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

# Short note: Monopole antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2080 Asoj, 3 marks
- Video: [Monopole Antenna (Imaginary Ground Concept, Radiation, Pros & Cons) Explained](https://www.youtube.com/watch?v=VYuATkzrnSw) — 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/monopole-antenna-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on monopole antenna.

## Short answer

A λ/4 monopole over a ground plane, by image theory, behaves like a λ/2 dipole. Radiation pattern is upper half of a dipole, omnidirectional in horizontal plane, vertical polarization, radiation resistance 36.5 Ω, directivity 5.15 dBi. Used in AM broadcasting, car whips, and mobile stations.

## Answer

A **monopole antenna** is a single straight vertical conductor mounted over a conducting ground plane and fed between its base and the ground. The most common is the **quarter-wave (λ/4) monopole** (Marconi antenna).

```text
      |  λ/4 monopole
      |
      |
  ====o==== ground plane / radials
     feed
      :   image λ/4 (formed by ground)
```

**Principle**: by **image theory**, the ground plane creates an image of the monopole, so above ground it behaves like a λ/2 dipole fed at the centre.

**Characteristics**
- Radiation pattern: upper half of a dipole pattern; omnidirectional in the horizontal plane, max along the ground, null overhead.
- Polarization: vertical.
- Radiation resistance ≈ **36.5 Ω** (half of 73 Ω); directivity ≈ 3.28 (5.15 dBi), twice a dipole's.
- Needs a good ground: earth with buried radials, metal car body, or 3–4 radial wires (ground-plane antenna).
- Can be shortened by top loading or loading coils.

**Applications**: MF AM broadcasting towers, car whip antennas, walkie-talkies, mobile base stations, Wi-Fi routers.

## Check yourself

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

1. Why does a quarter-wave monopole over a ground plane behave like a half-wave dipole above ground?
   A. The monopole is made twice as long
   B. The ground absorbs the radiation
   C. The ground creates an image of the monopole forming the other half
   D. The ground changes the polarization to circular
   Answer: C. By image theory, the ground plane gives an image lambda/4 below, completing a lambda/2 dipole in the upper half-space. (Key point: Image theory)

2. Compared with a half-wave dipole, a quarter-wave monopole has:
   A. Twice the radiation resistance, half the directivity
   B. Radiation resistance 146 Ω and lower directivity
   C. The same radiation resistance and gain
   D. Radiation resistance about 36.5 Ω and directivity about twice (3.28)
   Answer: D. Only the upper half-space is radiated into, so resistance is half of 73 Ω and directivity doubles. (Key point: Radiation resistance and gain)

3. What is the monopole's pattern and polarization?
   A. Omnidirectional horizontally, null overhead, vertical polarization
   B. Figure-8 in horizontal plane, horizontal polarization
   C. Maximum overhead, circular polarization
   D. Unidirectional, horizontal polarization
   Answer: A. It is the upper half of a dipole pattern: omnidirectional in azimuth, max along ground, null overhead, vertically polarized. (Key point: Pattern and polarization)

## Related in Propagation and Antenna

- [Ground system in vertical monopole](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Amonopole-ground-system) — Propagation and Antenna, chapter 3

## Related in other subjects

- [Numerical: λ/4 monopole and two-ray received power](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amonopole-two-ray-received-power) — Wireless Communication, chapter 3