Utilization of the near-millimeter-wave band is limited by atmospheric effects on propagation. Although this paper is restricted to clear-air effects, it is pointed out that these phenomena, which are molecular in origin, are also present to a significant degree in adverse weather. The paper surveys theoretical and observational knowledge concerning absorption, emission, refraction, and turbulence effects in clear air and in the spectral range 90 to 1000 GHz. Modeling practices are also reviewed.

This paper describes coherent sources of radiation useful in the near-millimeter-wavelength spectrum, generally taken to span the range of about 80 to 1000 GHz; however, both optically pumped and discharge pumped lasers are excluded from this treatment, since these devices are adequately covered elsewhere. Included in this discussion are solid-state sources such as IMPATTs and Gunn devices; tube sources such as klystrons, magnetrons, and backward-wave oscillators; and newer sources including TUNNETTs, gyrotrons, and relativistic electron-beam devices. Because of the large amount of material to be covered, the treatment is limited to a brief description of each device and an enumeration of its capabilities; no attempt is made to give a detailed discussion of device physics, as references are available for this purpose. Phase and frequency control of near-millimeter-wave sources is becoming increasingly important because of applications of this spectrum to Doppler radar, communication, and measurement systems. The paper concludes with a presentation of phase and frequency control results for both tube and solid-state sources.