Antenna Basics(Radiation Pattern)

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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 antenna’s far field.

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As an example, consider the 3-dimensional radiation pattern in Figure 1, plotted in decibels (dB) .

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three dimensional radiation pattern

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Figure 1. Example radiation pattern for an Antenna (generated with FEKO software).

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This is an example of a donut shaped or toroidal radiation 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.

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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 polar angle theta for plots in spherical coordinates is the angle measured off the z-axis, and phi, the azimuth angle of spherical coordinates in antenna patterns is the angle measured counterclockwise off the x-axis.

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2d or two dimensional radiation patterns for antennas

 Figure 2. Two-dimensional Radiation Patterns.

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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 radiation pattern as shown in Figure 1. The radiation 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 radiation pattern is minimum at 0 and 180 degrees and becomes maximum broadside to the antenna (90 degrees off the z-axis). This corresponds to the plot on the left in Figure 2.

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The radiation pattern 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 radiation pattern in Figure 1 is symmetrical around the z-axis, this plot appears as a constant in Figure 2.

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A pattern is “isotropic” if the radiation pattern is the same in all directions. Antennas with isotropic radiation patterns don’t exist in practice, but are sometimes discussed as a means of comparison with real antennas.

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Some antennas may also be described as “omnidirectional”, which for an actual antenna means that the radiation pattern is isotropic in a single plane (as in Figure 1 above for the x-y plane, or the radiation pattern on the right in Figure 2). Examples of omnidirectional antennas include the dipole antenna and the slot antenna.

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The third category of antennas are “directional”, which do not have a symmetry in the radiation pattern. These antennas typically have a single peak direction in the radiation pattern; this is the direction where the bulk of the radiated power travels. These antennas are very common; examples of antennas with highly directional radiation patterns include the dish antenna and the slotted waveguide antenna. An example of a highly directional radiation pattern (from a dish antenna) is shown in Figure 3:

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 Figure 3. Directional Radiation Pattern for the Dish Antenna.

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In summary, the radiation pattern is a plot which allows us to visualize where the antenna transmits or receives power.