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.
Due to their high frequency they do not propagate well down coax cables.  For this reason they are primarily transmitted through waveguide. A waveguide is a structure which guides energy waves, such as electromagnetic waves or sound waves. Waveguides are metallic lines, similar in fashion to transmission lines, which are used at microwave frequencies. They are typically utilized to connect receivers and transmitters with antennas.
There are several advantages to waveguide:

• A waveguide is completely shielded.
• Waveguides have no trouble transmitting extremely high peak powers.
• The waveguide has an almost non-existent loss at microwave frequencies.
• Waveguides can also be pressurized with special gases to increase maximum power levels. They are typically lined with silver plating to cut down on the resistance loss, which makes your average copper or aluminum waveguide that much more efficient.