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Monopole Over Real Ground

A monopole is a vertical element connected to a ground plane and with the feed point at its base. In this example we will simulate a radio mast on an imperfect ground, which is used for broadcasting in the LF and MF bands.

Step 1 | Setup

Go to the Setup tabsheet and set an operating frequency of 3 MHz in the Frequency panel. Then, go to the Environment panel > Ground Plane box and select Real, Fig. 1. Select Radial wire ground screen and the Poor ground options. Note that the soil conductivity will automatically be set to 0.001 S/m and the permittivity (dielectric constant) to 5.

Finally, set the number of radials, their length and radius as shown in Fig. 1. In radio masts it is customary to use a constant input power as a reference, for example 1 kW. Go to the Excitation panel, select Discrete Sources, Set Input Power and enter 1,000 W, Fig. 2.

Fig. 1: Setting a radial wire ground screen.
Fig. 2: Setting discrete sources as the excitation with 1,000 W of input power.
Step 2 | Draw

Right click on the workspace and select Line from the displayed pop-up menu >. Specify a vertical wire 25 m in height (1/4 of a wavelength at 3 MHz) and with a triangular cross section as shown in Fig. 3. Although the recommended minimum number of segments is 3, we will divide the wire into 10 segments to obtain greater resolution in the current distribution. Note that the wire will be automatically connected to the ground at the origin (0,0,0).

Right click on the wire, select the Source/Load command from the pop-up menu and put a voltage source on the first segment, so the source will be connected to the base of the mast. Refer to Adding Sources >.

Fig. 3(a): Specifying a vertical wire in the Line page.
Fig. 3(b): Specifying a triangular cross section.
Step 3 | Run

Click on the Run Currents and Far-Field (F11) button on the toolbar. After the calculations are complete, click on the Far-Field 3D Plot button on the toolbar to display the radiation pattern. Choose Radiation Pattern under the Plot menu in AN-3D Pattern to plot the normalized radiation pattern (dimensionless). Then, choose the Radiation Pattern [dB] option to see the pattern in decibel scale. Note that the far field has a null on the xy-plane due to the losses in the ground plane, Fig. 4.

The antenna efficiency is the radiated to the input power ratio. Go to the Results tabsheet to see the input impedance, VSWR, Directivity, Gain, and Efficiency, Fig. 5. Note that the efficiency is low and therefore the gain too since most of the input power is lost to the ground. In this example we have chosen a Poor soil. Try different soils and increasing the number of radial wires and their length to improve the antenna efficiency.

Fig. 4: Radiation pattern of a quarter-wave monopole over a radial wire ground screen.
Fig. 5: Results tabsheet for a quarter-wave monopole over a radial wire ground screen.
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