This study investigates the theoretical system ca-pacity of single-carrier orbital angular momentum (OAM) multiplexing considering the radiation pattern and polarization in the presence of ground reflection. Assuming that the adoption of a highly directional antenna is required to overcome the large propagation loss in the extremely high-frequency bands, we incorporate the impact of radiation pattern into the system-capacity analysis of single-carrier broadband OAM multiplexing. Moreover, because the impact of reflected waves depends on the polarization plane, which determines the reflection coefficient, we demonstrate its difference in the system capacity by assuming vertical and horizontal polarization. Regarding the analytical evaluation in the presence of reflected wave, the consideration of either the radiation pattern or difference in the polarization planes has been unfortunately lacking. As for radio propagation, we utilize the 3GPP technical report (TR) 38.901 as a radiation pattern and clarify its impact with and without frequency domain equalization (FDE) compared to the isotropic-antenna case using analytical approaches.