antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

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 radiation pattern of most antennas shows a pattern of "''lobes''" at various angles, directions where the radiated signal strength reaches a maximum, separated by "'' nulls''", angles at which the radiated signal strength falls to zero. This can be viewed as the diffraction pattern of the antenna. In a

directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance ...

in which the objective is to emit the radio waves in one direction, the lobe in that direction is designed to have a larger field strength than the others; this is the "'' main lobe''". The other lobes are called "''sidelobes''", and usually represent unwanted radiation in undesired directions. The sidelobe directly behind the main lobe is called the back lobe. 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. 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. It is generally desirable to minimize the sidelobe level (SLL), which is measured in

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...

s relative to the peak of the main beam. The main lobe and sidelobes occur for both transmitting and receiving. The concepts of main and sidelobes, radiation pattern, aperture shapes, and aperture weighting, apply to optics (another branch of electromagnetics) and in

acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...

fields such as loudspeaker and sonar design, as well as antenna design. Because an antenna's far field radiation pattern is a

Fourier Transform A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...

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).

Sidelobes for case of uniformly-illuminated aperture

For a rectangular aperture antenna having a uniform amplitude distribution (or uniform weighting), the first sidelobe is relative to the peak of the main beam. For such antennas the radiation pattern has a canonical form of Simple substitutions of various values of into the canonical equation yield the following results: style="margin-left: auto; margin-right: auto; border: none;" ! ! Radiation Pattern ! Explanation , - , 0 , style="text-align:right;" , , peak of main beam , - ,

\displaystyle 3.14=\pi

, style="text-align:right;" , −∞ dB , first null , - ,

\displaystyle 4.49\approx\frac

, style="text-align:right;" , , peak of first sidelobe , - ,

\displaystyle 6.28=2\pi

, style="text-align:right;" , −∞ dB , second null , - ,

\displaystyle 7.72\approx\frac

, style="text-align:right;" , , peak of second sidelobe , - For a circular aperture antenna, also having a uniform amplitude distribution, the first sidelobe level is relative to the peak of the main beam. In this case, the radiation pattern has a canonical form of where

\displaystyle J_1(x)

is the Bessel function of the first kind of order 1. This is known as the

Airy pattern In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best- focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics, ...

. Simple substitutions of various values of into the canonical equation yield the following results: style="margin-left: auto; margin-right: auto; border: none;" ! ! Radiation Pattern ! Explanation , - , 0 , style="text-align:right;" , , peak of main beam , - , 3.83 , style="text-align:right;" , −∞ dB , first null , - , 5.14 , style="text-align:right;" , , peak of first sidelobe , - , 7.02 , style="text-align:right;" , −∞ dB , second null , - , 8.42 , style="text-align:right;" , , peak of second sidelobe , - A uniform aperture distribution, as provided in the two examples above, gives the maximum possible

directivity In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction from ...

for a given aperture size, but it also produces the maximum sidelobe level. Sidelobe levels can be reduced by tapering the edges of the aperture distribution (changing from uniformity) at the expense of reduced

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

complex plane In mathematics, the complex plane is the plane formed by the complex numbers, with a Cartesian coordinate system such that the -axis, called the real axis, is formed by the real numbers, and the -axis, called the imaginary axis, is formed by the ...

. Hence, adjacent sidelobes are generally 180° out of phase to each other.

Grating lobes

{{Main, Grating lobes For discrete aperture antennas (such as

phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...

s) 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. Grating lobes are a special case of a sidelobe. 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 have larger amplitudes than most, if not all, of the other sidelobes. The mathematics of grating lobes is the same as of

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

External links

Sidelobes and Beamwidths - An Antenna Tutorial
Antennas Radio frequency propagation de:Antennendiagramm#Elemente eines Antennendiagramms