Marconi antenna characteristics

A Marconi antenna is a vertical quarter-wave (λ/4) monopole whose lower end is connected to ground through the transmitter. The conducting earth produces an image of the antenna, so a λ/4 monopole behaves like a λ/2 dipole (Hertz antenna) in the upper half-space.

      ^  I = 0 (top)
      |  current
 λ/4  |  distribution        Pattern (vertical plane)
      |  max at base               _ _
      |                         .'  |  '.
  ~ TX|________ ground       __/____|____\__ ground
 ////////////////////////     max along ground,
   image (λ/4) below          null overhead

Major characteristics

1. Length: λ/4 physical height; the image supplies the other λ/4.
2. Current distribution: maximum at the base (feed), zero at the top; voltage maximum at the top.
3. Radiation pattern: same as the upper half of a half-wave dipole; omnidirectional in the horizontal plane, maximum along the ground, null overhead.
4. Polarization: vertical, ideal for ground-wave propagation at LF/MF.
5. Input (radiation) resistance: ≈ 36.5 Ω, half of a dipole's 73 Ω, because it radiates into a half space only.
6. Directivity: 3.28 (5.15 dBi), double that of a half-wave dipole, since power goes into half space.
7. Ground system: needs good earth; buried radial wires, or a counterpoise (wire mesh above ground) where soil is poor. Ground loss reduces efficiency.
8. Height reduction: at MF, λ/4 is tall (e.g. 75 m at 1 MHz), so top loading (capacitance hat, T or inverted-L) or base loading coil is used.

Marconi vs Hertz

Applications: AM (MF) broadcasting towers, car whip antennas, mobile and handheld radios.