Sidelobe
Understanding Antenna Sidelobes
Sidelobes are an inherent consequence of finite aperture antennas. They represent directions where the antenna has significant gain, potentially receiving interference or transmitting energy outside the intended coverage area. Managing sidelobes is critical for radar (clutter rejection), satellite (adjacent satellite interference), and 5G (inter-cell interference).
Sidelobe Control
- Uniform illumination: Maximum gain but -13.2 dB first sidelobes.
- Cosine taper: -23 dB sidelobes, 0.9 dB gain loss, 1.3x beamwidth increase.
- Taylor (n-bar=5, -25 dB): -25 dB sidelobes, 0.3 dB gain loss. Good compromise.
- Gaussian: Very low sidelobes but significant gain loss and beam broadening.
Sidelobe Impact
- Radar: High sidelobes receive clutter from off-axis directions, masking weak targets.
- Satellite: High sidelobes couple energy to adjacent satellites, causing interference.
- EW: Electronic warfare systems detect and exploit sidelobes for jamming.
Uniform: -13.2 dB
Cosine: -23.0 dB
Cosine-squared: -31.5 dB
Taylor (SLL=-25): -25.0 dB
Taylor (SLL=-30): -30.0 dB
Trade-off: lower SLL = wider beam, lower gain
Each 5 dB SLL reduction costs ~0.2-0.5 dB gain
Frequently Asked Questions
What are antenna sidelobes?
Sidelobes are secondary peaks in the antenna radiation pattern outside the main beam. They are inherent in any finite-aperture antenna. The first sidelobe of a uniformly illuminated aperture is 13.2 dB below the main beam peak.
Why are low sidelobes important?
Low sidelobes reduce reception of interference from off-axis directions, minimize clutter in radar, reduce interference to adjacent satellite systems, and improve security by limiting unintended radiation. Satellite and military specifications typically require -25 to -35 dB sidelobes.
How do you reduce sidelobes?
Apply amplitude taper (illumination weighting) across the aperture. The edge elements receive less power than the center elements. This reduces sidelobes at the cost of slightly wider main beam and lower gain. Taylor, Chebyshev, and cosine tapers are common approaches.