Designing an Open-Ended Quadrifilar Helix

Self-guided learning

This demonstration discusses the design of an open-ended quadrifilar helix antenna in Antenna Magus. The antenna template used here is available in the Antenna Magus student edition.

Image of the Open-Ended Quadrifilar Helix
Figure 1: Image of the open-ended quadrifilar helix



From the start page go directly to 'Find Mode' by following the Find Antenna link

Figure 2: Find Antenna button on the Start Page


Find the template of the open-ended quadrifilar helix antenna by first entering the keyword ‘helix’ in the Keyword list. Note that when typing the keyword, suggested terms and their definitions according to the Keyword Dictionary are shown. Add "open-ended" as a search term to further refine the search.

Figure 3: Keyword List in Find Mode


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.

Templates grouped according to the entered keywords
Figure 4 : Templates grouped according to the entered keywords


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

Figure 5: Add, Design and Info actions on hover over template


Click on Info to open the info browser which contains more information about this antenna.

Information document
Figure 6: Information document

The info browser is used to view additional information on devices. Devices may also be compared to others in a side-by-side view by using the New Comparison button within the info browser.


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. It is saved automatically and loaded at start-up when Antenna Magus is launched.

Collection of prototypes the user is working on
Figure 7: Collection of prototypes the user is working on

Select the prototype in the collection to start designing. Antenna Magus will switch to Design Mode and automatically start a new design. The design is located in the Prototype Designs and Tweaks expander located at the top of the pallet.

Smart Design

The antenna can be designed without entering any design objective values or only specifying some objectives. In such cases, Antenna Magus will calculate the necessary values to produce a valid design, if possible. If an invalid value is entered, an error message will be displayed stipulating the reasons why the input cannot be accepted. Enter 5 GHz and click on Design. This will design the antenna for the specified frequency. The design will automatically be named ‘Design 1’. It is, however, possible to rename a design to a more suitable name.

Figure 8: Changing the objective input method

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 parametrically accurate model preview is generated and displayed in the workspace.

Figure 9: Parameters, Sketches and 3D Preview

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 perspective once the simulation has completed to view the results. Shown below are the results displayed in the workspace.

Figure 10: Estimated Performance results

By double clicking any of the windows in the workspace, we can expand them to get a better view. More information is available in the expanded view. Different representations are also available e.g. Impedance, VSWR, Lefthand circular gain, Ludwig III etc.

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 ribbon.

This will start a new design. Change the input method for the Radiation Pattern group in order to enter specific values. Select ‘LHC’ for polarization and specify a 3dB beamwidth of 170 degrees. Click Design.

Figure 11: Updated objectives

The antenna is now designed to take the additional design objectives into consideration. 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).

Figure 12: 2D radiation patterns for the Design 1 and Design 2. Note the increase in 3dB beamwidth for the second design

Tweaking the Design

From the Reflection coefficient for the previous two designs we note that the minimum reflection is slightly shifted from the desired 5 GHz center frequency. In this scenario we can tweak the design to obtain the desired results. From the Design Guidelines in the Overview perspective we determine that the frequency may be increased by decreasing the radius of the QHA.

Navigate to the Sketches or Overview perspective to view parameters on the antenna sketches. The radius we are interested in tweaking is denoted as Rh.

Select Design 1. Click on New Tweak at the bottom of the parameters expander or on the ribbon. The tweak is named automatically, but can be renamed.

In the Parameters expander, set the values for Rh to 4.0 mm and click on Estimate Performance to view the tweaked results. The plot below shows that the resonant frequency has been shifted to 5 GHz.

Reflection coefficient results for designs and tweak
Figure 13: Reflection coefficient results for designs and tweak


Ensure that the tweak is selected. Now navigate to Export Mode by using the Home ribbon button and select the CST Studio Suite tab.

Export Mode button on home ribbon

Figure 14: Export Mode button on home ribbon


Inspect the parameters to be exported and click on Export Model located in the Export ribbon.

Name the file and save it to a convenient location. Minimize Antenna Magus and open the exported file. The Antenna Magus prototype is now ready for editing and simulation in CST Studio Suite.