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A four-leaf-clover-shaped antenna composed of a highly resonant radiation element and a stable DC feed element and mounted on an extended hemispherical lens is described. The proposed antenna was optimized to have maximum total efficiency and maximum directivity on an extended hemispherical lens. Compared to a full-wavelength dipole antenna on a similar lens, we found a six-fold increase in the input resistance and an improvement in the total efficiency by a factor of 2.7. This is expected to reduce the impedance mismatch between the photomixer and antenna and lead to a significant increase in the THz output power.