RF and Microwave Engineering: Fundamentals of Wireless Communications

7.4 Wire Antennas

7.4.1 Half-Wave Dipole

The Hertzian dipole has an infinitesimal length with a homogeneous current distribution and cannot therefore be realized. We will look now at thin, centre-fed dipoles (see Figure 7.9a) with practical length and non-homogeneous current distribution. A dipole consists of two metallic rods and is considered to be thin if the diameter d of the rod wire is small compared to the dipole length (d ).

Figure 7.9 (a) Thin centre-fed dipole, (b) real- and imaginary part of input impedance, (c) Nyquist plot and (d) current distribution on dipole antennas of different lengths compared to wavelength.

To start with, we consider—as an example—a practical dipole length of ( = 53 mm). With an EM simulator we calculate the input impedance at the centre feedpoint. Figure 7.9b and c show the real and imaginary part of the antenna input impedance Z_A as well as the Nyquist plot (locus) in the complex impedance plane. The arrow designated by the letter f indicates the ...