Radiation pattern is the variation of the field intensity of an antenna, as
an angular function, with respect to the axis.
All antennas are measured against what is known as an isotropic antenna,
which is a theoretical antenna. This is the basis for all other antennas, as
shown in Figure
. Isotropic
antenna coverage can be thought of as a balloon that extends in all directions
equally. When an omnidirectional antenna is designed to have gain, coverage is
lost in certain areas.
Imagine pressing in the top and bottom of a
balloon. This causes the balloon to expand in an outward direction, which
covers more area in the horizontal pattern. It also reduces the coverage area
above and below the balloon. This yields a higher gain, as the balloon, which
represents the antenna, appears to extend to a larger horizontal coverage area.
This can be seen in Figure
.
Remember the higher the gain, the smaller the vertical beamwidth.
Some important antenna types include the following:
-
Isotropic antenna – This is a hypothetical antenna that radiates or
receives energy equally in all directions. Isotropic antennas do not exist
physically, but they represent convenient reference antennas for expressing
directional properties of physical antennas
-
Dipole antenna – This is usually a straight, center-fed, one-half
wavelength antenna, which is pictured in Figure
.
-
Antenna array – This is an assembly of antenna elements with
dimensions, spacing, and illumination sequence arranged in such a way that the
fields for the individual elements combine. This combination produces a maximum
intensity in a particular direction and minimum field intensities in other
directions.