Figure 4-10.Comparison of rectangular- and polar-coordinate graphs for an isotropic source.
In the rectangular-coordinate graph, points are located by projection from a pair of stationary,
perpendicular axes. In the polar-coordinate graph, points are located by projection along a rotating axis
(radius) to an intersection with one of several concentric, equally-spaced circles. The horizontal axis on
the rectangular-coordinate graph corresponds to the circles on the polar-coordinate graph. The vertical
axis on the rectangular-coordinate graph corresponds to the rotating axis (radius) on the polar-coordinate
Look at view A of figure 4-10. The numbered positions around the circle are laid out on the
HORIZONTAL AXIS of the graph from 0 to 7 units. The measured radiation is laid out on the
VERTICAL AXIS of the graph from 0 to 10 units. The units on both axes are chosen so the pattern
occupies a convenient part of the graph.
The horizontal and vertical axes are at a right angle to each other. The point where the axes cross
each other is known as the ORIGIN. In this case, the origin is 0 on both axes. Now, assume that a
radiation value of 7 units view B is measured at position 2. From position 2 on the horizontal axis, a
dotted line is projected upwards that runs parallel to the vertical axis. From position 7 on the vertical axis,
a line is projected to the right that runs parallel to the horizontal axis. The point where the two lines cross
(INTERCEPT) represents a value of 7 radiation units at position 2. This is the only point on the graph that
can represent this value.
As you can see from the figure, the lines used to plot the point form a rectangle. For this reason, this
type of plot is called a rectangular-coordinate graph. A new rectangle is formed for each different point
plotted. In this example, the points plotted lie in a straight line extending from 7 units on the vertical scale
to the projection of position 7 on the horizontal scale. This is the characteristic pattern in rectangular
coordinates of an isotropic source of radiation.
The polar-coordinate graph has proved to be of great use in studying radiation patterns. Compare
views A and B of figure 4-10. Note the great difference in the shape of the radiation pattern when it is