Editing Radiation pattern (section)

==Typical patterns==
[[Image:Sidelobes en.svg|thumb|Typical polar radiation plot.  Most antennas show a pattern of "lobes" or maxima of radiation.  In a [[Directional antenna|directive antenna]], shown here, the largest lobe, in the desired direction of propagation, is called the "[[main lobe]]".  The other lobes are called "[[sidelobe]]s" and usually represent radiation in unwanted directions.]]

Since [[electromagnetic radiation]] is [[dipole radiation]], it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical [[isotropic antenna]] is used as a reference to calculate [[antenna gain]].

The simplest antennas, [[Monopole antenna|monopole]] and [[dipole antenna]]s, consist of one or two straight metal rods along a common axis. These [[Axial symmetry|axially symmetric]] antennas have radiation patterns with a similar symmetry, called [[Omnidirectional antenna|omnidirectional]] patterns; they radiate equal power in all directions perpendicular to the antenna, with the power varying only with the angle to the axis, dropping off to zero on the antenna's axis. This illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry.

In most antennas, the radiation from the different parts of the antenna [[Interference (wave propagation)|interferes]] at some angles; the radiation pattern of the antenna can be considered an [[interference pattern]]. This results in minimum or zero radiation at certain angles where the radio waves from the different parts arrive [[out of phase]], and  [[local maximum|local maxima]] of radiation at other angles where the radio waves arrive [[in phase]]. Therefore, the radiation plot of most antennas shows a pattern of maxima called "''lobes''" at various angles, separated by "''[[null (radio)|null]]s''" at which the radiation goes to zero. The larger the antenna is compared to a wavelength, the more lobes there will be.
[[Image:Typical Antenna Pattern.jpg|thumb|right|A rectangular radiation plot, an alternative presentation method to a polar plot]]

In a [[directional antenna]] in which the objective is to emit the radio waves in one particular direction, the antenna is designed to radiate most of its power in the lobe directed in the desired direction. Therefore, in the radiation plot this lobe appears larger than the others; it is called the "''[[main lobe]]''". The axis of maximum radiation, passing through the center of the main lobe, is called the "''beam axis''" or ''[[Antenna boresight|boresight axis]]''". In some antennas, such as split-beam antennas, there may exist more than one major lobe. The other lobes beside the main lobe, representing unwanted radiation in other directions, are called minor lobes. The minor lobes oriented at an angle to the main lobe are called  "''[[side lobe]]s''". The minor lobe in the opposite direction (180°) from the main lobe is called the "''back lobe''".

Minor lobes usually represent radiation in undesired directions, so in directional antennas a design goal is usually to reduce the minor lobes. Side lobes are normally the largest of the minor lobes. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level. Side lobe levels of −20&nbsp;dB or greater are usually not desirable in many applications. Attainment of a side lobe level smaller than −30&nbsp;dB usually requires very careful design and construction. In most radar systems, for example, low side lobe ratios are very important to minimize false target indications through the side lobes.