Sidelobe

A sidelobe is a secondary lobe in a directional pattern of radiated or received energy that lies outside the main lobe, which is the region of highest response in the intended direction. In antennas, the main lobe points toward the desired direction, while sidelobes are smaller peaks at other angles. The strongest sidelobe is typically called the first sidelobe; a lobe opposite the main lobe is referred to as a backlobe. The level of sidelobes is usually expressed in decibels relative to the main lobe peak, for example as a sidelobe level (SLL) in dB.

Causes and implications: Sidelobes arise from the finite size and shape of the aperture, discontinuities in excitation across the aperture, element patterns in arrays, and mutual coupling between elements. They can lead to undesirable radiation in unintended directions, contributing to interference, reduced dynamic range, or false detections in radar and communications systems.

Control and design: Sidelobe levels can be reduced by apodizing or tapering the aperture illumination, using windowed or optimized excitation profiles (such as Hanning, Hamming, Blackman, or Dolph-Cagr chose windows). There is a trade-off: lowering sidelobes often broadens the main lobe, increasing beamwidth. In array design, careful element placement, feeding, and impedance considerations also influence sidelobe performance.

Related areas: In signal processing, sidelobes appear in the frequency response of finite-length filters and can be mitigated with windowing techniques, at the cost of reduced frequency resolution. Sidelobes are relevant in acoustics, sonar, radio astronomy, and wireless communications.