Antenna Engineering, Peter Knott Tutorial Patch Antenna Design

Patch Antenna Design using MICROWAVE STUDIO

1 What is CST MICROWAVE STUDIO ?

CST MICROWAVE STUDIO is a full-featured software package for electromagnetic anal-ysis and design in the high frequency range. It simplifies the process of inputting the struc-ture by providing a powerful solid 3D modelling front end, see figure ??. Strong graphicfeedback simplifies the definition of your device even further. After the component hasbeen modelled, a fully automatic meshing procedure is applied before a simulation engineis started.

CST MICROWAVE STUDIO is part of the CST DESIGN STUDIO suite [?] and offersa number of different solvers for different types of application. Since no method worksequally well in all application domains, the software contains four different simulationtechniques (transient solver, frequency domain solver, integral equation solver, eigenmodesolver) to best fit their particular applications.

The most flexible tool is the transient solver, which can obtain the entire broadbandfrequency behaviour of the simulated device from only one calculation run (in contrastto the frequency step approach of many other simulators). It is based on the FiniteIntegration Technique (FIT) introduced in electrodynamics more than three decades ago[?]. This solver is efficient for most kinds of high frequency applications such as connectors,transmission lines, filters, antennas and more. In this tutorial we will make use of thetransient solver for designing a microstrip patch antenna as an example.

Figure 3: Graphical User Interface of CST MICROWAVE STUDIO

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2 Simulation Workflow

After starting CST DESIGN ENVIRONMENT, choose to create a new CST MICROWAVESTUDIO project. You will be asked to select a template for a structure which is closestto your device of interest, but you can also start from scratch opening an empty project.An interesting feature of the on-line help system is the Quick Start Guide, an electronicassistant that will guide you through your simulation. You can open this assistant byselecting Help→Quick Start Guide if it does not show up automatically.

If you are unsure of how to access a certain operation, click on the correspondingline. The Quick Start Guide will then either run an animation showing the location ofthe related menu entry or open the corresponding help page. As shown in the QuickStart-dialog box which should now be positioned in the upper right corner of the mainview, the following steps have to be accomplished for a successful simulation:

• Define the Units

Choose the settings which make defining the dimensions, frequencies and time stepsfor your problem most comfortable. The defaults for this structure type are geo-metrical lengths in mm and frequencies in GHz.

• Define the Background Material

By default, the modelled structure will be described within a perfectly conductingworld. For an antenna problem, these settings have to be modified because thestructure typically radiates in an unbounded (“open”) space or half-space. In orderto change these settings, you can make changes in the corresponding dialogue box(Solve→Background Material).

• Model the Structure

Now the actual antenna structure has to be built. For modelling the antenna struc-ture, a number of different geometrical design tools for typical geometries suchas plates, cylinders, spheres etc. are provided in the CAD section of CST MI-CROWAVE STUDIO. These shapes can be added or intersected using boolean op-erators to build up more complex shapes. An overview of the different methodsavailable in the tool-set and their properties is included in the on-line help.

• Define the Frequency Range

The next setting for the simulation is the frequency range of interest. You can specifythe frequency by choosing Solve→Frequency from the main menu: Since you havealready set the frequency units (to GHz for example), you need to define only theabsolute numbers here (i.e. without units). The frequency settings are importantbecause the mesh generator will adjust the mesh refinement (spatial sampling) tothe frequency range specified.

• Define Ports

Every antenna structure needs a source of high-frequency energy for excitation ofthe desired electromagnetic waves. Structures may be excited e.g. using impressedcurrents or voltages between discrete points or by wave-guide ports. The latter arepre-defined surfaces in which a limited number of eigenmodes are calculated and

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may be stimulated. The correct definition of ports is very important for obtainingaccurate S-parameters.

• Define Boundary and Symmetry Conditions

The simulation of this structure will only be performed within the bounding box ofthe structure. You may, however, specify certain boundary conditions for each plane(xmin, xmax, ymin etc.) of the bounding box taking advantage of the symmetry inyour specific problem. The boundary conditions are specified in a dialogue box thatopens by choosing Solve→Boundary Conditions from the main menu.

• Set Field Monitors

In addition to the port impedance and S-parameters which are calculated automati-cally 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 thecalculation of these output data, use the command Solve→ Field Monitors.

• 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 shouldbe calculated. Therefore, select the Source type port for which the couplings to allother ports will then be calculated during a single simulation run.

2.1 Using Parameters

CST MICROWAVE STUDIO has a built-in parametric optimizer that can help to findappropriate dimensions in your design. To take advantage of this feature you need todeclare one or more parameters in the parameter list (bottom left part of the programwindow) and use the symbols in almost every input field of the program (dimensions, portsettings etc.) Also simple calculations using these pre-defined symbols are possible (e.g.4*x+y).

3 Simulation Results

After a successful simulation run, you will be able to access various calculation resultsand retrieve the obtained output data from the problem object tree at the right hand sideof the program window.

3.1 Analyse the Port Modes

After the solver has completed the port mode calculation, you can view the results (evenif the transient analysis is still running). In order to visualize a particular port mode, youmust choose the solution from the navigation tree. If you open the specific sub-folder, youmay select the electric or the magnetic mode field. Selecting the folder for the electric fieldof the first mode e1 will display the port mode and its relevant parameters in the mainview: Besides information on the type of mode, you will also find the propagation constant

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Figure 4: Typical Patch Antenna Geometry and Dimensions

β at the central frequency. Additionally, the port impedance is calculated automatically(line impedance).

3.2 Analyse S-Parameters and Field Quantities

At the end of a successful simulation run you may also retrieve the other output datafrom the navigation tree, e.g. S-Parameters and electromagnetic field quantities.

References

[1] Computer Simulation Technology (CST), “CST Design Studio”,http://www.cst.com/Content/Products/DS/Overview.aspx

[2] T. Weiland, “ Discretization Method for the Solution of Maxwell’s Equations forSix-Component Fields”, Electron. Commun. (AEU), Vol. 31, No. 3, pp. 116-120,1977

3.3 Exercises

3.3.1 Rectangular Patch Antenna for WLAN application

The aim of this tutorial is the design of a microstrip patch antenna for a practical WirelessLocal Area Network (WLAN) application operating at 2.4 GHz as well as connecting andmatching the antenna to the system via a microstrip transmission line. The typicalgeometry of a patch antenna and the dimension parameters important for specifying adesign are shown in figure ??.

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Figure 5: Antenna Matching Techniques: Asymmetric Feed, Recessed Feed and Quarter-Wavelength Transformer

The antenna should be built on a dielectric substrate of industrial FR-4 material1

(εr = 4.9, µr = 1) of height hS = 1.0 mm. The PCB has a copper cladding of thicknesshC = 35 µm on top and bottom. At the beginning, the substrate should be consideredof infinite extent (open boundary conditions at the sides) and lossless, i.e. loss tangenttanδ = 0. Also the conductor material should be considered perfectly conducting (PEC).

3.3.2 Coarse Design

The initial design should be a very simple microstrip-to-patch transition without anyrecesses or other impedance transformer (t = 0). Before you start modelling the an-tenna with CST MICROWAVE STUDIO, use the Transmission Line Model (TLM) designmethod illustrated in the script to calculate the approximate dimensions of the microstripline width and patch width and height.

3.3.3 CST Simulation

Start CST MICROWAVE STUDIO, build a CAD model of the patch antenna (comprisingfeed line and rectangular patch on infinite PCB substrate) based on the approximateddimensions from the previous exercise and make the appropriate settings in the programfor units, frequency, boundary conditions etc. If necessary, you can make use of the QuickStart Guide to lead you through the different steps.

Use the transient solver to calculate the antenna properties. What are the resultingfrequency of resonance and input reflection coefficient S11?

3.3.4 Improved Antenna Matching

Since the input impedance of the patch antenna is different from that of the feedingmicrostrip line, the mismatch will cause a certain amount of reflected waves at the inputport. With additional matching techniques (e.g. asymmetric feeding, recessed feed orquarter-wavelength transformer, see figure ??) you can reduce the mismatch and improvereflection coefficient S11.

Try out different matching techniques and see how you can improve antenna matching.What is the improvement of the 10 dB-bandwidth that you can achieve? Does it satisfy thebandwidth requirements for operation in an IEEE 802.11b/g system (2.4 - 2.4835 GHz)?

1Flame Retardant 4, a glass reinforced epoxy laminate for manufacturing printed circuit boards

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3.3.5 Antenna Far-field and Polarisation

Use a field monitor to calculate the far-field radiated by the antenna. What is the far-fieldpolarisation and maximum antenna gain in the direction normal to the antenna? Howdoes changing the patch dimensions (W,L) affect these values?

3.3.6 More Realism

For the sake of simplicity, some idealised assumptions have been made in the previousdesign exercises (infinite and lossless substrate material). How do the results of thesimulation change if

a) the PCB has a finite size with a margin of 1 cm around the antenna and transmissionline structure?

b) the copper material is considered lossy (σ = 5.8 · 107 S/m)?

3.3.7 Additional Exercises

• Design a quadratic patch antenna with coaxial transmission line feed and two po-larization ports.

• Design a quadratic patch antenna with truncated edges for circular polarisation.

• Design an array consisting of 5 patch antennas with the beam scanned to 30◦ off-boresight.

• Design an array of series fed patch antennas (see figure ??) for the same scanningdirection.

Figure 6: Series Fed Patch Antenna Array

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