Millimeter wave Near-field focusing Cassegrain reflector antennas

The most common application of millimeter wave is the imaging from buried metals. 94 GHz is one of the best choices due to the signal loss curves. In this paper the antenna of a millimeter wave imaging system is studied. The main purpose is to provide an image with 2 cm × 2 cm resolution at 4 m distance (near-field). The power at this distance with 1 cm offset should be half of the power on the antenna axis without offset. Therefore the antenna diameter should be about 70 cm. The feed antenna has the -12dB beamwidth of 24° and its phase center is approximately placed on a single point for both e-plane and h-plane patterns. Different sub-reflectors are used to obtain the best efficiency for the near-field focusing antenna. Finally the effect of rotating the sub-reflector of antenna system is indicated for three angles on the form of field distribution and the efficiency of antenna system.