Antenna theory

AN4190

10/29

Doc ID 023812 Rev 1

application. Usually there is a distinction made between pattern and input impedance

variations. Accordingly, pattern bandwidth and impedance bandwidth are used to

emphasize this distinction. Associated with pattern bandwidth are gain, side lobe level, and

polarization, while input impedance and radiation efficiency are related to impedance

bandwidth.

1.7

Antenna polarization

Polarization of an antenna in a given direction is defined as the polarization of the wave

transmitted by the antenna. When the direction is not stated, the polarization is taken to be

the polarization in the direction of maximum gain. In practice, polarization of the radiated

energy varies with the direction from the center of the antenna, so that different parts of the

pattern may have different polarizations.

Polarization of a radiated wave is defined as that property of an electromagnetic wave

describing the time varying direction and relative magnitude of the electric filed vector;

specifically, the figure traced as a function of time by the extremity of the vector at a fixed

location in space, and the sense in which it is traced, as observed along the direction of

propagation. Polarization, then, is the curve traced by the end point of the arrow

representing the instantaneous electric field.

Polarization may be classified as linear, circular or elliptical (see Figure 6). If the vector that

describes the electric field at a point in space as a function of time is always directed along

a line, the field is said to be linearly polarized (horizontally or vertically). In general, however,

the figure that the electric field traces is an ellipse, and the field is said to be elliptically

polarized. Linear (see Figure 6) and circular (see Figure 7) are special cases of elliptical

polarization, and they can be obtained when the ellipse becomes a straight line or a circle,

respectively. The figure of the electric field is traced in a clockwise or counterclockwise

sense. Clockwise rotation of the electric field vector is designated as right-hand polarization

and counterclockwise as left-hand polarization.