The Dipole Antenna

In this section, the dipole antenna with a very thin radius is considered. The dipole is similar to the short dipole except it is not required to be small compared to the wavelength (of the frequency the antenna is operating at). For a dipole antenna of length L oriented along the z-axis and centered at z=0, the current flows in the z-direction with amplitude which closely follows the following function: Note that this current is also oscillating in time sinusoidally at frequency f. The current distributions for a quarter-wavelength (left) and full-wavelength (right) dipoles are given in Figure 1. Note that the peak value of the current is not reached along the dipole unless the length is greater than half a wavelength. Figure 1. Current distributions on finite-length dipoles. Before examining the fields radiated by a dipole, consider the input impedance of a dipole as a function of its length, plotted in Figure 2 below. Note that the input impedance is specified as Z=R + jX, where R is the resistance and X is the reactance. Figure 2. Input impedance as a function of the length (L) of a dipole antenna. Note that for very small dipoles, the input impedance is capacitive, which means the impedance is dominated by a negative reactance value (and a relatively small real impedance or resistance). As the dipole gets larger, the input resistance increases, along with the reactance. At slightly less than 0.5 the antenna has zero imaginary component to the impedance (reactance X=0), and the antenna is said to be resonant. If the dipole antenna's length becomes close to one wavelength, the input impedance becomes infinite. This wild change in input impedance can be understood by studying high frequency transmission line theory. As a simpler explanation, consider the one wavelength dipole shown in Figure 1. If a voltage is applied to the terminals on the right antenna in Figure 1, the current distribution will be as shown. Since the current at the terminals is zero, the input impedance (given by Z=V/I) will necessarily be infinite. Consequently, infinite impedance occurs whenever the dipole is an integer multiple of a wavelength. In the next section, we'll consider the radiation pattern of dipole antennas.