Extremely high frequency 1

Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low (or far) infrared light, also referred to as terahertz radiation. This band has a wavelength of ten to one millimetre, giving it the name millimeter band or millimetre wave, sometimes abbreviated MMW or mmW.

Compared to lower bands, terrestrial radio signals in this band are extremely prone to atmospheric attenuation, making them of very little use over long distances. In particular, signals in the 57–64 GHz region are subject to a resonance of the oxygen molecule and are severely attenuated. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength. In climates other than deserts absorption due to humidity also has an impact on propagation. While this absorption limits potential communications range, it also allows for smaller frequency reuse distances than lower frequencies. The small wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.

Applications

Scientific research

This band is commonly used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues. Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The ITU non-exclusive passive frequency allocation at 57-59.3 is used for atmospheric monitoring in meteorological and climate sensing applications, and is important for these purposes due to the properties of oxygen absorption and emission in Earth’s atmosphere. Currently operational U.S. satellite sensors such as the Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15-18) satellites and the Special Sensor Microwave Imager Sounder (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range.