This paper presents a terahertz high gain beam steering transmitarray antenna (BSTA) working at 340 GHz. Substrateless double hexagon ring slots unit-cells which present low loss characteristics at THz band are used to constitute the layout of THz BSTA. To improve the beam steering performance, bifocal technique is used to design the layout of BSTA. Because the fabrication risk of the THz BSTA prototype increases a lot as the aperture
dimension is enlarged, four inch silicon wafer is chosen after weighting the risk and gain of the BSTA. Micromachining process is used to fabricate the large aperture THz BSTA to ensure the machining accuracy of the unit-cells. The measured results of the prototype show that the THz BSTA could realize -15°~15° range beam scanning with gain > 38.3 dB, scanning loss < 1.2 dB and side lobe level < -17.8 dB, by moving the feed along the focal plane of the BSTA.
