In antenna engineering, side lobes are the lobes of the far field radiation pattern that are not the main beam, where the terms "beam" and "lobe" are synonyms. An antenna radiation pattern is more commonly called a beam pattern. The power density in the side lobes is generally much less than that in the main beam. It is generally desirable to minimize the sidelobe level (SLL), which is measured in decibels relative to the peak of the main beam. The main lobe and side lobes occur for both conditions of transmit, and for receive. The concepts of main and side lobes, aperture shapes, and aperture weighting, apply to problems in radar and optics (two specific applications of electromagnetics) and in sonar.
For a rectangular aperture antenna having a uniform amplitude (or uniform weighting), the first sidelobe is -13.26 dB relative to the peak of the main beam because for such antennas the radiation pattern has a canonical form of
A typical antenna radiation pattern showing sidelobes.
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