Antenna Pattern of the Quasi-optical Hot-electron Bolometric Mixer at THz Frequencies

An investigation of the antenna pattern of a quasi-optical hot-electron bolometric mixer at terahertz (THz) frequencies is presented. The antenna consists of a logarithmic-spiral planar feed antenna which was mounted on the back side of a hemispherical lens with an extension length optimized for 2.5 THz. The power patterns were measured at several frequencies between 0.7 THz and 4.3 THz for two orthogonal polarizations. The level of the first sidelobe is smallest at 2.5 THz. The sidelobes are different for both polarizations and the main lobes are wider than predicted by diffraction at the diameter of the lens. This indicates that the pattern is not only determined by the lens but also by the feed antenna. The antenna pattern measured by heterodyne detection has almost the same width as the power pattern but significantly higher sidelobes. At 1.6 THz the phase pattern of the hybrid antenna was measured with a quasi-optical interferometric technique.