ohmic resistance is about 1 ohm for conductors of large diameter. The ohmic

resistance does not become important until the radiation resistance drops to

a value less than 10 ohms. This may be the case when several antennas are

coupled together.

4-52. The energy radiated from an antenna forms a field having a definite

radiation pattern. A radiation pattern is a plot of the radiated energy from an

antenna. This energy is measured at various angles at a constant distance

from the antenna. The shape of this pattern depends on the type of antenna

used. The following sections introduce the basic types of radiation (isotropic

and anisotropic) and their radiation patterns.

4-53. Some antenna sources radiate energy equally in all directions.

Radiation of this type is known as isotropic radiation. We all know the sun

radiates energy in all directions. The energy radiated from the sun measured

at any fixed distance and from any angle will be approximately the same.

Assume that a measuring device is moved around the sun and stopped at the

points indicated in figure 4-10 to make a measurement of the amount of

radiation. At any point around the circle, the distance from the measuring

device to the sun is the same. The measured radiation will also be the same.

The sun is therefore considered an isotropic radiator.

4-54. To plot the pattern shown in figure 4-10, assume that the radiation is

measured on a scale of 0 to 10 units and that the measured amount of

radiation is 7 units at all points. The measurements can be plotted on two

different types of graphs: rectangular-and polar-coordinate graphs. The