Monopole
Understanding Monopole Antennas
The monopole is conceptually half a dipole with the ground plane providing the missing half through its electrical image. Because the radiation is concentrated in the upper hemisphere (above the ground plane) rather than spreading into both hemispheres, the monopole achieves 3 dB higher gain than a dipole.
Monopole Types
- Quarter-wave monopole: Length = lambda/4. Standard monopole. 5.15 dBi gain over infinite ground plane.
- Stub antenna: Short, thick monopole. Lower gain but wider bandwidth. Used on handsets.
- Whip antenna: Flexible quarter-wave wire. Used on portable radios.
- Loaded monopole: Top-loaded or helically loaded to reduce physical length while maintaining resonance.
Ground Plane Effects
The ground plane size significantly affects monopole performance. An infinite ground plane gives the ideal 5.15 dBi gain. A small ground plane (1-2 lambda) tilts the beam upward and reduces gain. For mobile devices, the phone chassis acts as the ground plane.
Length = lambda/4 = c / (4f)
At 900 MHz: L = 83 mm
At 2.4 GHz: L = 31 mm
At 5.8 GHz: L = 13 mm
Gain (infinite GP): 5.15 dBi
Input impedance: ~36 ohms
Bandwidth: ~10% (VSWR < 2)
Frequently Asked Questions
What is a monopole antenna?
A monopole is a vertical antenna element (typically lambda/4 long) mounted on a ground plane. The ground plane creates an electrical mirror image, making it function like a half dipole. It produces an omnidirectional pattern with 5.15 dBi gain.
How does ground plane size affect a monopole?
A larger ground plane provides better performance closer to the ideal 5.15 dBi. When the ground plane is small (comparable to a wavelength), the radiation pattern tilts upward, gain decreases, and the input impedance changes. The ground plane should ideally be at least 1 lambda in radius.
What is the difference between a monopole and dipole?
A dipole uses two equal-length elements radiating in both directions. A monopole uses one element with a ground plane that mirrors it. The monopole is half the length but achieves 3 dB more gain because radiation is concentrated in one hemisphere.