---
title: "Marconi antenna working and design"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:marconi-working-design"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/marconi-working-design
site: BCT Engineering
---

# Marconi antenna working and design

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2071 Bhadra, 5 marks
- Video: [Antenna Lect20 Telescopic or Marconi Antenna](https://www.youtube.com/watch?v=qjpV1pMQgx0) — Mahesh Pingle
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/marconi-working-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Explain the working principle and design of Marconi antenna.

## Short answer

A Marconi antenna is a vertical quarter-wave monopole fed against ground; the ground image makes it act like a half-wave dipole. It has Rr = 36.5 Ω, directivity 3.28 (5.16 dBi), vertically polarised omnidirectional pattern. Practical height is about 71/f(MHz) m; loading coils, top hats, and ground radials improve short LF/MF masts.

## Answer

A **Marconi antenna** is a vertical quarter-wave (λ/4) monopole fed at its base against the ground. The ground acts as a mirror, so the antenna together with its image behaves like a vertical half-wave dipole.

```text
        |  λ/4 radiator
        |
        |
  ~~~~[ ~ ]~~~~~~~ ground (conducting)
        :
        :  λ/4 image
        :
```

#### Working principle
- The RF source is connected between the base of the vertical wire and the ground.
- The current is maximum at the base and zero at the top (quarter-wave standing-wave distribution).
- By image theory, a perfectly conducting ground produces an image current in the same direction below the ground. The real wire plus its image form a λ/2 dipole, so the field above ground is the same as that of a half-wave dipole.
- Radiation exists only in the upper half space, so the total radiated power is half that of the dipole for the same current. Hence:
  - Radiation resistance Rᵣ = 73/2 ≈ **36.5 Ω**
  - Directivity = 2 × 1.64 = **3.28 (≈ 5.16 dBi)**
- Pattern: omnidirectional in the horizontal plane, maximum along the ground, zero overhead; **vertically polarised**. This suits ground-wave (surface-wave) broadcasting in LF/MF.

#### Design
1. Physical height: h = λ/4 = 75/f(MHz) m. Allowing about 5% end effect, h ≈ 71/f(MHz) m.
   Example: at 1 MHz, λ = 300 m, so h ≈ 75 m (≈ 71 m practical).
2. **Short heights:** at LF/MF, λ/4 is very tall. A shorter mast is used and brought to resonance by:
   - a **loading coil** (series inductance) at the base to cancel capacitive reactance;
   - **top loading** (capacity hat, T or inverted-L wire) to make the current more uniform and raise radiation resistance.
3. **Ground system:** since half of the "antenna" is the ground, a good ground is essential. Usually 60–120 copper radials, each about λ/4 long, are buried a few cm deep around the base; a counterpoise (wire mesh above ground) is used where soil is poor or rocky.
4. **Efficiency:** η = Rᵣ/(Rᵣ + R_loss); ground loss is the main loss, so a good radial system raises η.
5. **Matching:** a feeder (50–75 Ω coax) is matched to the base impedance (≈ 36 Ω) through an L-network.

Uses: AM (MW) broadcasting, mobile whips, VLF/LF transmitters.

## Check yourself

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

1. Why does a Marconi (λ/4) antenna behave like a half-wave dipole?
   A. The ground acts as a mirror whose image current is in the same direction, so antenna plus image form a λ/2 dipole
   B. The antenna is physically λ/2 long
   C. The ground absorbs the lower half
   D. The image current is opposite and cancels the field
   Answer: A. A perfectly conducting ground gives an image of the same current direction below it, forming an equivalent λ/2 dipole above ground. (Key point: Image theory)

2. What are the radiation resistance and directivity of a Marconi monopole?
   A. 73 Ω and 1.64
   B. 36.5 Ω and 3.28 (5.16 dBi)
   C. 36.5 Ω and 1.64
   D. 146 Ω and 3.28
   Answer: B. Radiation is only into the upper half space, so Rr = 73/2 = 36.5 Ω and directivity = 2 x 1.64 = 3.28. (Key point: Radiation resistance and directivity)

3. At LF/MF the λ/4 mast is very tall. How is a shorter mast brought to resonance?
   A. Remove the ground system
   B. Use a loading coil at the base and top loading (capacity hat)
   C. Shorten the radials
   D. Use horizontal polarisation
   Answer: B. A series loading coil cancels the capacitive reactance, and top loading makes current more uniform and raises radiation resistance. (Key point: Short antenna loading)

4. Why is a good radial ground system essential for a Marconi antenna?
   A. Ground loss is the main loss, and efficiency is η = Rr/(Rr + Rloss)
   B. It is needed to increase the height
   C. It makes the polarisation horizontal
   D. It changes the frequency
   Answer: A. Half of the antenna is the ground, and its resistance reduces efficiency; 60-120 radials of about λ/4 lower the loss. (Key point: Ground system and efficiency)

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