20 millimeter wavelength is about 15 GHz
called the super high frequency (SHF) band which encompasses frequencies from 3–30 GHz and wavelengths from 99.9–10.0 mm. It is used for radio astronomy, microwave devices/communications, wireless LAN, DSRC, most modern radars, communications satellites, cable and satellite television broadcasting, DBS, amateur radio, satellite radio.
Here's an article on the SHF band.
en.wikipedia.org/wiki/Super_high_frequency