Photoconductive antenna as continuous-wave terahertz photomixer has attracted much attention in these years. Its main restriction is the limited terahertz radiant power. In this paper, terahertz emission from dipole photoconductive antenna structures based on the photomixing mechanism has been studied. The terahertz output power is proportional to the radiation resistance of the antenna, the square of dc conductance and that of bias voltage. The photoconductive antenna radiation impedance as a function of frequency has been calculated numerically. And results reveal that the dipole photoconductive antenna design dominates the properties of the radiated output at resonant frequencies below 1THz, while the efficiency at higher frequencies is additionally dependent on the design of the interdigitated fingers, the effect of which is modeled by a lumped capacitance in parallel with the photoconductive area.