The purpose of this paper is to present the complex antenna factor of an EMI dipole antenna with a balun consisting of two coaxial feeders(coaxial cable balun) for measuring electromagnetic fields. A new formula of complex antenna factors for an EMI antenna with coaxial cable balun is derived using by power loss concepts. The complex antenna factor shows that the present result in this study was identical with that of the result from S-parameters. The theoretical complex antenna factors derived by power loss concepts are in good agreement with the experiments.