Development of millimeter-waves in 1960s and 70s-its background and limitation

Millimeter-waves have been attracting new attention for versatile radio communications use. In the 1960s and 70s, the transmission of millimeter-waves by use of a circular waveguide was developed for post-coaxial and post-microwave transmission medium. After the invention of a semiconductor laser and an optical fiber in 1970, however, the millimeter-waves have handed over the mission as a main high-capacity and long-distance transmission medium to optical fibers. We review the millimeter-wave technology developed in the 60s and 70s, which gives new ideas for future further progress