Impedance Loaded Dipole

This example illustrates the evaluation of the transient behaviour of a loaded impedance dipole.

The example shown has been studied by Kanda[4]. The resistive loading of the dipole elements is graduated from the feed to the end points of each element. At the feed the loading is approximately 5kWm-1, in the centre of each element it is around 10kWm-1 and at the tip it is effectively infinite although a value of 200kWm-1 has was used in this example, as is consistent with Kanda.

The loading on the antenna is designed is to allow the antenna to perform well for impulse excitation. It has been shown by Wu and King[5] that a one way travelling wave can be excited on the arms of a dipole providing the loading on the dipole obeys a specific profile. Physically this can be interpreted as the energy of the wave being dissipated in the load before reaching the tip of the antenna where it would otherwise be reflected. While the profile satisfying this criterion is complex (i.e. consisting real and imaginary parts) and so is not realisable in practice, an approximation can be arrived at for a purely resistive loading, proposed by Kanda. It is shown in Figure 1 for one arm of a 15 cm dipole, the feed point is at the left of the graph.

 

Figure 1 - Resistance Loading Profile of Dipole Arms

 

The point of loading the antenna is that while reducing the efficiency substantially, it prevents resonant behaviour and allows a short duration transient pulse to be radiated. Kanda provides computed and experimental data on a number of parameters associated with the loaded antenna. Figures 2 and 3 show the current distribution along the dipole arms at 100MHz and 1GHz respectively.

 

Figure 2 - Current Distribution at 100MHz Along Dipole Arm

 

Figure 3 -Current Distribution at 1GHz Along Dipole Arm

 

Figures 4 and 5 shows the input admittance (conductance and susceptance respectively) of the loaded dipole. Kanda points out that the reason for the clear difference between the susceptance of the experimental configuration and the ideal case, as calculated, is the feed point capacitance. In the experiments it is estimated that the connector used to attach the antenna has a capacitance of around 1pF which causes the susceptance to increase rapidly with frequency. This effect can be accounted for in the Celia model, although for this example it has not beeen demonstrated.

 

Figure 4 - Feed Point Conductance

 

Figure 5 - Feed Point Susceptance

 

Continue to example 2 - Loaded Dipole Above a soil Interface