Marconi antenna working and design

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.

        |  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.