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

# Short note: Aperture antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2081 Chaitra, 3 marks
- Video: [Aperture Antennas -  An Introduction](https://www.youtube.com/watch?v=_sVsStcxwjQ) — Padmasri Naban
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/short-note-aperture-antenna
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on aperture antenna.

## Short answer

Aperture antennas radiate through an opening several wavelengths large (horns, slots, reflectors). Effective aperture Ae = ηA, gain G = 4πAe/λ², beamwidth ~ λ/D. They provide high gain and narrow beams at microwave frequencies for radar, satellite links, and feeds.

## Answer

An **aperture antenna** is one that radiates through an opening (aperture) whose size is several wavelengths, rather than from currents on thin wires. The fields across the aperture act as secondary sources (Huygens' principle), and the radiation pattern is found from the aperture field distribution.

```text
 waveguide   aperture (E-field across opening)
  ______   .‾‾‾‾‾‾‾‾‾|
 |______|=<  horn    |  ===> beam
           '_________|
```

**Examples**
- Open-ended waveguide and **horn antennas** (sectoral, pyramidal, conical).
- **Slot antennas** cut in a metal sheet or waveguide wall.
- **Reflector antennas** (parabolic dish) – the dish mouth is the aperture.
- **Lens antennas** and microstrip patches (radiating edges).

**Key relations**

```text
Effective aperture  A_e = η A    (η ≈ 0.5-0.8)
Gain                G = 4π A_e / λ²
Beamwidth           ≈ k λ / D   (k ≈ 50-70 degrees)
```

**Features**: high gain and narrow beam because the aperture is large compared with λ; wide bandwidth; easily flush-mounted on aircraft and vehicles; used mainly at **microwave frequencies** (above ~1 GHz). **Applications**: radar, satellite links, microwave relays, feeds for reflectors and gain standards.

## Check yourself

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

1. How does an aperture antenna radiate?
   A. Through currents on thin wires
   B. Through fields across an opening acting as Huygens sources
   C. Through the ground wave only
   D. Through resonance of a loop
   Answer: B. Fields across the aperture act as secondary sources whose pattern follows the aperture distribution. (Key point: Radiating mechanism)

2. Which relation gives the gain of an aperture antenna?
   A. G = eta lambda / A
   B. G = lambda^2 / (4 pi A)
   C. G = 4 pi A_e / lambda^2 with A_e = eta A
   D. G = A_e / 4 pi
   Answer: C. Gain is 4 pi times effective aperture over lambda squared. (Key point: Gain formula)

3. Which list contains only aperture antennas?
   A. Horn, loop, monopole, rhombic
   B. Dipole, monopole, loop, Yagi
   C. Helix, rhombic, V, dipole
   D. Horn, slot, parabolic dish, lens
   Answer: D. Horn, slot, reflector and lens antennas radiate through apertures; dipoles, loops and rhombics are wire types. (Key point: Examples and features)

## Related in Propagation and Antenna

- [Aperture antenna and parabolic feeding](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aaperture-antenna-parabolic-feeding) — 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
- [Fibre types and numerical aperture](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-types-define-na) — Propagation and Antenna, chapter 6
- [Short note: Numerical aperture](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Anumerical-aperture-short-note) — Propagation and Antenna, chapter 6
- [Numerical aperture and dispersion](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ana-and-dispersion) — Propagation and Antenna, chapter 6