Why does a quarter-wave monopole over a ground plane behave like a half-wave dipole above ground?

The ground creates an image of the monopole forming the other half

The monopole is made twice as long

The ground absorbs the radiation

The ground changes the polarization to circular

Compared with a half-wave dipole, a quarter-wave monopole has:

Radiation resistance about 36.5 Ω and directivity about twice (3.28)

Twice the radiation resistance, half the directivity

Radiation resistance 146 Ω and lower directivity

The same radiation resistance and gain

What is the monopole's pattern and polarization?

Omnidirectional horizontally, null overhead, vertical polarization

Figure-8 in horizontal plane, horizontal polarization

Maximum overhead, circular polarization

Unidirectional, horizontal polarization

2080 Asoj · 3 marks

Short note: Monopole antenna

Short answer

A λ/4 monopole over a ground plane, by image theory, behaves like a λ/2 dipole. Radiation pattern is upper half of a dipole, omnidirectional in horizontal plane, vertical polarization, radiation resistance 36.5 Ω, directivity 5.15 dBi. Used in AM broadcasting, car whips, and mobile stations.

Write a short note on monopole antenna.

A monopole antenna is a single straight vertical conductor mounted over a conducting ground plane and fed between its base and the ground. The most common is the quarter-wave (λ/4) monopole (Marconi antenna).

      |  λ/4 monopole
      |
      |
  ====o==== ground plane / radials
     feed
      :   image λ/4 (formed by ground)

Principle: by image theory, the ground plane creates an image of the monopole, so above ground it behaves like a λ/2 dipole fed at the centre.

Characteristics

Radiation pattern: upper half of a dipole pattern; omnidirectional in the horizontal plane, max along the ground, null overhead.
Polarization: vertical.
Radiation resistance ≈ 36.5 Ω (half of 73 Ω); directivity ≈ 3.28 (5.15 dBi), twice a dipole's.
Needs a good ground: earth with buried radials, metal car body, or 3–4 radial wires (ground-plane antenna).
Can be shortened by top loading or loading coils.

Applications: MF AM broadcasting towers, car whip antennas, walkie-talkies, mobile base stations, Wi-Fi routers.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Short note: Monopole antenna

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