Propagation
Millimeter waves travel solely by line-of-sight, and are blocked by building walls and attenuated by foliage. The high free space loss and atmospheric absorption limits propagation to a few kilometers. Thus they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse.
They show "optical" propagation characteristics and can be reflected and focused by small metal surfaces around 1 ft. diameter, and diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading. Doppler shift of frequency can be significant even at pedestrian speeds. In portable devices shadowing due to the human body is a problem.
Read more about this topic: Extremely High Frequency