The Dipole Antenna (continued)

The normalized radiation patterns for dipole antennas of various lengths are shown in Figure 1.

Figure 1. Normalized radiation patterns for dipoles of specified length.

The full-wavelength dipole is more directional than the shorter quarter-wavelength dipole. This is a typical result in antenna theory: it takes a larger antenna in general to increase directivity. However, the results are not always obvious. The 1.5-wavelength dipole pattern is also plotted in Figure 1. Note that this pattern is maximum at approximately +45 and -45 degrees.

The dipole is symmetric when viewed azimuthally; as a result the radiation pattern is not a function of the azimuthal angle . Hence, the dipole antenna is an example of an omnidirectional antenna. Further, the E-field only has one vector component and consequently the fields are linearly polarized. When viewed in the x-y plane (for a dipole oriented along the z-axis), the E-field is in the -y direction, and consequently the dipole antenna is vertically polarized.

The 3D pattern for the 1-wavelength dipole is shown in Figure 2. This pattern is similar to the pattern for the quarter- and half-wave dipole.

Figure 2. Normalized 3d radiation pattern for the 1-wavelength dipole.

The 3D radiation pattern for the 1.5-wavelength dipole is significantly different, and is shown in Figure 3.

Figure 3. Normalized 3d radiation pattern for the 1.5-wavelength dipole.

The (peak) directivity of the dipole varies as shown in Figure 4.

Figure 4. Dipole directivity as a function of dipole length.

Figure 4 indicates that up until approximately L=1.25 the directivity increases with length. However, for longer lengths the directivity has an upward trend but is no longer monotonic.