Custom Antenna Templates in Antenna Magus
Adding your own antenna information to Antenna Magus
This note illustrates the process of adding a new antenna template to your Antenna Magus database. Due to copyright restrictions the reference for the antenna used is not distributed as part of this application note. The paper may be downloaded from IEEE Xplore (https://ieeexplore.ieee.org/document/407911):
S. T. Jellett and M. E. Bialkowski, “A circularly polarised MSAT antenna element with increased operational bandwidth”, Proceedings of IEEE Antennas and Propagation Society International Symposium and URSI National Radio Science Meeting, Seattle, WA, USA, 1994, pp. 1044-1047, Vol. 2.
A zip file containing an existing template and a CST Studio Suite model has been provided as described below.
Creating an Antenna Template in Antenna Magus
Antenna templates are used to capture antenna information in Antenna Magus. To create a new template click New Template on the home tab ribbon. This will add a blank card to the Find mode workspace and automatically switch to template editing mode. For this application note, however, it is recommended to work from the supplied partial template.
Importing the Partial Template
Select Import Templates on the ribbon on the home tab. This will open a file dialog where the template file ‘AM-CT-Example.am-templates’ should be selected. There are two templates in the file – one partial and one complete. Select only the partial template and press Import. Once imported, the template will be visible in Find mode. Select the template and press Edit template to switch to template-editing mode.
Changing the Thumbnail and the Name
The Antenna Magus logo has been used as a place holder in the partial template. To change it, hover over the existing thumbnail and select Replace. Choose the ‘thumbnail.png’ file. At the top of the workspace, change the name of the antenna template to ‘Circularly polarized stacked patch for wider band array element’.
Adding Search Terms
Search terms are used in Antenna Magus to search through the database. Two types of search terms (called ‘Keywords’) can be added to templates. The first type are pre-defined keywords contained in the Antenna Magus keyword dictionary, while the second type include custom, user-defined keywords. According to the paper, this antenna has a bandwidth of 15 %. In Antenna Magus, this qualifies as ‘Moderate Bandwidth’, used to describe all antennas with an operational bandwidth between 10 % and 70 %. To add the ‘Moderate bandwidth’ keyword to this template, click the Browse keywords button and navigate to ‘Bandwidth’. Alternatively, use the textbox to search for keywords.
The paper states that this antenna can be used in TM11 mode. To add this keyword, simply type it into the text box and press enter. Any keywords not found in the Antenna Magus keyword dictionary are assumed to be custom keywords.
The following text can be added as Background information for this antenna. It is recommended to copy and paste this text. Note that it is taken verbatim from the paper.
“With recent developments in mobile satellite (MSAT) communications, there has been a lot of activity in the area of antennas which would be suitable for this particular application. There are many requirements for the MSAT antennas, and they include low profile, circular polarization, sufficiently high gain and operational bandwidth of about 8 %. Microstrip patch antennas have been found to be one type that is suitable for MSAT applications provided that their typical bandwidth of less than 4 % is increased. One solution to improve the bandwidth is to increase the patch antenna height above the conducting ground plane. This however leads to the use of thick substrates which inevitably results in increased manufacturing cost of the antenna system. Another problem associated with antennas of increased height is that there is often a reduction in the isolation between the individual array elements (when no shielding is used). Such a lack of isolation can result in a difficulty to predict the operation of the whole antenna system.”
Go to the Parameters, Sketches & Guidelines tab of the template. On the Palette there are two sections – Objectives and Parameters. The frequency (f1) is used by Antenna Magus to scale prototypes. All other parameters that are length-type will be scaled by the ratio between this frequency and the design frequency of the prototype. In this case, the antenna was designed around 1.6 GHz. Edit the frequency by double-clicking the field box, or by clicking the edit button that appears when the cursor is hovering over the field box. Set the value to 1.6 GHz and change the description Scaling frequency to Centre frequency so that it is more descriptive.
Two parameters are missing in the partial template – the diameter of the top patch and the relative permittivity of the feed disc dielectric. To add these parameters, press the Add Parameter button. Take care to set the parameter type correctly – length-type parameters are scaled on design of prototypes. The diameter of the top patch is 81 mm and the relative permittivity of the disc is not given in the paper, so it is set at 4.6.
Adding Sketches and Design Guidelines
Sketches can be added in much the same way as the thumbnail was changed. Add the ‘sketch.png’ file as a sketch and give it a suitable caption. This particular example paper only has one sketch, but several can be added. It is most effective if sketches displaying parameter names are used. Some design guidelines found in the paper have been added under this section already. The following guideline can be added too: “The coupling value depends on the size and height of the two patches. When there is insufficient isolation between the two feeds of a circularly polarized antenna, the connection of a 3dB coupler results in a very poor return loss at the input to the coupler. To overcome this problem, four feeds can be used to cancel out the higher order modes.”
Switch to the Models tab of the template. A parametric, CST MWS model for this antenna has been prepared and distributed with this application note. Press Add Model and browse to the model that is to be imported. The workspace will display some properties of the model, the associated model information and the mapping of Model parameters to Template parameters. The Model parameters are imported from the file.
Add the following information for the model:
“This model is created for demonstration purposes and has not been validated to work correctly at all. The reference paper does not provide all the parameter values needed to replicate the performance of this antenna. The geometry will update and the solver should run, but at least a mesh convergence study must be done before use.”
In the Details section, the Status must be changed from Draft to Complete. This status determines if a model can be exported from Antenna Magus. The frequency and length units must be correctly set! If this is not done, the values will be exported incorrectly ((GHz, mm) in this case).
There is also provision to add captions (see figure above). This is useful when there are multiple models for the antenna.
Update the associations table to associate the correct template parameter with the correct model parameter. Any model parameters that are not associated with a template parameter will not be updated when a prototype is designed. The red lock icon indicates that these parameter names are in the model and cannot be edited from Antenna Magus. New parameters can be added to the model if needed.
Adding Performance Charts
Performance charts are displayed when looking at the Performance Estimate tab of a prototype. The reference paper gives 4 result graphs – Return loss without the branch coupler, Port isolation, Return loss with the branch coupler and a radiation pattern. The return loss with and without the branch coupler will be put onto the same graph to facilitate comparison.
Add a new chart to the template by pressing Add Chart and choosing Cartesian. Make the title of the chart ‘Return loss’, label the X-axis ‘frequency’ and the Y-axis ‘Return loss [dB]’. Since the X-axis is frequency, the values should change when a prototype is synthesized using the scaling feature. To enable this, select the Scale axis with design frequency checkbox. These results are for the antenna scaled to work at 1.6 GHz, and this should be completed in the appropriate box. In the chart, the entered data will be in Hz, so the frequency unit box should be left unchanged.
The data for the first trace is in the file ‘S11-no_coupler.tsv’ file. Press the Add Traces button and import the data. Change the name of this trace to ‘No branch coupler’. The data for the second graph still needs to be digitized. To do this, change to the Toolbox tab in Antenna Magus and open the Chart tracer tool from the Data Conversion dropdown menu. Click the ‘Load’ button and select ‘From file’ to navigate to the file ‘S22-with_coupler.png’. Move the two red anchor points to known value locations on the image. Enter the values at these known coordinates (be sure to enter the x-axis values in Hz). Change the interpolation scheme to Spline. Now select points along the trace, until you have adequate resolution to reproduce the trace.
On the Export button, select the To Clipboard option. Switch back to the template view, and select add the trace from the clipboard. Name the trace ‘With branch coupler’.
Now add new charts and create the traces for the port isolation and the pattern, using the chart tracing tool as described for the previous graph. The port isolation graph is in the file ‘port_isolation.png’ and the pattern is in the file ‘pattern.png’. Note that when adding the pattern, the x-axis should not scale with frequency.
The setup of the template is now complete and ready for use. Close the template by clicking Close template on the ribbon.
Exporting and Loading Custom Templates
Custom Antenna Magus templates can be exported and saved to an external location for backup and distribution purposes. From the Home tab, click the Export Templates button to open the Export Templates dialog, displaying all the custom templates in the Antenna Database. Select one of the icons and click Export to save the template to a convenient location.
The antenna template that has been added can now be used in the usual way. At this point, add the antenna to the collection and design it. Enter the scaling frequency of 1.6 GHz . All the parameter values, etc. should be as provided in the paper. Now try a scaling frequency of 800 MHz. All the length-type parameters will double in value. Export the models and check that the parameter values are set correctly. A complete template for this antenna is also supplied.