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

# Antenna definition and single wire radiation conditions

Propagation and Antenna · Chapter 1: Radiation and Antenna Fundamentals

- Asked in: 2081 Chaitra, 2+4 marks; 2079 Chaitra, 2+4 marks; 2075 Bhadra, 2+4 marks; 2075 Baisakh, 2+4 marks (asked 4 times)
- Video: [Radiation Mechanism -  Single Wire](https://www.youtube.com/watch?v=o6zXXmikK58) — ASN Vision
- 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-conditions
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-1

## Question

Define an antenna. Explain the radiation mechanism and conditions of a single wire antenna that can radiate electromagnetic waves or not.

## Short answer

Radiation needs time-varying current, equivalent to accelerated charges: l dI/dt = l q dv/dt. Static charge or uniform velocity on a straight infinite wire does not radiate; bends, ends, and discontinuities radiate. An oscillating AC current radiates, most strongly for lengths comparable to wavelength.

## Answer

An **antenna** is a metallic structure (wire, rod, horn, dish) that converts guided electromagnetic energy from a transmission line into free-space waves, and vice versa. IEEE defines it as "a means for radiating or receiving radio waves".

#### Radiation mechanism in a single wire
Radiation is caused by **time-varying current**, i.e. by **accelerated or decelerated charges**. For a wire of length l carrying charge density q (C/m) moving with velocity v:

```text
I = q · v
dI/dt = q · dv/dt
l · dI/dt = l · q · dv/dt      (basic radiation equation)
```

So a time-changing current (left side) is the same as accelerated charge (right side). Both mean radiation.

#### Conditions for radiation from a single wire
1. **No charge motion:** no current, no radiation.
2. **Charge moving with uniform velocity:**
   - on a straight, infinitely long, smooth wire – **no radiation**;
   - on a wire that is **curved, bent, discontinuous, terminated or truncated** – radiation occurs, because the charge changes direction or speed there.
3. **Charge oscillating in time** (time-harmonic current), even on a straight wire – **radiation occurs**.

```text
uniform v, straight  ------->   no radiation
bent wire            ---->\     radiation at bend
                           \
end of wire          ------>|   radiation at end
oscillating current  <-> <->    radiation all along
```

| Condition | Radiates? |
|---|---|
| Static charge | No |
| Uniform velocity, straight infinite wire | No |
| Uniform velocity, bent/ended wire | Yes |
| Time-varying (AC) current | Yes |

So a practical antenna is fed with an **alternating current**; the charges accelerate continuously, and the wire radiates EM waves. Radiation is strongest for wire lengths comparable to the wavelength (e.g. λ/2 dipole).

## Check yourself

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

1. What fundamentally causes a wire to radiate?
   A. A large static charge on it
   B. A steady DC current
   C. A high voltage across it
   D. Time-varying current, i.e. accelerated or decelerated charges
   Answer: D. Radiation arises from time-varying current, expressed by l·dI/dt = l·q·dv/dt, so accelerated charge radiates. (Key point: Cause of radiation)

2. In which case does a wire NOT radiate?
   A. Charge moving at uniform velocity on a straight, infinitely long, smooth wire
   B. Charge moving uniformly on a bent wire
   C. Charge moving uniformly on a wire with an open end
   D. Charge oscillating in time on a straight wire
   Answer: A. Uniform velocity on a straight infinite wire involves no acceleration, so there is no radiation. (Key point: No-radiation case)

3. Why does a bent or truncated wire radiate even with uniform charge speed?
   A. The wire resistance rises
   B. The charge changes direction or speed at the bend or end
   C. The wire becomes a capacitor
   D. The voltage becomes infinite
   Answer: B. At a bend, discontinuity or end the charge is accelerated or decelerated, which produces radiation. (Key point: Radiation at bends and ends)

4. How is a practical antenna fed so that it radiates continuously?
   A. With DC
   B. With a static charge
   C. With alternating current so charges accelerate continuously
   D. With a uniform-velocity beam
   Answer: C. AC makes the charges oscillate and accelerate all the time, so the wire radiates EM waves. (Key point: Practical feeding)

## Related in Propagation and Antenna

- [Single wire radiation and omnidirectional antenna characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-radiation-omnidirectional) — Propagation and Antenna, chapter 1
- [Single wire radiation and travelling wave long wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-travelling-long-wire) — Propagation and Antenna, chapter 1
- [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×
- [Antenna radiation pattern and lobes](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aradiation-pattern) — Propagation and Antenna, chapter 2, asked 4×

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