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Patch Antenna Design using MICROWAVE STUDIO

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					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 graphic
feedback simplifies the definition of your device even further. After the component has
been modelled, a fully automatic meshing procedure is applied before a simulation engine
is started.
    CST MICROWAVE STUDIO is part of the CST DESIGN STUDIO suite [?] and offers
a number of different solvers for different types of application. Since no method works
equally well in all application domains, the software contains four different simulation
techniques (transient solver, frequency domain solver, integral equation solver, eigenmode
solver) to best fit their particular applications.
    The most flexible tool is the transient solver, which can obtain the entire broadband
frequency behaviour of the simulated device from only one calculation run (in contrast
to the frequency step approach of many other simulators). It is based on the Finite
Integration 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 the
transient solver for designing a microstrip patch antenna as an example.




          Figure 3: Graphical User Interface of CST MICROWAVE STUDIO




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Antenna Engineering, Peter Knott                            Tutorial Patch Antenna Design


2     Simulation Workflow
After starting CST DESIGN ENVIRONMENT, choose to create a new CST MICROWAVE
STUDIO project. You will be asked to select a template for a structure which is closest
to 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 electronic
assistant that will guide you through your simulation. You can open this assistant by
selecting 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 corresponding
line. The Quick Start Guide will then either run an animation showing the location of
the related menu entry or open the corresponding help page. As shown in the Quick
Start-dialog box which should now be positioned in the upper right corner of the main
view, 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 steps
      for 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 conducting
      world. For an antenna problem, these settings have to be modified because the
      structure typically radiates in an unbounded (“open”) space or half-space. In order
      to 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 such
      as 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 methods
      available 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 specify
      the frequency by choosing Solve→Frequency from the main menu: Since you have
      already set the frequency units (to GHz for example), you need to define only the
      absolute numbers here (i.e. without units). The frequency settings are important
      because the mesh generator will adjust the mesh refinement (spatial sampling) to
      the frequency range specified.

    • Define Ports
      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


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Antenna Engineering, Peter Knott                            Tutorial Patch Antenna Design


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

    • Define Boundary and Symmetry Conditions
      The simulation of this structure will only be performed within the bounding box of
      the structure. You may, however, specify certain boundary conditions for each plane
      (xmin , xmax , ymin etc.) of the bounding box taking advantage of the symmetry in
      your specific problem. The boundary conditions are specified in a dialogue box that
      opens 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 the
      calculation 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 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.

2.1    Using Parameters
CST MICROWAVE STUDIO has a 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 list (bottom left part of the program
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).


3     Simulation Results
After a successful simulation run, you will be able to access various calculation results
and retrieve the obtained output data from the problem object tree at the right hand side
of the program window.

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


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Antenna Engineering, Peter Knott                            Tutorial Patch Antenna Design




              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 data
from 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 for
                                                     ¨
     Six-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 Wireless
Local Area Network (WLAN) application operating at 2.4 GHz as well as connecting and
matching the antenna to the system via a microstrip transmission line. The typical
geometry of a patch antenna and the dimension parameters important for specifying a
design are shown in figure ??.


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Antenna Engineering, Peter Knott                                   Tutorial Patch Antenna Design




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
( r = 4.9, µr = 1) of height hS = 1.0 mm. The PCB has a copper cladding of thickness
hC = 35 µm on top and bottom. At the beginning, the substrate should be considered
of infinite extent (open boundary conditions at the sides) and lossless, i.e. loss tangent
tanδ = 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 any
recesses or other impedance transformer (t = 0). Before you start modelling the an-
tenna with CST MICROWAVE STUDIO, use the Transmission Line Model (TLM) design
method illustrated in the script to calculate the approximate dimensions of the microstrip
line width and patch width and height.

3.3.3      CST Simulation
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 ?

3.3.4      Improved Antenna Matching
Since the input impedance of the patch antenna is different from that of the feeding
microstrip line, the mismatch will cause a certain amount of reflected waves at the input
port. With additional matching techniques (e.g. asymmetric feeding, recessed feed or
quarter-wavelength transformer, see figure ??) you can reduce the mismatch and improve
reflection 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 the
bandwidth requirements for operation in an IEEE 802.11b/g system (2.4 - 2.4835 GHz)?
  1
      Flame Retardant 4, a glass reinforced epoxy laminate for manufacturing printed circuit boards




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Antenna Engineering, Peter Knott                          Tutorial Patch Antenna Design


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-field
polarisation and maximum antenna gain in the direction normal to the antenna? How
does 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 previous
design exercises (infinite and lossless substrate material). How do the results of the
simulation change if

a) the PCB has a finite size with a margin of 1 cm around the antenna and transmission
   line 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 scanning
     direction.




                       Figure 6: Series Fed Patch Antenna Array




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