Also known as: atmospheric absorption, gaseous absorption, atmospheric attenuation
Atmospheric absorption is the loss of radio energy to the molecules of the atmosphere itself — chiefly oxygen and water vapour — as a wave travels through clear air.1 Unlike rain fade, which depends on weather, this is an ever-present, frequency-dependent loss that stays negligible below about 10 GHz but rises into sharp resonance peaks at millimetre-wave frequencies. Those peaks and the low-loss gaps between them — the atmospheric windows — shape which bands are usable for terrestrial and satellite links.
How it works
Gas molecules have quantised rotational energy states. When the radio frequency matches the energy of a transition, the molecule absorbs a photon, and the wave loses energy — a resonance line. Two gases dominate at radio frequencies:
- Oxygen (O₂). A dense cluster of magnetic-dipole lines near 60 GHz produces a very strong absorption band (roughly 15 dB/km at sea level), plus an isolated line at 118 GHz.
- Water vapour (H₂O). Electric-dipole lines at about 22 GHz and a much stronger one near 183 GHz, with absorption scaling with humidity.
Between these lines lie atmospheric windows — bands around 35, 94, 140, and 220 GHz where loss is comparatively low. Absorption also depends on path geometry: a satellite link at low elevation cuts a long slant through the dense lower atmosphere and suffers far more than one straight overhead. The ITU-R P.676 model gives the specific attenuation of oxygen and water vapour line by line for link planning.
Variants
The 60 GHz oxygen peak is a double-edged tool. Its heavy loss makes it useless for long-range links, but ideal for short-range, frequency-reuse-dense systems: 60 GHz Wi-Fi (802.11ad/ay) and indoor mm-wave backhaul exploit the fact that signals die out within a room or a city block, so the same channel can be reused nearby with little interference and good security. The windows, by contrast, are chosen for Earth–space links, mm-wave imaging, and radio astronomy.
Relevance to SDR
For the VHF/UHF land-mobile bands that trunking scanners monitor, atmospheric absorption is utterly negligible — P25, DMR, TETRA, and NXDN operate three orders of magnitude below the first significant resonance, so molecular loss never enters their link budget. It becomes relevant only when an SDR user reaches up into the microwave and millimetre-wave world: high-band satellite downlinks, 5G mm-wave, and experimental links must respect these peaks and windows in their band planning.
GopherTrunk decodes terrestrial land-mobile signals and does not model gaseous absorption; it is included here as the fundamental clear-air loss that, together with free-space path loss and rain fade, determines the total attenuation of any high-frequency link. The key intuition for band planners: absorption sets which millimetre-wave frequencies are even worth using.
Sources
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Radio propagation — Wikipedia, on gaseous absorption by oxygen and water vapour and the resulting atmospheric windows. ↩