Imaging quality depends on radiation performance of a transmitter and receiver, including beam width and side lobe levels. For imaging systems, side lobes of radiation sources should be minimum for optimum scanning and array configurations. A high sidelobe level can potentially disturb proper power detection in receivers affected by some radiation power emitted from side lobe radiation of an unexpected source. In this paper, we study a technique of side lobe level suppression in a THz bowtie antenna design on an extended hemispherical dielectric lens. Two methods are applied and reviewed in this research to reduce sidelobe level, i.e., antenna cross design to provide quasi-isotropic radiation pattern and optimum matching layers to collimate radiated waves from the substrate to free space. CST Microwave studio is used to simulate the model. The optimum side lobe level obtained from five optimum matching layers can be at-12.1 dB and-11.9 dB for E-plane and H-plane, respectively. This research is expected to contribute to THz imaging systems for enhancing image quality.