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Wire Antenna – Half-wavelength Dipole

E-field phase animation at 10 GHz
Figure 1: E-field phase animation of a dipole antenna showing the broadside radiation of this antenna.

The Physics

The half-wavelength dipole antenna is a simple dipole which length is half-wavelength of the operation frequency. It is a balanced antenna and its characteristics are well known by the theory and can be found in [1]. It has a broadside radiation with a torus like radiation pattern (Fig. 2) with a maximum theoretical directivity of 1.643 and an input impedance of 73 Ω.

Radiation pattern of a half-wavelength dipole antenna
Figure 2: Radiation pattern of a half-wavelength dipole antenna.

The Model

The half-wavelength dipole antenna operating at 1 GHz can be modeled in CST STUDIO SUITE® as two cylinders separated by a small gap as shown in Fig. 3, driven by discrete face port between the two. The parameters used in the final model are shown in Table 1. The dipole length has been adjusted to tune the antenna to 1 GHz. For more information on this tuning process, see the Construction Help.

Parameter Value Description
L 135.6 mm Dipole length
Gap 20 mm Feeding gap of the antenna
D 5 mm Diameter of the conductor
Z0 73 Ω Port impedance
Half-wavelength dipole modeled in CST STUDIO SUITE, showing the main dimensions and the face port.
Figure 3: Half-wavelength dipole modeled in CST STUDIO SUITE® using simple geometric primitives, showing the main dimensions and the face port.

 

Model Construction Watch Video

Download Model File (Student Edition) Download Model File

Discussion of Results

Fig. 1 shows the E-field phase animation. On that animation we can see the broadside radiation of the dipole. In Fig. 4 the S-parameter of the dipole is shown and its directivity is plotted on Fig. 5. The S-parameter shows that the antenna is resonant at 1 GHz and the farfield at this frequency shows the torus-like radiation pattern and a maximum directivity of 1.67, close to the number predicted by the theory.

Farfield plot of half-wavelength dipole simulated in CST STUDIO SUITE ®
Figure 4: S-parameter result of a half-wavelength dipole antenna designed to operate at 1 GHz.

 

S-parameter result of the half-wavelength dipole antenna
Figure 5: Farfield plot of the directivity of a half-wavelength dipole. On the left side the farfield directivity in linear scale is plotted on the elevation plane. On the right side the 3D pattern of the farfield is shown in dBi

Additional Tasks

  • According to theory, the dipole impedance is ~ 73Ω. Parametrize the discrete face port impedance and perform a parameter sweep on it, adjusting the impedance at 25Ω intervals. Observe what happens with the S11. How does impedance correlate with the directivity and realized-gain plots?
  • A plane wave is used to illuminate a dipole antenna as shown in Fig. 6. The antenna is a dipole with resonant frequency of 250 MHz. The voltage results in the antenna terminals are shown in Fig.7 against the S-parameter result. Looking at the farfield results of the dipole explain why the antenna does not pick up significant energy from the plane wave at higher frequencies, even though there are resonances in the S-parameters.
Plane wave configuration used to illuminate the dipole
Figure 6: Plane wave used to illuminate the dipole. Note that the E-field is linearly polarized and propagates in the X direction, with the E-field vector pointing in the Z direction.

 

Plot of the S11 against the terminal voltages of the dipole illuminated by a plane wave
Figure 7: Broadband S-parameter result of a dipole with central frequency at 250 MHz in red. In blue is the voltage at the antenna terminals when it is illuminated by a plane wave with 1 V/m.

References

  • [1] C.A. Balanis, Antenna Theory: Analysis and Design, 3rd Edition, Wiley-Interscience, pp. 182-184
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