ELEC0048 - Antennas and Propagation Lab Session
1. Background
This antenna simulation lab aims at giving you some hand-on experience with a 3D electromagnetic simulator. There are some other simulators available in the literature, and the ways of using these simulators are similar.
CST MICROWAVE STUDIO [1] is a commercial software package for electromagnetic analysis and design for high frequency devices in the GHz range. It provides a 3D modeling front end and basically have four different solvers; transient solver, frequency domain solver, integral equation solver, eigenmode solver. In this microstrip patch antenna design lab, we will use the transient solver.
The transient solver can obtain the entire broadband frequency results of the simulated device from only one calculation run. It is based on the Finite Integration Technique (FIT). This solver is suitable for most kinds of high frequency applications such as connectors, transmission lines, filters, antennas and more.
This lab script is based on [1] and modified for the use of this lab, so only those are useful for the lab are extracted. For more information, you can visit https://www.3ds.com/products-services/simulia/products/cst-studio-suite/
2. Simulation Procedures
The tutorial will take you step by step through the construction of your model, and relevant screen shots will be provided so that you can double-check your entries along the way.
2.1 Select a Template
Once you have started CST DESIGN ENVIRONMENT™ and have chosen to create a new CST MICROWAVE STUDIO project, you are requested to select a template that best fits the device you want to design, but you can also start from scratch opening an empty project. You can open this assistant by selecting Help > Quick Start Guide if it does not show up automatically.
• Choose the “Antenna (Planar)” template.
As shown in the Description box, choosing this template, the following setting will be automatically defined for you,
o Units: millimeter, GHz
You can define your own setting by clicking Solve > Units and other units will be used in the simulation can be set in the Units windows as well
o Background Material: Normal. It means the modeling space is air
Similarly, it can be modified by choosing Solve > Background Material.
2.2 Define Frequency
Define the frequency of interest. You are suggested not to define the frequency too narrow or too wide, as an over-narrow frequency band simulation may not give you much information, while an over-wide simulation will waste time.
2.3 Define Boundary
Choose “open (add space) for antenna simulation ”. For the moment, define open (add space) in our antenna design project, so you can see the correct radiation patterns. By proper defining the symmetry planes, it will save you a lot of time. You can also define the thermal boundaries and boundary temperature if necessary.
2.4 Draw the Structure
Now the actual antenna structure can be built. For modeling the antenna structure, a number of different geometrical design tools for typical geometries such as plates, cylinders, spheres etc. are provided in the CAD section.
But before you draw anything, do a little sketch for a clearer idea of your structure you want to model. Here are some
o How many layers are there?
o What materials are used? What kinds of conductors? What kinds of dielectric materials (dielectric constant and thickness)?
o What the shapes are? Rectangular blocks? Circular cylinders? Spheres?
o Where is the center of reference of your structure?
Here is an example for drawing a rectangular block,
• Choose Objects > Basic Shapes > Brick…
• Move the cursor to the Draw window, and double click to drop the first corner, then the second corner for the width and length and third corner for the height of the brick. A red wire brick will be shown to show you roughly the scale. Don’t worry too much how the brick looks like at this step.
• Then a dialog box will pop up to ask you the detail dimensions of the brick you just created. Enter the name of the brick, use something carry some meaning, such as “grd_plane” for the ground plane or “Sub” for the substrate, etc. You can enter the exact values of the length, width and height of the brick, but you are suggested to use parameters, such as Xgrd, Xsub instead of the exact values.
• The parametric function in CST can speed up your work by allowing you to directly modify the dimensions of your structure in the parameter list. Moreover, the built-in parametric optimizer that can help to find appropriate dimensions in your design. To take advantage of this feature you need to declare one or more parameters in the Parameter Window and use the symbols in almost every input field of the program (dimensions, port settings etc.) Also, simple calculations using these pre-defined symbols are possible (e.g. 4*x+y).
• Then choose the material of the brick, such as PEC (perfect electric conductor) or dielectric material. You can also click choose from library for some of the commonly used material.
• The shapes can be added or intersected using Boolean operators to build up more complex shapes. An overview of the different methods available in the tool-set and their properties is included in the on-line help.
2.5 Deine Excitation
Every antenna structure needs a source of high-frequency energy for excitation of the desired electromagnetic waves. Structures may be excited e.g. using impressed currents or voltages between discrete points or by wave-guide ports. The latter are pre-defined surfaces in which a limited number of eigenmodes are calculated and may be stimulated. The correct definition of ports is very important for obtaining accurate S-parameters.
• Use Objects > Pick to help you to define the surface for the waveguide port , or
• Type in the parameters for defining the waveguide port
2.6 Set Field Monitors
In addition to the port impedance and S-parameters which are calculated automatically for each port, field quantities such as electric or magnetic currents, power flow, equivalent currents density or radiated far-field may be calculated. To invoke the calculation of these output data, use the command Solve > Field Monitors.
2.7 Start the Simulation
After defining all necessary parameters, you are ready to start your first simulation. Start the simulation from the transient solver control dialogue box: Solve >Transient Solver. In this dialogue box, you can specify which column of the S-matrix should be calculated. Therefore, select the Source type port for which the couplings to all other ports will then be calculated during a single simulation run.
3. Design Exercise I - Rectangular Patch Antenna for WLAN application
Refer to the lecture notes for the typical geometry of a microstrip line fed microstrip patch antenna. Design an antenna for a Wireless Local Area Network (WLAN) router as well as connecting and matching the antenna to the system via a microstrip transmission line. The data sheet of the microwave substrate to be used is attached at the back of the lab script.
Before you start modeling the antenna with CST MICROWAVE STUDIO, estimate the length and width of the microstrip antenna and the approximate dimensions of the microstrip line width.
Start CST MICROWAVE STUDIO, build a CAD model of the patch antenna (comprising feed line and rectangular patch on infinite PCB substrate) based on the approximated dimensions from the previous exercise and make the appropriate settings in the program for units, frequency, boundary conditions etc. If necessary, you can make use of the Quick Start Guide to lead you through the different steps.
Use the transient solver to calculate the antenna properties. What are the resulting frequency of resonance and input reflection coefficient, S11?
Use Field Monitor to calculate the far-field radiated by the antenna. What is the far-field
polarisation and maximum antenna gain in the direction normal to the antenna? How does changing the patch dimensions (W, L) affect these values?
Comment on the surface current of the microstrip patch antenna.
4. Report
Write a 4-page report which is in line with the IEEE format for letters. For your reference,
https://www.ieee.org/conferences/publishing/templates.html . Please note that I only meant the format of your report, nothing about the procedure for paper submission, IEEE Graphics Checker, editorial policy, publication principles or so are relevant to your report writing.
Report structure:
Maximum 4 pages, it should have
- Abstract: max. 150 words
- Introduction: max. half of a page
- Antenna Design: max. 1 page
- Simulation results and figures: max. 1.5 page
- Analysis and discussion: max. 1 page
- Conclusion: max. 400 words.
Reference:
[1] CST user manual
[2]https://rogerscorp.com/-/media/project/rogerscorp/documents/advanced-
connectivitysolutions/english/data-sheets/rt-duroid-5870---5880-data-sheet.pdf