Designing an Open-Ended Quadrifilar Helix in Antenna Magus


Image of the Open-Ended Quadrifilar Helix

This application note discusses the design of an Open-Ended Quadrifilar Helix antenna in Antenna Magus. The antenna template used here is available in the free evaluation version of Antenna Magus.

We start by looking at navigation of the Antenna Database and selecting the antenna for design. Then we go through the process of designing the antenna prototype according to certain design specifications using the design options offered. Antenna Magus is used to estimate performance of the prototype and evaluate how tweaking designs affects the antenna's performance.

A significant feature of Antenna Magus is the capability to export ready-to-run parameterized antenna models to various leading commercial full wave packages. After the prototypes have been designed we look at how they can be exported to these packages.

User Interface

If you are unfamiliar with Antenna Magus it is recommended that you briefly familiarize yourself with the basic layout of the UI before continuing with the rest of the application note. A description of the main areas in UI is given below.

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  • Ribbon - Offers Access to commonly used tools and operations. It also allows the user to switch between different modes in the application.
  • Palette - The pallet consist of expanders that generally follow workflow of the application, however, strict adherence to this workflow is not required. The expanders generally consist of parameters and operations.
  • Workspace - Typically displays results and additional information. Here the user is able to view and refine results.
  • Collections - Consists of the users antenna prototypes.

Locating the Template

Before we can start the design process, we have to locate the template to use. When launching Antenna Magus, it starts up in "Find mode" where the antenna database is displayed in the 'Workspace'. Here the user can browse and explore the templates available in Antenna Magus. Using the search bar located in the 'Pallet' the user can enter keywords to search for antennas based on the certain criteria. Enter the keyword "Helix" into the search bar. Note that when typing the keyword suggested terms and their definitions according to the 'Keyword Dictionary' are shown.

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Press enter to start searching for antennas that match the word "Helix". Also enter "Open-Ended" as a search term.

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The Antennas in the database are grouped according to how well they match these search terms. The top group will contain the required Open-Ended Quadrifilar Helix template. .

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Hover the mouse over the template. Three buttons will appear: 'Add', 'Design' and 'Info'.

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Click on 'Info' to open the 'Info Browser' which contains more information about this antenna. (shown below.)

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The 'Info Browser' is used to view additional information on antennas or compare them with other antennas. Click on the 'New Comparison' button within the info browser to open a side-by-side view. The right-hand pane is empty and we have to add another antenna to the 'Info Browser' to start comparing. Navigate back to the antenna database and select another template to compare with; click on the 'Info' button of the template. This will open another page in the 'Info Browser'. Now open the comparison page again and select the other template in the right-hand pane. A side-by-side of the two antennas are now shown on the comparison page.

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In the comparison view different properties of the two antennas can be compared with ease.

After inspecting the information on the antenna, close the info browser and navigate back to the main Antenna Magus window.

Design

We can now start with the design of the antenna. Hover the mouse over the Open-Ended Quadrifilar Helix template and click on the 'Add' button. This will add a new prototype to the 'Collection' using the selected template. The 'Collection' (located at the bottom of the window) contains all the prototypes that the user is working on and is saved automatically and loaded when Antenna Magus is started.

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Click on the prototype in the 'Collection' to start designing. Antenna Magus will switch to 'Design Mode' and automatically start a new design. The design ("Design 1") is located in the 'Designs and Tweaks' expander located at the top of the pallet.

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Locate the 'Design for:' drop down in the 'Design Objectives' expander. This drop down shows all the design methods available for the antenna. If we expand the dropdown we note that the design methods available are 'operating frequency' and 'all options'.

Design for Operating Frequency

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First we are going to look at designing the antenna by only specifying the operating frequency. Rename the design (located in the 'Designs and Tweaks' expander) from "Design 1" to "Design - Operating Frequency".

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From the 'Design for:' drop-down select 'Operating Frequency'. We are presented with the input parameters for the design method. In this case the only input parameter is the centre frequency (f0). Hover the mouse over the input field and note the dialog displaying the valid input range (200 MHz to 20 GHz).

If we enter an invalid value an error message will be displayed stipulating the reasons why the input cannot be accepted. Set the centre frequency to 5GHz and click on the 'Design' button. This will design the antenna with the specified centre frequency as the only design objective. The parameters of the antenna are now calculated and shown in the 'Parameters' expander. Note how these designed parameters correlate with the parameters in the 'Sketches' displayed in the workspace. A model preview is generated form the parameters and also displayed in the workspace.

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The designed parameters can be adjusted, however, leave them unaltered for the time being. After inspecting the parameters, click on the 'Estimate Performance' button located at the bottom of the 'Parameters' expander to simulate the designed antenna. Navigate to the 'Estimated Performance' tab once the simulation has completed to view the results. Shown below is the results displayed in the workspace.

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This is typically what the results will look like for most antennas. By double clicking any of the windows in the workspace, we can expand them to get a better view. Double click on the 2D radiation plot to expand it (shown below). Note the additional information that is displayed in the expanded view of the plot.

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In the expanded view we can switch between different representations by using the spinner arrows at the top.

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Use the right arrow to switch to Left-hand circular gain.

In the top right of the chart window are two icons to switch between Cartesian and Polar plot. Use the 'Cartesian' button to switch to Cartesian plot. The representation can also be changed by using the options presented on the 'Format' ribbon.

After inspecting the radiation patterns, restore the maximized window to its original size by clicking on the icon in the top right corner.

Designing for Increased Beamwidth

To start a second design for the same prototype click on the 'New Design' button located in the pallet or on the 'Home' ribbon.

This will start a new design. Rename the design from "Design 2" to "Design - Increased Beamwidth". From the design for drop down, select 'all options'.

Additional design objectives are displayed. In addition to the centre frequency we can specify the polarisation, beamwidth and input resistance. Set the polarisation to 'LHC'; the 3db beamwidth to 170 degrees and the input resistance to 50 Ohm and click 'Design'.

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The antenna is now designed to take the additional design objectives into consideration. Inspect the designed parameters, leave them unaltered and click on 'Estimate Performance'. Once the simulation has completed the results are visible on the 'Estimated Performance' tab. We can compare the two antennas in the workspace. Ensure the radiation pattern is set to 'Total Gain' and note the increased beamwidth of the new design (shown below).

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Tweaking the Design

From the Reflection coefficient for the previous two designs (shown below) we note that the minimum reflection is below the desired 5GHz location. In this scenario we can tweak the design to obtain desired results.

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Navigate to the 'Sketches and Design Guidelines' tab to view parameters on the antenna sketches. For this tweak we are changing the value of Rh.

We are using "Design - Operating Frequency" as the base design that we want to tweak, select it in the "Designs and Tweaks" expander. Click on 'New Tweak' at the bottom of the parameters expander. Rename the tweak to "Tweak - Rh = 3.8mm". We can now tweak the parameters and compare the results to the original design.

In the 'Parameters' expander, set the values for Rh to 3.8mm and click on 'Estimate Performance' to view the tweaked results. The plot below shows that the resonant frequency has increased too much.

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Start a new tweak, rename it to "Tweak - Rh = 4mm" and repeat the process for Rh = 4mm. Shown below are the results for the new tweak.

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We note that the resonant frequency after the last tweak is considerably closer to 5GHz.

Exporting CEM models of designs to FEKO, CST Microwave Studio

Antenna Magus' export functionality lets users export fully parametric, ready-to-run models to FEKO, CST MWS of their designed antennas. The advantage of the models being parametric is that they can be easily edited within these packages. Since the models are ready-to-run any changes can be simulated in the respective packages without the need to specify mesh properties, ports, far field locations or boundary conditions etc.

Ensure that "Tweak - Rh = 4mm" is selected form the 'Designs and Tweaks' expander. Open 'Export Mode' by clicking on 'Export Mode' button in the 'Home' ribbon. The tabs in 'Export Mode' show which packages have available export models. Follow the links below to the respective sections.


Exporting to FEKO
Exporting to CST MWS

[Note: A licenced version of the chosen CEM tool is required to open and work with the models exported by Antenna Magus, and required to complete the steps illustrated below]

Exporting to FEKO

Select the FEKO tab in the Export View of Antenna Magus. Review the parameters and click on the 'Export Model' button. Ensure that the file type is set to "CADFEKO model", name the file "Open-ended quadrifilar helix.cfx" and save it to a convenient location. Minimize Antenna Magus and open the exported file. The Antenna Magus prototype is now open for editing in CADFEKO.

Before we start to simulate the model in FEKO a surface mesh of the model has to be created. This mesh is required to run the solver. In CADFEKO navigate to the 'Mesh' tab and click on 'Create Mesh' to open the 'Create mesh' dialog. Inspect the parameters in the dialog an click on 'Mesh' to create a mesh for the model. The model along with its mesh is shown below.

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Once the mesh has been created we can open POSTFEKO to start the solver. In CADFEKO navigate to the 'Solve/Run' tab and click on the 'POSTFEKO' button. POSTFEKO will load with the solver ready to run. In the 'Home' ribbon click on 'FEKO solver' to run the simulation. The simulation will require some time to run.

Shown below is the 3D farfield as simulated by FEKO.

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Exporting to CST MWS

Ensure that 'Export Mode' is open and that the 'CST MICROWAVE STUDIO' tab is selected. Inspect the parameters to be exported and click on 'Export Model' located in the 'Export' ribbon.

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Name the file "Open-ended quadrifilar helix.cst" and save it to a convenient location. Minimize Antenna Magus and open the exported file. The Antenna Magus prototype is now open for editing and simulation in CST MSW as shown below.

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To run the solver in CST MWS, click on the 'Solve' drop-down menu and select 'Time Domain Solver'. This opens the 'Time Domain Solver' dialog. Inspect the settings on the dialog and click 'Start'. The simulation will require some time to run.

Once the simulation has completed the results can be viewed by using the 'Navigation Tree' located on the left of the window. Expand the 'Farfields' node to and select the farfield at centre frequency (Shown below).

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We can view the S11 plot by expanding the '1D Results' and 'S-Parameters' nodes in the 'Navigation Tree'.

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Conclusion

This application note discussed the process of designing an Open-Ended Quadrifilar Helix antenna using Antenna Magus. The Open-Ended Quadrifilar antenna template has two design objective groups which were used to design an antenna for a specific frequency, beamwidth, polarization and input resistance. Once these designs were completed we showed how tweaking a design could achieve optimal performance. Next we described the process of exporting ready-to-run antenna simulation models to FEKO and CST MSW and how to run these models. Antenna Magus

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