---
title: "Duct propagation"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:duct-propagation"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/duct-propagation
site: BCT Engineering
---

# Duct propagation

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

- Asked in: 2073 Magh, 5 marks; 2072 Asoj, 5 marks; 2070 Magh, 4 marks (asked 3 times)
- 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/duct-propagation
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

Explain the phenomenon of Duct propagation.

## Short answer

Duct propagation occurs when a temperature or humidity inversion creates a layer where the refractive index falls faster than 0.157 N-units/km, causing microwaves to be trapped and guided along the earth’s curvature, extending range far beyond line-of-sight. It is unreliable for planned links, causing anomalous propagation and interference.

## Answer

**Duct propagation** is a form of space-wave propagation in which a microwave signal is trapped in a thin layer of the lower atmosphere (a **duct**) and guided along the earth's curvature, far beyond the normal line-of-sight range, like a wave in a waveguide.

#### Cause: abnormal refractive index profile
In a standard atmosphere the refractive index *n* falls slowly with height, so rays bend down slightly (effective earth radius = 4/3 R). Sometimes *n* falls much faster with height, due to:
- **Temperature inversion** (warm air over cold air), e.g. over cool sea, or after sunset over land;
- **Sharp fall of humidity** with height (moist air near sea surface, dry air above);
- Subsidence of air in high-pressure areas.

The **modified refractive index** is M = (n − 1 + h/R) × 10⁶. Normally M increases with height. Where **dM/dh < 0** (n falls faster than 0.157 N-units per metre, i.e. dN/dh < −157 per km), the ray curvature becomes greater than the earth's curvature, and a duct forms.

```text
   height          upper boundary (n jumps)
     |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     |   /\    /\    /\    /\    /\
     |  /  \  /  \  /  \  /  \  /  \     trapped ray
     | /    \/    \/    \/    \/    \
  ===Tx==========================Rx===  earth / sea
        surface duct (few m to 100s m)
```

#### Mechanism
1. A ray launched at a low angle bends downward more strongly than the earth curves.
2. It returns to the earth (or the lower boundary of an elevated duct), reflects, goes up again, and is bent down again.
3. The wave keeps zig-zagging inside the layer and travels hundreds or even thousands of km with low loss.

#### Types
- **Surface duct:** lower boundary is the earth or sea surface (common over warm seas).
- **Elevated duct:** layer lies above the ground, between two heights.

#### Conditions
- Only wavelengths small compared with the duct thickness are trapped (like waveguide cut-off). A duct of thickness *d* supports roughly λ_max ≈ 2.5 d √(ΔM × 10⁻⁶); so mainly VHF high end, UHF and microwaves are ducted.
- Both antennas should be inside or near the duct, and the wave launched at a small angle.

#### Effects
- Very long, unexpected ranges for microwave and VHF/UHF signals (TV and radar "anomalous propagation").
- **Interference** between distant stations on the same frequency; false radar echoes.
- Not reliable for planned links, because ducts depend on weather.

## Check yourself

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

1. What causes duct propagation?
   A. Refractive index falling much faster with height than normal, e.g. by temperature inversion or sharp humidity drop
   B. Ionisation in the E layer
   C. Strong ground conductivity
   D. Rain scatter
   Answer: A. Temperature inversion or rapid humidity fall makes n decrease abnormally fast with height. (Key point: Cause of duct)

2. What condition on the modified refractive index M gives a duct?
   A. dM/dh > 0
   B. dM/dh < 0
   C. M constant
   D. M = 0
   Answer: B. Where dM/dh < 0 (dN/dh < −157 per km) the ray curvature exceeds the earth's curvature and the ray is trapped. (Key point: Condition on M)

3. How does the trapped wave travel?
   A. It zig-zags inside the layer, bending down and reflecting from the earth or lower boundary, travelling far with low loss
   B. It travels straight into space
   C. It is absorbed by the D layer
   D. It follows the ionosphere
   Answer: A. The ray bends back down, reflects, rises and bends down again repeatedly, like in a waveguide. (Key point: Mechanism)

4. Which signals are ducted, and what is the effect?
   A. Short wavelengths (VHF high end, UHF, microwaves); unexpected long range and interference
   B. MF and LF; reliable planned links
   C. HF only; skip zone
   D. VLF only; submarine links
   Answer: A. Only wavelengths small compared with duct thickness are trapped, giving anomalous propagation and co-channel interference; ducts depend on weather so are unreliable. (Key point: Frequencies and effects)

## Related in Propagation and Antenna

- [Duct propagation; critical frequency and MUF](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aduct-critical-frequency-muf) — Propagation and Antenna, chapter 4