---
title: "Short note: Horn antenna"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:horn-antenna-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/horn-antenna-short-note
site: BCT Engineering
---

# Short note: Horn antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2079 Chaitra, 3 marks; 2072 Magh, 3 marks (asked 2 times)
- Video: [Horn Antenna (Basics, Structure, Flaring, Types, Designing Parameters, Applications) Explained](https://www.youtube.com/watch?v=MOvVkotklfE) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/horn-antenna-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on horn antenna.

## Short answer

A horn antenna is a flared waveguide that transitions from waveguide impedance to free space impedance, reducing reflections and forming a directive beam. Types include sectoral, pyramidal, and conical. It offers wide bandwidth, moderate gain 10-25 dBi, and is used as a reflector feed and gain standard.

## Answer

A **horn antenna** is a hollow waveguide whose end is flared out into a larger opening. The flare gives a gradual transition from the waveguide impedance to free-space impedance (≈ 377 Ω), reducing reflection and producing a directive beam. It is the most common **aperture antenna** at microwave frequencies.

```text
 waveguide       flare        aperture
  ______      .-'‾‾‾‾‾‾‾‾‾‾|
 |______|===<     flare    |  ---> beam
             '-.__________|
               length L, aperture a x b
```

**Types**
- **Sectoral horn**: flared in one plane only — **E-plane** sectoral (flare in E direction) or **H-plane** sectoral.
- **Pyramidal horn**: flared in both planes; most common, used as gain standard.
- **Conical horn**: flared circular waveguide, used with circular polarization.

**Design points**
- If the flare angle is too large, the path from throat to the aperture edge differs from the axial path by δ, giving phase error. Optimum horn: δ ≈ 0.25λ (E-plane) and ≈ 0.4λ (H-plane).
- Gain: G ≈ 4πA_e/λ² ≈ 7.5 A/λ² (aperture efficiency ≈ 0.6).
- HPBW (optimum horn): E-plane ≈ 56λ/a_E deg, H-plane ≈ 67λ/a_H deg.

**Advantages**: wide bandwidth, simple, moderate gain (10–25 dBi), low VSWR, easy to make. **Applications**: feed for parabolic reflectors, standard gain antenna for measurement, radar, microwave links.

## Check yourself

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

1. The flare of a horn
   A. increases VSWR
   B. blocks radiation
   C. reduces bandwidth
   D. gives gradual transition from waveguide impedance to free space, reducing reflection
   Answer: D. The gradual match to 377 ohm gives a directive beam with low reflection. (Key point: Role of flare)

2. A pyramidal horn is flared
   A. in both E and H planes
   B. in the E-plane only
   C. in the H-plane only
   D. in neither plane
   Answer: A. Sectoral horns flare in one plane; pyramidal horns flare in both. (Key point: Pyramidal horn)

3. If the flare angle is too large, there is
   A. no effect
   B. phase error between axial and edge path
   C. lower VSWR
   D. higher efficiency
   Answer: B. Path difference delta across the aperture gives phase error. (Key point: Flare angle)

## Related in Propagation and Antenna

- [Short note: Pyramidal horn antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Apyramidal-horn-short-note) — Propagation and Antenna, chapter 3
- [Aperture antenna advantages and horn types](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aaperture-advantages-horn-types) — Propagation and Antenna, chapter 3
- [Aperture antennas and rectangular horn radiation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aaperture-antenna-rectangular-horn) — Propagation and Antenna, chapter 3
- [Pyramidal horn flare angle and length](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Apyramidal-horn-flare-angle) — Propagation and Antenna, chapter 3