A novel broadband antenna designed for the terahertz (THz) frequency range is proposed and developed for the THz emitter on a photomixer chip. This THz emitter comprises an ultra-high-speed indium phosphide photodetector integrated with a planar THz antenna. This paper presents a novel broadband antenna configuration comprising a combination of bowtie and circular patch elements designed for the frequency range of 150 GHz to 500 GHz. Detailed parametric analysis of the antenna’s design parameters is also provided. The simulation results demonstrate that the optimized antenna achieves an impedance bandwidth of 350 GHz, satisfying the |S11| ≤ −10 dB condition, and exhibits a relative bandwidth of 107% within the 150 GHz to 500 GHz frequency range. This novel broadband terahertz antenna showcases an exceptional wideband performance and is highly suitable for high-speed transmission systems.