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

# Short note: Pyramidal horn antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2073 Bhadra, 3 marks
- Video: [4.5.3 Pyramidal Horn Antenna | EC602 |](https://www.youtube.com/watch?v=RbHkPJE73jM) — RGPV Official
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/pyramidal-horn-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on pyramidal horn antenna.

## Short answer

Pyramidal horn is a rectangular waveguide flared in both E and H planes, matching impedance to free space. Optimum horn has gain ≈ 7.5 aE aH/λ². It is used as a gain standard and feed for reflectors.

## Answer

A **pyramidal horn** is a rectangular waveguide flared in **both** the E-plane and the H-plane, so that its mouth is a larger rectangle (a_H × a_E). It is the most widely used horn antenna and the standard reference for gain measurement at microwave frequencies.

```text
 waveguide a x b        aperture a_H x a_E
   ____         ___________
  |____|=====> |           |  ===> beam
  TE₁₀ mode    |___________|
           flare in both planes, length L
```

**Working**: the TE₁₀ wave in the guide spreads gradually through the flare, which matches the waveguide impedance to free space (377 Ω). The aperture field (uniform in E-plane, cosine in H-plane) radiates a directive beam along the axis.

**Design**
- Path-difference (phase error) limits for an optimum horn: δ ≈ 0.25λ (E-plane), δ ≈ 0.4λ (H-plane), with δ ≈ a²/(8L).
- Both flares must give the same throat point so the horn can be built (physical realizability).

**Performance (optimum horn)**

```text
Gain       G ≈ 7.5 a_E a_H / λ²   (η ≈ 0.6)
HPBW (E) ≈ 56 λ/a_E deg,  HPBW (H) ≈ 67 λ/a_H deg
```

**Advantages**: high gain (15–25 dBi), wide bandwidth, low VSWR, simple, accurately predictable gain. **Applications**: gain standard, feed for reflectors, radar, microwave test benches.

## Check yourself

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

1. What is a pyramidal horn?
   A. A circular waveguide flared into a cone
   B. A rectangular waveguide flared in both E-plane and H-plane
   C. A rectangular waveguide flared in the E-plane only
   D. A waveguide with ridges inside for wider bandwidth
   Answer: B. A pyramidal horn flares a rectangular waveguide in both planes so the mouth is a larger rectangle. (Key point: Definition of pyramidal horn)

2. What is the main purpose of the flare in a horn?
   A. To raise the cutoff frequency
   B. To make the guide mode change from TE10 to TM11
   C. To match the waveguide impedance gradually to free space (377 Ω)
   D. To make the beam omnidirectional
   Answer: C. The gradual flare transforms the waveguide impedance to that of free space, giving low reflection and a directive beam. (Key point: Why the flare is used)

3. For an optimum pyramidal horn, which statement is correct?
   A. δ is 0.4λ in E-plane and 0.25λ in H-plane; G ≈ 7.5 a_E a_H/λ²
   B. δ is 0.25λ in E-plane and 0.4λ in H-plane; G ≈ 7.5 a_E a_H/λ²
   C. δ is 0.25λ in both planes; G ≈ 4 a_E a_H/λ²
   D. δ is 0.4λ in both planes; G ≈ 0.6 a_E a_H
   Answer: B. Path difference limits are 0.25λ (E-plane) and 0.4λ (H-plane), and gain is about 7.5 times aperture area over λ² (efficiency about 0.6). (Key point: Optimum horn limits and gain)

## Related in Propagation and Antenna

- [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
- [Short note: Horn antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ahorn-antenna-short-note) — Propagation and Antenna, chapter 3, asked 2×
- [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