There is a strong need for wideband and sensitive receivers in the terahertz (THz) region. This chapter focuses on the detection of continuous waves (CW) in the THz, using innovative bolometric sensors working as mixers for heterodyne reception with down conversion to the GHz range, as needed for radio astronomy, remote sensing, or passive imaging. In this case, the coupling to the incident THz radiation is accomplished by means of multi-octave planar micro-antennas. After selecting the antenna shape as angular, self-complementary, and/or self-similar, various antenna geometries are studied, namely, bow tie, Sierpinski fractal, sinuous, and log-periodic. Delivered power, directivity, and radiation pattern are discussed. According to seven criteria, the log-periodic antenna is preferred.