Orbital angular momentum (OAM) beam emitters with compact structure and high performance are highly desirable for wireless communication and radar technology. Here, we propose a compact emitter that only consists of a ring resonator and a feed line. Continuously-variable-mode OAM beams are generated by adjusting the wavelength and transmission path. The basic design principle and specific evaluation index are discussed. Both the simulated and experimental results demonstrate that the proposed emitter obtains the capacity of generating variable-modes. This approach opens a way for designing novel OAM beam emitter with desired properties.