monopole-like radiation pattern when placed near a two-dimensional periodic patch-loaded grounded slab (PLGS). Such a structure has been proposed as a low-profile surface-wave antenna. In this letter, we examine the radiation mechanism of such an antenna. It is indicated in this letter that the vertical monopole-like pattern is generated by the vertical common-mode current flowing on the outside of the coaxial cable feeding the short dipole, and not from the dipole itself. In this manner, the unbalanced horizontal dipole closely resembles a classic T antenna. Furthermore, we examine the propagation characteristics of the surface wave on the PLGS and discuss its contribution to the overall radiation of this antenna. We find that, in the lower frequency range, the PLGS appears insignificant to the antenna radiation. The investigation is carried out by the use of a full wave method-of-moments simulation.