Clear-Air Fading
Understanding Clear-Air Fading
The Earth's atmosphere has a refractive index slightly greater than 1.0 (n ≈ 1.0003 at sea level), decreasing with altitude as temperature, pressure, and humidity change. This gradient causes the microwave beam to curve slightly downward under "standard" conditions, extending the effective radio horizon beyond the geometric horizon. When the refractivity gradient departs from standard (dN/dh ≠ -40 N-units/km), the beam bending changes: subrefraction (dN/dh > -40) causes the beam to rise above normal, potentially missing the receive antenna on long paths. Superrefraction (dN/dh < -157) bends the beam toward the ground, and extreme superrefraction creates atmospheric ducts that trap the signal near the surface.
The most common clear-air fading mechanism on well-engineered paths is multipath. During temperature inversions (warm air over cool surface, common at night and near water bodies), elevated layers with sharp refractivity gradients partially reflect the microwave signal. The reflected ray arrives at the receiver with a path delay of a few nanoseconds, creating frequency-selective fading across the channel bandwidth. When the direct and reflected rays are nearly equal in amplitude and 180° out of phase, destructive interference creates deep narrowband fades that can sweep across the channel as atmospheric conditions change. For wideband digital systems, this causes intersymbol interference and increased bit error rates even before the total signal level drops below the receiver threshold.
Clear-Air Fading Parameters
P(A) = K · d3.1 · (1 + |ε|)-1.29 · f0.89 · 10-A/10
Refractivity:
N = (n - 1) × 106 = (77.6/T)(P + 4810 · e/T)
Standard Gradient:
dN/dh = -40 N-units/km (effective Earth radius factor k = 4/3)
Where d = path length (km), f = frequency (GHz), ε = path inclination (mrad), A = fade depth (dB), K = geoclimatic factor, T = temperature (K), P = pressure (hPa), e = water vapor pressure (hPa).
Clear-Air vs Rain Fading
| Parameter | Clear-Air Fading | Rain Fading |
|---|---|---|
| Cause | Refractivity layers, multipath | Raindrop absorption/scattering |
| Frequency response | Selective (notch) | Flat across channel |
| Dominant range | 4 to 11 GHz | Above 10 to 15 GHz |
| Worst conditions | Night, warm, humid, coastal | Heavy convective rain |
| Key countermeasure | Space diversity, equalization | Adaptive modulation, ATPC |
Frequently Asked Questions
What causes clear-air multipath fading?
Temperature inversions create elevated refractive layers that partially reflect the signal. The reflected ray arrives with different phase than the direct ray. When they're nearly equal amplitude and 180° out of phase, destructive interference creates deep fades (>40 dB). Fading is frequency-selective and sweeps across the channel as conditions change.
How is fade margin designed?
ITU-R P.530 calculates multipath fade probability as a function of path length, frequency, climate, and terrain. Typical targets: 99.99% availability requires 30 to 40 dB fade margin; 99.999% requires 40 to 50 dB. Countermeasures: space diversity, frequency diversity, equalization, ATPC.
How does it differ from rain fading?
Clear-air is frequency-selective, dominant at 4 to 11 GHz, worst at night in humid coastal areas. Rain fading is flat, dominant above 15 GHz, worst during convective storms. At 10 to 15 GHz both matter. Space diversity helps both; rain fading also uses adaptive modulation.