Radiation Pattern
Understanding Radiation Patterns
The radiation pattern is the most complete description of an antenna's directional properties. It reveals not only where the antenna transmits/receives most effectively (main beam), but also where it may pick up interference (sidelobes) and where it has zero response (nulls).
Pattern Features
- Main lobe/beam: The direction of maximum radiation. The gain is measured at the peak of the main lobe.
- Half-power beamwidth (HPBW): The angular width of the main beam between the -3 dB points.
- Sidelobes: Secondary radiation peaks outside the main beam. First sidelobe is typically 13 dB below main beam for a uniform aperture (20-30 dB with tapering).
- Nulls: Directions of zero radiation between lobes.
- Back lobe: Radiation in the direction opposite the main beam.
Pattern Types
- Omnidirectional: Uniform pattern in one plane (e.g., dipole H-plane). Used for broadcast and mobile base stations.
- Directional: Concentrated main beam. Used for point-to-point links, satellite, and radar.
- Shaped beam: Custom pattern for specific coverage requirements (e.g., cosecant-squared for ground illumination).
theta_3dB = 51 x lambda / D (degrees)
where D = aperture diameter
First sidelobe level:
Uniform: -13.2 dB
Taylor (n-bar=5, SLL=-25): -25 dB
Cosine taper: -23 dB
Gain from HPBW:
G = 41,253 / (theta_E x theta_H) (approximate)
Frequently Asked Questions
What is a radiation pattern?
A radiation pattern shows the radiated field intensity of an antenna vs. angle, revealing the main beam, sidelobes, nulls, and back lobe. It is the fundamental characterization of how an antenna distributes energy in space.
What determines sidelobe level?
Sidelobe level is primarily determined by the amplitude distribution across the antenna aperture. A uniform distribution gives the highest gain but -13.2 dB sidelobes. Tapered distributions (Taylor, cosine) reduce sidelobes at the cost of slightly lower gain and wider main beam.
How is a radiation pattern measured?
antenna radiation patterns are measured in an anechoic chamber (indoor) or on an outdoor antenna range. The antenna under test is rotated while a probe antenna measures the received signal vs. angle. Near-field scanning is used for large antennas, with mathematical transformation to far-field.