A method to extract terahertz radiation from surface plasmon polaritons excited in the surface of a uniform graphene structure by an electron beam is presented. A sidewall configuration is proposed to lift the surface plasmon mode to be close to the light line, so that some of its harmonics have chances to go above the light line and become radiative. The harmonics are considered to be excited by a train of periodic electron bunches. The physical mechanism is analyzed with three-dimensional theory, and the harmonics excitation and radiation are demonstrated through numerical calculations.