_______________________________________________________________________ Antennas
on the polar-coordinate graph. The vertical axis on the rectangular-coordinate
graph corresponds to the rotating axis (radius) on the polar-coordinate graph.
Rectangular-Coordinate Pattern
4-56. Look at figure 4-11, view A. 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.
4-57. 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 is
measured at position 2. From position 2 on the horizontal axis, a dotted line
that runs parallel to the vertical axis is projected upwards. From position 7
on the vertical axis, a line that runs parallel to the horizontal axis is
projected to the right. 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.
4-58. 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.
Polar-Coordinate Pattern
4-59. The polar-coordinate graph has proved to be of great use in studying
radiation patterns. Compare views A and B of figure 4-11. Note the great
difference in the shape of the radiation pattern when it is 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.
4-60. Look at figure 4-11, view B, 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 outermost 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.
4-61. 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.
4-62. Notice that the rotating axis is always drawn from the center of the
graph to some position on the edge of the graph (figure 4-11, view B). As the
axis moves toward the edge of the graph, it passes through a set of equally
4-17
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