Electromagnetic field generators based on circular ring resonators,whose perimeters are integer times of equivalent wavelength,are well known to have attractive potential for producing radio vortexes carrying orbital angular momentum (OAM).We study the radiation characteristics of the generators based on radiation vector and antenna array theory.The behaviors of radiation pattes,field intensity and phase distribution are investigated in detail,and show classical features of OAM beams.The evolution of the generators performance versus the OAM state is also analyzed.The proposed generators can be realized by all kinds of microwave transmission lines,verified by two different prototypes.The discussions and conclusions drawn in this study are useful and meaningful for the radio OAM generator design.