Sidelobes

A typical antenna radiation pattern showing sidelobes.

In antenna engineering, side lobes are the lobes of the radiation pattern that are not the main beam. 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.

For an antenna with a uniformly illuminated rectangular aperture, the first sidelobe is 13.5 dB below the peak of the main beam. For an antenna under the same conditions but with a circular aperture, the first sidelobe level is 17 dB below the peak of the main beam. The sidelobe level can be reduced by shaping the aperture distribution (changing from uniformity) at the expense of reduced directivity.

The null between sidelobes occur when the radiation patterns passes through the origin in the complex plane. Hence, adjacent sidelobes are generally 180° out of phase to each other.

Because an antenna's radiation pattern is a Fourier Transform of its aperture distribution, most antennas will generally have sidelobes, unless the aperture distribution is a Gaussian, or if the antenna is so small, as to have no sidelobes in the visible space. Larger antennas have narrower main beams, as well as narrower sidelobes. Hence, larger antennas have more sidelobes in the visible space (as the antenna size is increased, sidelobes move from the evanescent space to the visible space).

A typical array radiation pattern showing both grating lobes and side lobes.

For discrete aperture antennas (such as phased arrays) in which the element spacing is greater than one wavelength, the aliasing effect causes some sidelobes to become grating lobes, which are identical copies of the main beams. In such a case, the sidelobes should be considered all the lobes lying between the main lobe and the first grating lobe, or between grating lobes. It is conceptually useful to distinguish between sidelobes and grating lobes because grating lobes are higher than all the other side lobes.

For antennas in receive mode, sidelobes make the antenna more vulnerable to noise from stray signals coming far away from the transmit source. For transmit antennas communicating classified information, sidelobes represent security vulnerability, as an unintended receiver may pick up the classified communication.