Yagi Array Example

Modelling an array of modified Yagi antennas using SuperNEC

The starting point for this tutorial is the Yagi antenna. SuperNEC has a number of built-in antennas that can be very easily constructed. These antenna assemblies are listed in the menu item Add | Assembly | antennas as shown in Figure 1.

To add a Yagi antenna to the structure, select the menu item Add | Assembly | antennas | snyagi.

Figure 1 : Adding a Yagi to the structure.

A dialog box requesting the parameters of the Yagi will appear (Figure 2).

Figure 2 : Entering the Yagi parameters.

The main parameters of the Yagi are the element spacing, element lengths and the radii of the wires making up the Yagi. The above dialog shows the default settings for a Yagi, which is a Yagi with 5 elements each spaced 0.2 m apart. On pushing the OK button, a segmented Yagi will appear in the structure editor.

Figure 3 : The default SuperNEC Yagi.

Notice that the second element has a yellow marker on its centre segment. This marker is the SuperNEC symbol for a source.

SuperNEC provides both zooming and panning functionality. To zoom in on the structure do the following :

· Push the shift key on the keyboard.

· Position the mouse pointer within the grid displaying the structure, but not over any part of the structure.

· Push the left mouse button and move the mouse away from you. This will invoke the zoom-in facility.

· Moving the mouse towards you will give you the effect of zooming out.

To pan the view of the structure :

· Push the control key on the keyboard.

· Position the mouse as was done for zooming.

· Mouse the mouse to the left, right, up or down to pan the view.

On the bottom left hand side of the structure editor window is the text ‘Model Freq :’. The number to the right of this text specifies the frequency at which the structure is modelled. In this case, it is set to 300 MHz. You can change this figure to generate a model of the antenna at a different frequency. Try changing the number to 400 and then push the ‘Set’ button. Note that the number of segments used to model the antenna increases.

The ‘Group Level’ setting for the model is currently set to high. Reduce the level to 2 by clicking on the button with the ‘<’ label. Now use the mouse to select part of the structure (left click whilst the mouse pointer sits on top of part of the structure). Note that you are able to select individual the wires making up the structure. Reduce the group level to ‘low’ and again select part of the structure. You are now able to select the individual segments.

Select the segment with the source attached and push the ‘Edit’ button. A dialog box for editing the segment will appear as shown in Figure 4.

Figure 4 : The segment edit box.

Note that this segment is excited with an applied field voltage source with a value 1+0i. Now cancel the edit box.

You can also edit the segment by double clicking on the segment in the structure editor.

Now increase the group level setting, select the last wire element of the antenna and push the ‘Edit’ button (or double click on the last element). The edit box for the subassembly ‘snwire’ appears. You can now change the length (or some other parameter) of the wire by changing the values in the edit box.

If you increase the group level back to high and double click on the structure, the edit box for the Yagi antenna will appear. If you push the OK button, then the Yagi will be regenerated and any changes that you have made to the elements of the Yagi will be lost.

Let us now assume that we want to connect the centre of the last two elements of the Yagi with a 45 W transmission line. This is achieved as follows :

1. Set the group level to ‘low’ so that we are able to select individual segments.

2. Left click on the centre segment of the last element of the Yagi (this should highlight the segment in red).

3. Hold the shift key down and left click on the centre segment of the second last element of the Yagi (you should now have two segments highlighted in red as shown in Figure 5).

Figure 5 : Selecting two segments in the model.

Figure 6: Adding a transmission line between the two highlighted segments.

4. Select the menu item Add | Primitive | Network | Transmission Line as shown in Figure 6.

5. The transmission line dialog box will appear. Fill it in as shown in Figure 7.

Figure 7 : Setting the properties of the transmission line.

6. Push the OK button and another dialog asking how the segments should be linked will be displayed.

Figure 8 : How should the segments be linked.

7. Push the ‘Pairs’ button and the transmission line will be added to the structure as shown in Figure 9.

Figure 9 : The new transmission line.

You can edit the parameters of the transmission line by double clicking on the centre portion of the transmission line symbol.

We will now put a lumped element load of 3 W onto the tips of reflector element. This is achieved as follows :

1. Select the two segments as shown below:

Figure 10 : Selecting the segments to load.

2. Call the menu item Add | Primitive | Load.

Figure 11 : Illustrating the menu item for adding a load.

3. Fill in the parameters of the load

.

Figure 12 : Adding a 3 W load

4. Push the OK button.

Figure 13 : The Yagi with two 3 W loads applied.

Note that the SuperNEC symbol for a load is a black mark as shown in Figure 13.

You can check the loading of parameters of each of the segments by double clicking on the loaded segments. The segment edit box for the left hand segment should look similar to Figure 14.

Figure 14 : Illustrating the settings for one of the loaded segments.

We will now illustrate some of the other structure manipulation features of the editor. First we will translate the entire structure 1 m in the x-direction and 1 m in the z-direction. To do this :

· Push the ‘Select All’ button at the bottom of the structure editor window.

· Push the ‘Translate’ button and fill in the resulting dialog box as follows :

Figure 15 : Translating the structure by the vector [1, 0, 1].

· Push the OK button and the structure is translated.

Figure 16 : The translated structure.

We will now create a stacked array of these Yagi’s. The stack will be vertically oriented and the spacing between antennas will be 0.5 m. To do this :

· Select all.

· Push the translate button and fill in the resulting dialog box as follows :

Figure 17 : Creating 2 duplicates of the structure.

· Push the OK button and observe the result.

Figure 18 : A 3-stack of Yagi’s.

We will now assume that this modification we made to the structure was incorrect. To undo this modification push <control-Z> (the control key and then the Z key).

To redo the operation push <control-Y>. The structure should now be restored to that shown in Figure 18.

The structure will now be reflected along the x-axis (y-z plane). To do this :

· Select all.

· Push the reflect button and check the x-axis check box.

· Push the OK button.

Figure 19 : The structure reflected along the x-axis.

Note that the reflected portion of the structure is grey. This grey colour means that these elements of the structure form an image of the original structure and that symmetry will be used in the simulation. If you modify the structure (other than to add transmission lines and excitations), then the symmetry of the model will be lost.

This concludes the generation of the model.

Setting the simulation settings

Let us assume that we wish to model this structure from 250 MHz to 300 MHz and determine its radiation pattern in this frequency range. To set these simulation settings call the Edit | Simulation Settings menu item.

Figure 20 : Calling the simulation settings editor.

Figure 21 shows the simulation settings editor with the simulation frequencies set to start at 250 MHz, end at 300 MHz and go up in 10 MHz intervals ([250:10:300]).

Figure 21 : Setting the simulation frequency range.

To set the radiation patterns, click on the last line of the summary list box (Radiation Patterns: <click here to add>).

The bottom part of the dialog box changes to the radiation pattern editor as shown in Figure 22

Figure 22 : The radiation pattern editor.

Select the 3D pattern in 4 degree increments from the predefined patterns list box and push the Add button.

The simulation settings dialog changes to that shown in Figure 23.

Figure 23 : Adding a radiation pattern.

This concludes the simulation settings requirement of the simulation and the OK button can be pushed.

You now return to the structure editor. To simulate the structure, push the simulate button at the bottom right hand side of the editor. You will be requested to save your work, after which the simulation will proceed.

The simulation takes place in a DOS box similar to Figure 24.

Figure 24 : Running the simulation in a DOS window.

To view the simulation results, use the Simulate | View Output menu item.

Figure 25 : Starting the output viewer.

The output viewer as shown in Figure 26 will appear.

Figure 26 : The output viewer.

Push the ‘All’ button below the ‘Filter Available Structures’ list box and also the ‘All’ button below the ‘Filter Available Frequencies’ list box.

The ‘Summary of available results’ list box will change as shown in Figure 27.

Figure 27 : Showing the available currents and charges for display.

The top line of the list box is highlighted. This corresponds to selecting all the current distributions for plotting. To see these current distributions push the ‘Plot’ button. A window similar to Figure 28 will appear.

Figure 28 : The current distribution.

The slider at the bottom right-hand side of the screen can be moved to view the currents at the other simulation frequencies.

Return to the output viewer and push the ‘Radiation Pattern’ tab at the top of the viewer.

The ‘Summary of available results’ list box will change as shown in Figure 29

Figure 29 : Showing available radiation patterns.

Tick the check box labelled ‘Structure’ at the bottom left hand side of the window. If the ‘Plot’ button is now pushed, then the radiation patterns overlaid on the structure will be plotted. This is illustrated in Figure 30.

Figure 30 : The radiation pattern and Yagi antenna.

The structure had to be magnified in order to see it properly. This was achieved using the View | Structure | Scale menu item.

The slider at the bottom right-hand side of the plotter can be moved to show the radiation pattern at different frequencies.

This concludes the modified Yagi array tutorial.

The structure file, SuperNEC input and output files can be found in the tutorial directory. They are called iptut.str, iptut.nec and iptut.out respectively.

There are a number of other example files located in the tutorial directory.

They are :

1. The files optut.* model a folded dipole Yagi antenna from 100 MHz to 300 MHz. Near fields, 2-D radiation patterns and 3-D patterns are computed for this antenna.

2. The files optutmom.* model a monopole on a finite size ground plate. The ground plate is modelled using MoM and 2-D radiation patterns are computed.

3. The files optututd.* model a monopole on the same size finite ground plate as the optutmom.* simulation. The radiation patterns requested are also identical.

You can compare the results obtained from the MoM and UTD monopole simulations as follows :

· Start the output SuperNEC utility by typing snecout at the Matlab command line.

· Load the optutmom.out file into the output utility.

· Push the ‘Load’ button and load the optututd.out file.

Figure 31: The SuperNEC output interface with both MoM and UTD output files loaded.

· Push both ‘All’ buttons to select both models and all frequencies.

· Push the ‘Radiation Pattern’ tag and select the 250 MHz radiation patterns as shown below :

Figure 32 : Selecting the 250 MHz radiation patterns.

· Check the ‘Overlay’ check box and push the ‘Plot’ button. The radiation patterns will appear overlaid in the radiation pattern viewer as shown in Figure 33.

Figure 33 : Comparing the MoM and UTD patterns.

· Normalise the radiation patterns as shown below :

Figure 34 : Normalising the radiation patterns.

· Compare the normalised radiation patterns at the other frequencies in a similar manner.

· The input impedances computed using the two techniques can be compared by setting the output interface as follows :

Figure 35 : Setting up the output interface for a comparison of input impedances.

· Check the ‘Overlay’ button and push the ‘Plot’ button. The input impedances are overlaid on the Smith chart as shown in Figure 36.

Figure 36 : Comparing the input impedance as computed by the two methods.

Home Price List Contact Us Download News Info Applications

Top of Page