---
title: "Short note: Super refraction"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:super-refraction-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/super-refraction-short-note
site: BCT Engineering
---

# Short note: Super refraction

Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum

- Asked in: 2074 Bhadra, 4 marks
- Video: [Duct Propagation in Antenna and Wave Propagation | Simplified | Super Refraction | AWP Module 6](https://www.youtube.com/watch?v=4XGi7HdfjBA) — THE BACKBENCH ENGINEERING COMMUNITY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/super-refraction-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

Write a short note on 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.

## Answer

**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 |

```text
            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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What happens to radio waves in super refraction?
   A. They bend upward and the range is reduced
   B. They bend downward more than normal, extending the radio horizon
   C. They travel in a straight line with no bending
   D. They are reflected by the ionosphere
   Answer: B. Super refraction occurs when refractive index falls with height faster than standard, so rays curve down more and the horizon extends. (Key point: Definition)

2. Which dN/dh (per km) corresponds to super refraction?
   A. Greater than 0
   B. About -40
   C. -79 to -157
   D. Below -157
   Answer: C. Standard is about -40, super refraction is -79 to -157, and below -157 the wave is trapped (ducting). (Key point: Gradient range)

3. Which condition causes super refraction?
   A. Temperature inversion with warm air above cool air
   B. Cold air above warm air with high turbulence
   C. Heavy rain at 10 GHz
   D. High electron density in the F2 layer
   Answer: A. Temperature inversion and rapid fall of humidity with height make refractive index drop quickly, causing super refraction. (Key point: Causes)

4. Which is an effect of super refraction?
   A. Effective earth radius factor k falls below 4/3
   B. Reduced co-channel interference
   C. Unexpected long-distance reception and co-channel interference
   D. Radar clutter disappears
   Answer: C. k exceeds 4/3, so distant stations are received and interfere; radar sees anomalous long-range clutter. (Key point: Effects)