4-15
transferred from the rectangular-coordinate graph in view A to the polar-coordinate graph in view B. The
scale of radiation values used in both graphs is identical, and the measurements taken are both the same.
However, the shape of the pattern is drastically different.
Look at view B of figure 4-10 and assume that the center of the concentric circles is the Sun. Assume
that a radius is drawn from the Sun (center of the circle) to position 0 of the circle. When you move to
position 1, the radius moves to position 1; when you move to position 2, the radius also moves to position
2, and so on.
The positions where a measurement was taken are marked as 0 through 7 on the graph. Note how the
position of the radius indicates the actual direction from the source at which the measurement was taken.
This is a distinct advantage over the rectangular-coordinate graph in which the position is indicated along
a straight-line axis and has no physical relation to the actual direction of measurement. Now that we have
a way to indicate the direction of measurement, we must devise a way to indicate the magnitude of the
radiation.
Notice that the rotating axis is always drawn from the center of the graph to some position on the
edge of the graph. As the axis moves toward the edge of the graph, it passes through a set of
equally-spaced, concentric circles. In this example view B, they are numbered successively from 1 to 10
from the center out. These circles are used to indicate the magnitude of the radiation.
The advantages of the polar-coordinate graph are immediately evident. The source, which is at the
center of the observation circles, is also at the center of the graph. By looking at a polar-coordinate plot of
a radiation pattern, you can immediately see the direction and strength of radiation put out by the source.
Therefore, the polar-coordinate graph is more useful than the rectangular-coordinate graph in plotting
radiation patterns.
Anisotropic Radiation
Most radiators emit (radiate) stronger radiation in one direction than in another. A radiator such as
this is referred to as ANISOTROPIC. An example of an anisotropic radiator is an ordinary flashlight. The
beam of the flashlight lights only a portion of the space surrounding it. If a circle is drawn with the
flashlight as the center, as shown in view B of figure 4-11, the radiated light can be measured at different
positions around the circle. Again, as with the isotropic radiator, all positions are the same distance from
the center, but at different angles. However, in this illustration the radiated light is measured at 16
different positions on the circle.
