What is a monopole antenna?
A monopole is a single conductor mounted over a ground plane, which by image theory behaves like a dipole of twice its length radiating only into the upper half-space. A quarter-wave monopole on an infinite perfect ground has half the dipole’s impedance, about 36.5 + j21.25 Ω, and twice its directivity, 3.28 (5.16 dBi).[1]
What does image theory say?
A perfect conductor can be replaced by the mirror image of the sources above it. A vertical current element over ground has an image carrying current in the same direction, so a λ/4 monopole plus its image forms a λ/2 dipole. Radiated power is confined to the upper half-space, halving radiation resistance and doubling directivity.[1]
How does a finite ground plane change things?
On a finite ground the beam tilts upward, the peak gain drops below 5.16 dBi, and currents on the ground plane (or on a vehicle body, PCB, or coaxial cable shield) become part of the radiator. A ground plane of radius well above λ/4 approaches the ideal; small grounds make the system behave more like an asymmetric dipole.[1][2] See ground-plane effects.
How are short monopoles made efficient?
Monopoles shorter than λ/4 are capacitive with low radiation resistance. Base or center loading coils, top hats and helical winding resonate them; top loading is most efficient because it raises the current along the vertical section. At LF and MF, ground losses dominate and large buried radial systems are used.[3]
Frequently asked questions
How long is a quarter-wave whip at 433 MHz?
About 17.3 cm free-space λ/4, trimmed slightly shorter in practice.
Why 5.16 dBi?
Directivity doubles (+3 dB) relative to the dipole’s 2.15 dBi because radiation is confined to a half-space.
Is a car antenna a monopole?
Yes; the car body acts as the ground plane.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Modern Small Antennas”. K. Fujimoto and H. Morishita, Cambridge University Press. 2013.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.