Atmospheric Loss
Understanding Atmospheric Loss
Every radio signal that passes through the Earth's atmosphere loses some energy. Even on a perfectly clear day with no rain, oxygen and water vapor molecules absorb electromagnetic energy at characteristic frequencies. Atmospheric loss is this baseline, always-present attenuation that must be included in every RF link budget calculation.
The Absorption Spectrum
Atmospheric absorption varies dramatically with frequency:
- Below 10 GHz: Atmospheric loss is negligible (< 0.01 dB/km). This is why radar, satellite, and cellular systems at lower frequencies are minimally affected by the atmosphere.
- 10–100 GHz: Loss increases with frequency, with sharp peaks at oxygen (60 GHz, ~15 dB/km) and water vapor (22 GHz, ~0.2 dB/km) resonances.
- Atmospheric windows: Between absorption peaks, relatively low-loss transmission windows exist at 35 GHz, 94 GHz, and 140 GHz, which are exploited by radar and communication systems.
Impact on Link Budgets
For satellite links at Ku-band and above, atmospheric loss is a significant link budget entry that must be carefully modeled. The loss depends on the elevation angle — a satellite at 10° elevation passes through much more atmosphere than one at 60° elevation. Low-elevation satellite links at Ka-band may experience 6–10 dB of clear-sky atmospheric loss, requiring correspondingly larger antennas or higher satellite transmit power.
Key Equations
γtotal = γO2 + γH2O dB/km
O2 absorption peak:
60 GHz: ~15 dB/km
H2O absorption peak:
22.2 GHz: ~0.2 dB/km
183 GHz: ~30 dB/km
Comparison
| Frequency | O2 | H2O | Total | Notes |
|---|---|---|---|---|
| 10 GHz | 0.01 | 0.01 | 0.02 dB/km | Clear window |
| 30 GHz | 0.03 | 0.05 | 0.08 dB/km | Ka-band |
| 60 GHz | 15 | 0.1 | 15.1 dB/km | O2 peak |
| 94 GHz | 0.4 | 0.4 | 0.8 dB/km | W-band window |
| 140 GHz | 0.1 | 3 | 3.1 dB/km | Marginal |
Frequently Asked Questions
What is the ITU-R P.676 model?
ITU-R Recommendation P.676 provides the internationally accepted model for computing atmospheric gaseous attenuation as a function of frequency (1–1000 GHz), air temperature, atmospheric pressure, and water vapor density. The model combines the absorption contributions of individual oxygen and water vapor spectral lines using a line-by-line calculation. It is the standard reference used by satellite operators, radio link designers, and regulatory bodies for atmospheric loss calculations.
How does altitude affect atmospheric loss?
Atmospheric density decreases exponentially with altitude. A satellite ground station at sea level sees more atmospheric loss than one on a mountaintop, because the lower atmosphere is denser and contains more water vapor. The ITU-R model accounts for station altitude. High-altitude observatory sites (like Mauna Kea at 4,200 meters) are chosen specifically because they are above much of the atmospheric water vapor, reducing loss at submillimeter wavelengths by a factor of 5–10 compared to sea-level sites.
Is atmospheric loss the same everywhere on Earth?
No. The gaseous loss from oxygen is relatively constant globally (since oxygen concentration is uniform). But water vapor concentration varies dramatically with climate — tropical coastal regions have much higher water vapor content (and therefore higher water vapor absorption) than dry, high-altitude continental interiors. Link budget calculations must use site-specific climate data (from ITU-R P.836 water vapor maps) rather than global averages.