Antenna Pattern Versus Electrical Length

Antennas do not radiate equally to all directions. The directional dependence of antenna radiation is aptly called the antenna pattern. An electric dipole that is 1/2 1 in length or smaller exhibits a fairly simple antenna pattern. The most power is radiated in the direction broadside to the antenna; that is, perpendicular to the antenna's length. The power decreases with decreasing angle such that at zero degrees, no power is radiated. In other words, no power is radiated in the direction of the end points. This behavior is fundamental to charges moving along a line. Referring to Figures 5.5D and 5.6B, you can see that no power is radiated in the direction that the charge moves. Figure 11.6 shows the three-dimensional plot of the radiated field power of a half-wavelength dipole.

Figure 11.6 The 3D radiation pattern of a half-wavelength dipole antenna, shown in both horizontal and vertical orientations.

Figure 11.6 The 3D radiation pattern of a half-wavelength dipole antenna, shown in both horizontal and vertical orientations.

In small antennas, the current goes to zero only at the end points. In large antennas with electrical length greater than l, other points of zero current appear. These locations of zero current that occur at intermediate distances along the antenna are standing wave nodes like those found on transmission lines. These nodes of zero current correspond to directions in which no radiation is received. The resulting antenna has regions, called lobes, of reception, which are separated by the nodes. Figure 11.7 shows the antenna pattern for electric dipoles of various lengths.

The magnetic dipole also exhibits an antenna pattern that is dependent on its electrical length. At small electrical lengths the majority of the power is radiated around the axis of symmetry. To understand what the axis of symmetry is, imagine a bicycle wheel. The axis of symmetry is the axle of the wheel. The radiation from the antenna is most prominent in the direction of the spokes that emanate from the axle. For electrically small loops, the radiation is zero in the direction of the axis of symmetry (in the direction of the axle).

As the loop is increased, a standing wave of current is created around the loop. The standing wave is quite complicated in form and causes the radiation pattern to lose its symmetry. The other interesting effect

Figure 11.7 Radiation patterns for electric dipoles of various electrical lengths. In each case, the antenna is driven with 1 A. The current on the antenna is shown for lengths of 1/2 and 3/2. The center plot shows the end-view pattern for all lengths.

Figure 11.7 Radiation patterns for electric dipoles of various electrical lengths. In each case, the antenna is driven with 1 A. The current on the antenna is shown for lengths of 1/2 and 3/2. The center plot shows the end-view pattern for all lengths.

of the standing wave is that it causes radiation to be emitted broadside to the loop (in the direction of the loop's axis of symmetry). Figure 11.8 shows the three-dimensional radiation patterns of loops of three different electrical sizes. Finally, the radiation from loop antennas contains waves of different polarizations.

Figure 11.8 Radiation patterns for magnetic loop antennas of three electrical sizes (given in terms of circumference). The top row shows the antenna with feed from each view.

Figure 11.8 Radiation patterns for magnetic loop antennas of three electrical sizes (given in terms of circumference). The top row shows the antenna with feed from each view.

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• SANTO
What is antenna electrical length?
7 years ago