---
title: "Single wire radiation and omnidirectional antenna characteristics"
subject: "Propagation and Antenna"
chapter: 1
key: "semester_6/propagation_and_antenna:single-wire-radiation-omnidirectional"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/single-wire-radiation-omnidirectional
site: BCT Engineering
---

# Single wire radiation and omnidirectional antenna characteristics

Propagation and Antenna · Chapter 1: Radiation and Antenna Fundamentals

- Asked in: 2072 Asoj, 4 marks
- Video: [Radiation Patterns of Antenna (Isotropic Antenna, Directional Antenna & Omnidirectional Antenna)](https://www.youtube.com/watch?v=gJZPX4OMn94) — 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/single-wire-radiation-omnidirectional
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-1

## Question

Explain how single wire can radiate electromagnetic waves with necessary radiation patterns and polarization. List the characteristics of omnidirectional antenna.

## Short answer

A single wire radiates when its charges are accelerated by a time-varying current; bends, ends, and oscillating current produce radiation. A vertical wire gives a figure-of-eight E-plane pattern, a circular H-plane pattern, and vertical linear polarization. An omnidirectional antenna radiates uniformly in one plane, has a doughnut-shaped 3-D pattern, nulls along its axis, and moderate gain (dipole 2.15 dBi, monopole 5.15 dBi).

## Answer

A single wire radiates when its charges are **accelerated**, i.e. when it carries a time-varying (AC) current: l·dI/dt = l·q·dv/dt. A static charge or a charge moving at uniform speed on a straight infinite wire does not radiate; bends, ends and oscillating current produce radiation. Field lines formed between the oscillating charges detach every half cycle and travel out as EM waves.

#### Pattern and polarization of a vertical wire (dipole)

```text
  E-plane (vertical)         H-plane (horizontal)
        z
        |                        .---.
   .-.  |  .-.                 /       \
  (   ) | (   )               |   (o)   |
   '-'  |  '-'                 \       /
        |                        '---'
  figure-of-eight            circle (omni)
```

- E ∝ sinθ: maximum broadside (θ = 90°), null along the wire.
- E-field lies along the wire, so a **vertical wire gives vertical (linear) polarization**; a horizontal wire gives horizontal polarization.

#### Characteristics of an omnidirectional antenna
- Radiates **uniformly in all directions of one plane** (usually the horizontal/azimuth plane).
- Directional in the orthogonal (elevation) plane; 3-D pattern is a doughnut.
- Has nulls along the axis of the antenna.
- Moderate gain, e.g. λ/2 dipole 2.15 dBi, λ/4 monopole 5.15 dBi.
- Usually vertically polarized.
- Examples: vertical dipole, monopole/whip, discone, normal-mode helix.
- Used in broadcasting, mobile phones, Wi-Fi access points, base stations where users are in all directions.

## Check yourself

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

1. When does a single wire radiate?
   A. When charges are accelerated, i.e. with time-varying current
   B. When it carries static charge
   C. When charge moves uniformly on a straight infinite wire
   D. When it is cooled
   Answer: A. Accelerated charges radiate; static or uniform straight motion does not. (Key point: Radiation condition)

2. What is the E-plane pattern of a vertical wire dipole?
   A. Circle
   B. Figure-of-eight with E ∝ sinθ, maximum at 90° and null along the wire
   C. Single narrow lobe along the wire
   D. Cardioid
   Answer: B. E ∝ sinθ gives maximum broadside and nulls along the axis. (Key point: Pattern of vertical dipole)

3. What polarization does a vertical wire give?
   A. Horizontal linear
   B. Circular
   C. Vertical linear, since E lies along the wire
   D. Elliptical
   Answer: C. The E-field is parallel to the wire, so a vertical wire is vertically polarized and a horizontal wire horizontally polarized. (Key point: Polarization)

4. Which describes an omnidirectional antenna?
   A. Radiates in one narrow direction
   B. Has no nulls anywhere
   C. Is always circularly polarized
   D. Radiates uniformly in all directions of one plane and is directional in the orthogonal plane (doughnut)
   Answer: D. It is uniform in the azimuth plane, directional in elevation, with nulls along its axis. (Key point: Omnidirectional definition)

## Related in Propagation and Antenna

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- [Single wire antenna and field line detachment](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-field-line-detachment) — Propagation and Antenna, chapter 1, asked 3×
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