Waveguide slot arrays are typically used at microwave frequencies and are particularly popular for radar applications. They are robust, have low-loss and are able to withstand high powers.
These antennas consist of a waveguide with slots cut into the broad side. The difference between the traveling wave and its resonant counterpart is in the spacing of the slots; with the resonant slotted-waveguide antenna the slots are spaced uniformly around the centreline and with the traveling-wave slotted-waveguide, the spacing is tapered to the end. The traveling-wave slotted-waveguide is also terminated with a matched waveguide load at the end.
In this application note we focus on the traveling-wave slotted-waveguide antenna and look at the design methods available in Antenna Magus and how they affect the estimated performance....
If you have no previous experience in designing an antenna with Antenna Magus it is advised that you familiarise yourself with the user interface (UI) sections and its uses. Click here for more information. The conical corrugated horn application note also demonstrates basic navigation, workflows and key concepts in Antenna Magus thoroughly and is available here.
To start our design process we have to locate the antennas that we want to work with. Ensure that Antenna Magus is in 'Find' mode so that the Antenna Database is visible in the workspace and type "slotted" and "waveguide" into the search bar in the palette. All the templates are grouped on how well they match the search terms. The top group in the workspace are the best matches and contain the two templates we require. Add both the "Linear resonant slotted-waveguide array" and "Linear traveling-wave slotted-waveguide" to the user collection by clicking or double-clicking the 'Add' button displayed while hovering over these templates.
Also add these templates to the information browser by clicking the 'Add to Info Browser' on the home ribbon. Now open the info browser by clicking the 'Open Info Browser' button on the home ribbon. In the info browser click the 'New Comparison' button to compare the two templates. Note the physical difference between the two variations of antenna.
Close the information browser or switch back to the main Antenna Magus window to start the design process.
We are going to look at three design methods available in Antenna Magus and investigate the effect of constraint changes in these methods.
Follow the links below to navigate to any of the methods.
Design for Operating Frequency
To compare the Linear resonant - and Linear travelling-wave slotted-waveguide antennas we design both for the same frequency and compare the estimated performance. Select the Linear resonant slotted-waveguide antenna in the collection. In the'design for:'drop down in the palette select operating frequency and set the frequency to 50 GHz and click'Design'. This will design the antenna with operating frequency as the only design constraint. Inspect the designed parameters and note how they correlate with the parameters in the sketches displayed in the workspace.
After inspecting the parameters, click on the 'Estimate Performance' button. This will run the simulation and the results can be viewed by navigating to the 'Estimated Performance' tab. The results of the estimation are shown below.
To compare the Linear resonant - and Linear travelling-wave slotted-waveguide antennas we design both for the same frequency and compare the estimated performance. Select the Linear resonant slotted-waveguide antenna in the collection. In the 'design for:' drop down in the palette select operating frequency and set the frequency to 50 GHz and click 'Design'. This will design the antenna with operating frequency as the only design constraint. Inspect the designed parameters and note how they correlate with the parameters in the sketches displayed in the workspace.
We want to design the Linear Travelling-wave slotted-waveguide array with the same design objectives. Click on the Linear travelling-wave slotted-waveguide array antenna in the collection, this should open it in Design Mode. Follow the same steps as above for this antenna. In the Design Objectives expander select 'operating frequency ' as our design objective and design the antenna for 50GHz. Run the estimation and inspect the results on the estimated performance tab.
If we compare the results we note the main difference is that the travelling-wave slotted-waveguide has a squint that in not present with the resonant slotted-waveguide. This squint can be attributed to the phase shift present in each pair of slots in the traveling-wave slotted-waveguide. For the travelling-wave the direction of the squint can be altered as discussed in the next section.
Design Traveling-Wave Slotted-Waveguide for squint
A feature of the travelling-wave slotted-waveguide is that the squint angle can be altered by changing physical attributes of the antenna.
To investigate how changing in the squint angle affects the physical structure, we can design the travelling-wave slotted-waveguide for two different squint values and compare the results.
Ensure that the travelling-wave slotted-waveguide is open in 'Design Mode' and click on 'New Design'. In the 'Design For' drop down select the 'all main beam properties and distribution taper' option. Set the Centre frequency to 50 GHz; the 3dB beamwidth to 5 °s; Taper to Uniform; Squint angle to 7 °s and load power fraction to 0.5. Now click on 'Design' to design our antenna for these objectives. Click on 'Estimate performance' and navigate to the 'Estimated Performance' tab when the simulation has completed. The results of the estimation are as shown below.
We repeat the same process as above for a different squint. Add a new design and select 'all main beam properties and distribution taper' from the 'design for' drop down. Keep all the design objectives the same while changing the squint angle to 15 °s. Design the new antenna and estimate the performance. The results of this simulation are shown below.
If we compare these results a clear difference in the squint angles visible in the polar plot.
If we navigate back to the 'Sketches and Design Guidelines' tab we can evaluate the differences in the designed parameters. Note how a lower squint angle requires a higher number of slots.
Design for Beamwidth
In this section we will look at how beamwidth as a design constraint will affect our physical design and performance. Select that the travelling-wave slotted-waveguide prototype from the user collection. Add a new design by clicking 'New Design' on the 'Home' ribbon or in the 'Design Objectives' expander. In the 'Design for' drop down select 'beamwidth' and set the centre frequency to 50 GHz, the 3dB beamwidth to 1 ° and the leave the taper on 'Uniform'. Now click on 'Design' to design our antenna for these design objectives. Inspect the designed parameters and note the high number of slots (99). We can now run the simulation, click on 'Estimate Performance' and navigate to the 'Estimated performance' tab, inspect the performance and note the high number of side lobes visible in the radiation patterns. This increase in the number of sidelobes is directly proportional to the number of slots. After inspecting the results navigate back to the 'Sketches and Design Guidelines' and repeat the process as above. Add a new design, select 'beamwidth' in the 'Design for' drop down. Leave all the parameters and only change the beamwidth to 11 °s. Design the antenna for the new design objectives and inspect the parameters. Note that the number of slots is significantly reduced from the previous design. Run the simulation again and view the results on the 'Estimated performance' tab. The graph below is the combined results of the two designs.
As expected the number of slots correlates strongly to the beamwidth as well as the number of sidelobes of this antenna. An increase in slots yields a narrower beamwidth and a higher number of sidelobes.
Exporting to CST MSW
A characteristic of the linear traveling-wave slotted-waveguide is that the squint angle changes with frequency. To evaluate this we export a model to CST MSW and do an analysis.
Select the 11° beamwidth design (as discussed earlier) of the linear travelling-wave slotted-waveguide prototype. Click on 'Export Mode' in the 'Home' ribbon. This opens 'Export Mode' and displays the export options for the model in the workspace and pallet. Switch to the 'CST MICROWAVE STUDIO' tab and note that in the export models info the use of the CST transient solver is suggested.
A beneficial feature of Antenna Magus is that exported models are completely parametric and these parameters can be altered in external full wave simulation packages. Inspect the parameters to be exported and click on 'Export Model' in the 'Export' ribbon. Save the model 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 MWS.
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'.
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 view the radiation patterns at different frequencies (shown below ). View the respective farfields and note how the squint changes with the frequency.
The S11 response can be viewed by expanding the '1D Results' - and 'S-Parameters' node and selecting S11.
This application note shows the process of designing slotted-waveguide array antennas configurations from their respective templates. We note the differences between the resonant - and traveling-wave slotted-waveguide with regards to physical design as well as radiation pattern. We also note the changes in physical configuration required to realize changes in the radiation pattern of the travelling-wave slotted-waveguide. Lastly the Export Mode of Antenna Magus was used to export a model to CST and investigate the frequency response and S11 of the exported model.