Sidelobes

In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the main lobe.

The other lobes are called "sidelobes", and usually represent unwanted radiation in undesired directions.

The longer the antenna relative to the radio wavelength, the more lobes its radiation pattern has.

In transmitting antennas, excessive sidelobe radiation wastes energy and may cause interference to other equipment.

Another disadvantage is that confidential information may be picked up by unintended receivers.

It is generally desirable to minimize the sidelobe level (SLL), which is 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 to optics (another branch of electromagnetics) and in acoustics fields such as loudspeaker and sonar design, as well as antenna design.

Simple substitutions of various values of X into the canonical equation yield the following results: For a circular aperture antenna, also having a uniform amplitude distribution, the first sidelobe level is −17.57 dB relative to the peak of the main beam.

Simple substitutions of various values of X into the canonical equation yield the following results: A uniform aperture distribution, as provided in the two examples above, gives the maximum possible directivity for a given aperture size, but it also produces the maximum sidelobe level.

The nulls between sidelobes occur when the radiation patterns passes through the origin in the complex plane.

For discrete aperture antennas (such as phased arrays) in which the element spacing is greater than a half wavelength, the spatial aliasing effect causes some sidelobes to become substantially larger in amplitude, and approaching the level of the main lobe; these are called grating lobes, and they are either identical, or nearly identical as shown in the figure, copies of the main beams.