dispersion diagram: cst microwave studio - دانشگاه صدا و...

[email protected] [Dispersion diagram: CST Microwave Studio]

1

Dispersion Diagram Example: Slow Wave Helix

(Thanks to Dr Miguel Navarro-Cía Imperial College London for his valuable comments)

General Description

This example demonstrates an eigenmode calculation using periodic boundaries in z-direction and a very meaningful example for showing how we can get the dispersion diagram in CST MWS for periodic structures. This example already is available in example folder of CST MWS in path of:

“C:\Program Files (x86)\CST STUDIO SUITE 2013\Examples\MWS\Eigenmode\TET\Slow Wave\helix.cst”

Structure description:

Inner conductor (Fig.1(a)) is a helical metal ribbon which has been hold by three dielectric bricks (d) inside a PEC cylindrical hole (b) as outer conductor.

Fig.1 (a) inner conductor (b) outer conductor (c) vacuum (d) dielectric holder

Frequency Range

The frequency range starts at DC and ends at 10 GHz.

Background Material:

Fig.2 Background material setting.


2

Mesh setting:

Click on the objects in navigation tree and R. Click on the object; then choose the “Local mesh properties” and set it as the following suggestions:

Vacuum: Meshgroup1

The automesh option is disabled for the two

inlets which are not parallel to the coordinate

axes.

Other elements: Meshgroup2

The automesh option is enabled.

(Inner&outer conductor, holders)

Fig.3 mesh setting of the model components.

Boundary Conditions

The boundary conditions for x and y planes are set to “electric” or Et=0, which is equivalent to PEC , while for the two boundary conditions in z-direction have been defined as “periodic” in order to model an infinite extent of the proposed helical inner conductor coax unit ell.

A parameter “phase” is assigned to the periodic boundary, so that the phase shift can be used in a parameter sweep, for measuring the frequency dispersive behavior of the present structure.

Fig.4 Boundary conditions and phase shift setting of the one dimensional periodic structure.

*A userdefined watch is already available and may serve as a template for building custom watches. The userdefined parameter sweep watch adds the group velocity, phase velocity and dispersion plots of the first mode to the navigation tree in the folder '1D Results'. Also, the power


3

flow and Pierce Impedance can be seen in the folder '1D Results'. In parameter sweep window, select "Edit" to view or modify the source code.

>

Fig.5 the user-defined VBA codes of the important reports of the simulation as dispersion curve, phase & group velocity and Pierce impedance.

But I preferred that the following method for setting the dispersion diagram.

Post Processing:

The dispersion diagram, which is a graph based on “frequency” vs. spatial phase variations of the predefined parameter of “phase”, could obtained by the following step by step process:

1) Recall the TBP and choose the “2D and 3D Field Results>3D Eigenmode Result”

Fig.6 the outline of defining the postprocessing setup of dispersion plot in CST

MWS.

2) For obtaining the dispersion data of the modes start one by one from the first mode, then after finishing simulation operation export the data in “txt” format(available in “Table” folder in Navigation tree) for our future postprocessing in Matlab environment.

3) Evaluate

Solver Setup

Whenever the “Eigenmode Solver” is started, a specific number of the lowest resonance frequencies of the structure are calculated. Since only the fundamental mode is of interest, the number of modes is reduced to “1”, and the JDM eigenmode solver is chosen, which is faster for the given example.


4

Fig.7 Eigenmode Solver setting for calculating the dispersion characteristics of the proposed periodic structure.

After the parameter sweep has been selected from the Eigenmode Solver's dialog, a new sequence is added, and the parameter "phase" is chosen to be swept from 0 to 180 degrees in 19 steps for an equal steps.

Fig.8 Parametric sweep setup for phase shift parameter of “phase” in the space periodicity direction.

Start:

If you set lower sweep limit at zero, the machine temporary would interrupt the process and alert we the parameter is zero, don’t care about it and click Ok to continue the operation.

>>

Fig.10


5

Results

During the run process the real time dispersion results is accessible from the Table folder, as a plot based on frequency vs. phase parameter, as following:

(a)

(b)

Fig. 11 The dispersion diagram based on (a) TBP result template, and (b) VBA userdefined.

As shown in Fig. 11, all the data are the same but in Fig.11(a) the horizontal axis is the phase.

Export the data of the Mode “1”

>>


6

>>

Fig. 12 extraction of table dispersion data

Open via excel

Open a new Excel file and open from it your txt file of “Mode1.txt”:

>

>

> >


7

Fig. 13 The preparation of the dispersion text data file for producing the compatible *.csv file

We should keep just the phase and value column and eliminate others, then edit the title of the value column as “Frequency(Mode1)”, finally save as a CSV file format, as shown in Fig. 13. We should repeat the simulation for remained modes frequencies as presented in Fig. 14, and extract the favorite data for updating the previous “mode1.csv” data file.

Fig. 15 How we can update the calculation for the higher mode?

Fig. 16 all the dispersion data gathered in one csv file for plotting the final dispersion diagram for first forth modes of the proposed 1D-periodic structure.


8

Mtalab Code for plotting the dispersion diagram

For publishing all of your plot result, especially in IEEE publications, we should import our data into Matlab environment, as a perfect engineering tool to sketch various plots in a unique plot in a high resolution view. Here we present a brief quick guide about it.

1)Run Matlab

2) import the dispersion CSV file.

>

4) disable the header and text box.

>>

5) Run the plot m.file(see the appendix)


9

Final result:

Fig. the final sketch of the dispersion diagram obtained by a MATLAB plotting program (Appendix).

0 20 40 60 80 100 120 140 160 1800

2

4

6

8

10

12

14

16

18

20

Phase(Deg)

Fre

quency(G

Hz)

Mode1

Mode2

Mode3

Mode4

BAND GAP


10

Appendix: Matlab Source code

clc;

x=data(:,1); %phase parameter

y=data(:,2); %Frequency (Mode1)

plot(x,y,'black');

hold on


plot(x,y,'Red');

hold on


plot(x,y,'Green');

hold on


plot(x,y,'blue');

legend('Mode1','Mode2','Mode3','Mode4');

xlabel('Phase(Deg)');

ylabel('Frequency(GHz)')

Good Luck!

An Old Fashion Iranian Home!

Me, my little son and Professor Yahya Rahmat Samii, IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting, Spokane, United States July 2011

dispersion diagram: cst microwave studio - دانشگاه صدا و...

Documents