Radiation Pattern
As an example, consider the 3-dimensional radiation pattern in Figure 1, plotted in
decibels (dB)
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A radiation pattern defines the variation of the power radiated by an antenna as a function
of the direction away from the antenna. This power variation as a function of the arrival angle
is observed in the far field.
Figure 1. Example radiation pattern (generated with FEKO software).
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This is an example of a donut shaped or toroidal pattern. In this case, along the z-axis,
which would correspond to the radiation directly overhead the antenna, there is very little
power transmitted. In the x-y plane (perpendicular to the z-axis), the radiation is
maximum. These plots are useful for visualizing which directions the antenna radiates.
Typically, because it is simpler, the radiation patterns are plotted in 2-d. In this case,
the patterns are given as "slices" through the 3d plane. The same pattern in Figure 1 is plotted
in Figure 2. Standard spherical coordinates are used, where
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is the angle measured off the z-axis, and
is the angle measured counterclockwise off the x-axis.
Figure 2. Two-dimensional radiation plots.
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If you're unfamiliar with radiation patterns or spherical coordinates, it may take a
while to see that Figure 2 represents the same pattern as shown in Figure 1. The pattern on
the left in Figure 2 is the elevation pattern, which represents the plot of the radiation
pattern as a function of the angle measured off the z-axis (for a fixed azimuth angle). Observing
Figure 1, we see that the pattern is minimum at 0 and 180 degrees and becomes maximum at
broadside to the antenna (90 degrees off the z-axis). This corresponds to the plot on the left
in Figure 2. The plot on the right in Figure 2 is the azimuthal plot. It is a function of the azimuthal angle for a fixed polar angle (90 degrees off the z-axis in this case). Since the pattern in Figure 1 is symmetrical around the z-axis, this plot appears as a constant in Figure 2. A pattern is "isotropic" if the radiation pattern is the same in all directions. These antennas don't exist in practice, but are sometimes discussed as a means of comparison with real antennas. Some antennas may also be described as "omnidirectional", which for an actual means that it is isotropic in a single plane (as in Figure 1 above for the x-y plane). The third category of antennas are "directional", which do not have a symmetry in the radiation pattern. |