- 2074 Bhadra · 4 marks
Short note: Super refraction
Short answer
Super refraction is a tropospheric condition where the refractive index decreases with height faster than normal (dN/dh < −79 per km), causing VHF/UHF waves to bend downward more and reach beyond the normal horizon. It results from temperature inversions or rapid humidity decrease; extreme values cause ducting and long-range interference.
Write a short note on super refraction.
Super refraction is the condition in which radio waves in the troposphere bend downward more than normal, because the refractive index of the air decreases with height faster than in the standard atmosphere. The radio horizon then extends beyond its normal value.
Refractivity gradients (N = (n − 1) × 10⁶):
| Condition | dN/dh (per km) | Effect |
|---|---|---|
| Sub-refraction | > 0 | Rays bend up; range reduced |
| Standard | ≈ −40 | k = 4/3; normal horizon |
| Super refraction | −79 to −157 | Rays bend down more; range extended |
| Ducting (trapping) | < −157 | Ray curvature exceeds earth's; wave trapped |
standard ray
Tx ____------------------
\ `--.___ super-refracted
\ `--.___ ray (bends more)
=====\=============`--.___Rx==== earth
Causes: temperature inversion (warm air above cool air), rapid fall of humidity with height, cool sea surface under warm dry air, clear calm nights.
Effects:
- Effective earth radius factor k becomes greater than 4/3 (even infinite at the limit), so VHF/UHF/microwave signals travel beyond the normal horizon.
- Unexpected long-distance reception; co-channel interference between distant TV/FM stations and microwave links.
- Radar "anomalous propagation": ground or sea clutter appears at long range.
- In extreme form it becomes duct propagation.
✓ Answer checked by Claude Opus 5.5 (high effort)