Oxygen Absorption of RF at 60 GHz
The unlicensed 60 GHz frequency band extends from 57 GHz to 64 GHz and provides unique propagation characteristics that facilitate cost-effective short-range wireless links with high throughput (1000 Mbps; “Gigabit Ethernet over Wireless”)
The laws of physics make it the case that RF signals in the 60 GHz band are unable to travel long distances without significant reduction in signal power. There are three factors that typically cause signal attenuation: free space path loss, oxygen absorption and rain fading.
Oxygen absorption relates to the structure and characteristics of molecular oxygen (02) as found in the atmosphere. Normally an oxygen molecule (which is composes of two oxygen atoms which share electrons between them) is in an electrical ground state (a low-energy state.) The nature of the electrons around the two atoms (electron “spin”) is such that the molecule has a permanent magnetic moment – it acts like a very tiny bar magnet. This magnetic characteristic of the molecule allows it to couple to the magnetic field of an electromagnetic wave (the RF signal.) When the oxygen molecule couples with the electromagnetic field the field energy absorbed by the oxygen molecule results in a reduction in RF signal strength. At a frequency of 63 GHz the oxygen molecule absorbs maximum energy and hence a reduction in RF signal strength due to magnetic coupling.
The reason that the 60 GHz band is unlicensed is that transmitted energy simply can’t propagate very far. Because of limited propagation capabilities in 60 GHz there is limited potential for interference between users. The 60 GHz band is “carved out” of surrounding licensed frequencies because of the limited potential for interference between users in those frequencies. 802.11ad divides the 60 GHz unlicensed band (57.00 to 66 GHz) into four 2.16 GHz wide channels (3 of which are used in the U.S. and Canada). 802.11ad Channel 2 (59.40 to 61.56 GHz) is available in all regions and is the default channel.