The half-wave dipole antenna is just a special case of the dipole antenna, but its important enough that it will have its own section. Note that the «half-wave» term means that the length of this dipole antenna is equal to a half-wavelength at the frequency of operation.
To make it crystal clear, if the antenna is to radiate at 600 MHz, what size should the half-wavelength dipole be?
One wavelength at 600 MHz is = c / f = 0.5 meters. Hence, the half-wavelength dipole antenna’s length is 0.25 meters.
The half-wave dipole antenna is as you may expect, a simple half-wavelength wire fed at the center as shown in Figure 1:
Figure 1. Electric Current on a half-wave dipole antenna.
The input impedance of the half-wavelength dipole antenna is given by Zin = 73 + j42.5 Ohms. The fields from the half-wave dipole antenna are given by:
The directivity of a half-wave dipole antenna is 1.64 (2.15 dB). The HPBW is 78 degrees.
In viewing the impedance as a function of the dipole length in the section on dipole antennas, it can be noted that by reducing the length slightly the antenna can become resonant. If the dipole’s length is reduced to 0.48 , the input impedance of the antenna becomes Zin = 70 Ohms, with no reactive component. This is a desirable property, and hence is often done in practice. The radiation pattern remains virtually the same.
The above length is valid if the dipole is very thin. In practice, dipoles are often made with fatter or thicker material, which tends to increase the bandwidth of the antenna. When this is the case, the resonant length reduces slightly depending on the thickness of the dipole, but will often be close to 0.47
.