🌫️ Atmospheric Gaseous Attenuation

ITU-R P.676 clear-air loss — oxygen + water vapour, specific / zenith / slant path

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Link

Surface Atmosphere

Results

Oxygen γ_o
– dB/km
Water-vapour γ_w
– dB/km
Total specific γ
– dB/km
Zenith attenuation
– dB
Slant-path @ elevation
– dB
Equivalent heights & breakdown
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📖 How to use

  1. Enter frequency (1–54 GHz) and elevation angle; pick an atmosphere preset if unsure.输入频率(1–54 GHz)和仰角;不确定就先选大气预设。
  2. Set surface pressure, temperature and water-vapour density for your site.设置站点的地面气压、温度和水汽密度。
  3. Read the specific (dB/km), zenith and slant-path gaseous attenuation.读取比衰减(dB/km)、天顶和斜路径气体衰减。
  4. Add the slant-path loss alongside rain fade in your link budget (they are separate effects).把斜路径衰减与雨衰一起加进链路预算(两者是独立效应)。
  5. Arrived via the ↗ link next to the Link Budget's atmospheric-loss field? Click the return bar at the top to carry the slant-path loss straight back (the bar never appears on a direct visit).如果是点链路预算大气损耗字段旁的 ↗ 链接跳转而来,点顶部的返回条即可把斜路径衰减直接带回(直接访问不会显示返回条)。

About gaseous (clear-air) attenuation

Even with no rain or cloud, the atmosphere absorbs radio energy through two gases: oxygen (with a strong absorption complex around 60 GHz and weaker effects below) and water vapour (with a notable absorption line at 22.235 GHz). This calculator implements the ITU-R P.676-11 Annex 2 method — the approximate equivalent-height method, pinned to that revision because Annex 2 was restructured in P.676-12.

What it computes

Specific attenuation γ = γ_o + γ_w (dB/km) at your surface conditions, then the zenith attenuation via equivalent heights (A = γ_o·h_o + γ_w·h_w) and the slant-path loss A / sin(θ) for elevation angle θ.

Gaseous loss is small compared with rain at Ku/Ka-band but is not negligible — especially near the 22 GHz water line and for low elevation angles or humid sites. Add it alongside the rain-fade term in your link budget. This implementation is valid for 1–54 GHz (below the 60 GHz oxygen complex) and for elevation angles of 5°–90°, which is the range over which Annex 2's cosecant law holds; lower angles need the full path integration of Annex 1, so an elevation below 5° is raised to 5° and flagged. Zenith attenuation from this method is accurate to about ±10% for dry air and ±5% for water vapour up to roughly 10 km altitude. All computation runs locally in your browser.

关于气体(晴空)衰减

即使没有降雨或云层,大气也会通过两种气体吸收无线电能量:氧气(在 60 GHz 附近有强吸收复合带,其下方也有较弱效应)和水汽(在 22.235 GHz 有一条显著吸收线)。本计算器实现 ITU-R P.676-11 附件 2 方法(等效高度近似法)。锁定该版本是因为附件 2 在 P.676-12 中已被重构。

计算内容

比衰减 γ = γ_o + γ_w(dB/km),基于你的地面条件;再通过等效高度得天顶衰减(A = γ_o·h_o + γ_w·h_w),以及仰角 θ 下的斜路径损耗 A / sin(θ)。

在 Ku/Ka 波段,气体损耗相比雨衰较小,但并非可忽略——尤其在 22 GHz 水汽线附近、低仰角或潮湿站点。把它与雨衰项一起加进你的链路预算。本实现适用于 1–54 GHz(60 GHz 氧气复合带以下)与 5°–90° 仰角——后者是附件 2 余割定律成立的范围;更低的仰角需要附件 1 的完整路径积分,因此低于 5° 的输入会被提升到 5° 并给出提示。本方法的天顶衰减在约 10 km 高度以内,干空气精度约 ±10%、水汽约 ±5%。全部计算在你的浏览器本地完成。