Sidelobes

Lore & Background

The radiation pattern of most antennas shows a pattern of lobes at various angles where radiated signal strength reaches a maximum, separated by nulls where it falls to zero. This can be viewed as the diffraction pattern of the antenna. The longer the antenna relative to the radio wavelength, the more lobes its radiation pattern has. Larger antennas have narrower main beams and narrower sidelobes, and as antenna size increases, sidelobes move from evanescent space to visible space, resulting in more sidelobes in the visible space.

Because an antenna's far field radiation pattern is a Fourier transform of its aperture distribution, most antennas will generally have sidelobes unless the aperture distribution is Gaussian or the antenna is so small as to have no sidelobes in visible space. For a uniformly illuminated rectangular aperture, the radiation pattern has a canonical form based on the sinc function, with the first sidelobe at −13.26 dB relative to the main beam peak. For a circular aperture with uniform amplitude distribution, the pattern follows the Airy pattern (based on the Bessel function J₁), with the first sidelobe at −17.57 dB.

A uniform aperture distribution gives maximum possible directivity for a given aperture size but also produces the maximum sidelobe level. Sidelobe levels can be reduced by tapering the edges of the aperture distribution at the expense of reduced directivity. The nulls between sidelobes occur when the radiation pattern passes through the origin in the complex plane, so adjacent sidelobes are generally 180° out of phase with each other.

Reader's Guide

Sidelobes are significant because in transmitting antennas, excessive sidelobe radiation wastes energy and may cause interference to other equipment; confidential information may also be picked up by unintended receivers. In receiving antennas, sidelobes may pick up interfering signals and increase the noise level in the receiver. The power density in the sidelobes is generally much less than that in the main beam, and it is generally desirable to minimize the sidelobe level (SLL), measured in decibels relative to the peak of the main beam.

The concepts of main and sidelobes, radiation pattern, aperture shapes, and aperture weighting apply not only to antenna design but also to optics (another branch of electromagnetics) and in acoustics fields such as loudspeaker and sonar design. For discrete aperture antennas such as phased arrays, when element spacing is greater than half a wavelength, spatial aliasing causes some sidelobes to become substantially larger and approach the level of the main lobe; these are called grating lobes. Grating lobes are a special case of sidelobe, and it is conceptually useful to distinguish between them because grating lobes have larger amplitudes than most other sidelobes. The mathematics of grating lobes is the same as that of X-ray diffraction.

Did You Know?

• For a uniformly illuminated rectangular aperture, the first sidelobe is −13.26 dB relative to the main beam peak.
• Grating lobes occur in phased arrays when inter-element spacing is greater than half a wavelength.