  Focus scanning is critically important in many terahertz (THz) imaging and
sensing applications. A traditional single focusing transmitarray can achieve a
good focus when the source is on-axis but moving the source off-axis produces a
significant aberration. This paper presents a novel approach to reducing coma
in off-axis scanning in the THz region. Here, a dual transmitarray solution is
proposed, in which a transmitarray with an optimized phase profile is placed
behind a regular phase profile transmitarray. A linearly polarized,
dual-transmitarray antenna was fabricated for validation, and the focusing
performances were experimentally characterized. The measured results are in
good agreement with the theoretical ones. The generated spot of the
dual-transmitarray antenna remains focused on an angle up to 50deg, with a -3
dB spot size of less than 4 mm at 290 GHz. The measured near-field sidelobes
are all below -10 dB within the whole scanning range.
