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# <CST MWS> A periodic structure+Plane wave problems

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##### Full Member level 4
cst network parameter extraction antenna

Hello all,

I'm simulating a periodic 2D planar structure. I am using a plane wave excitation to achieve the phase of the reflection coefficient.
As you'll see in the following pictures, I have used only one cell of the structure with periodic bundary conditions (sidewalls), Electric (bottom) and Open (initial wave-plane position). In addition i'm using several field-probes above the cell.

My main problem now is an unexplanable phase result (see picture). I get weird "jumps" of phase values. I understand that the angle is wrapped between -180 and 180 - but I cant explain that!

Why is this happening?
Do you have another idea of retrieving the phase of the ref. coeff. ?

Looking 4ward 2 your ideas and thoughts,
Tnx alot,
P.

#### irfan1

##### Full Member level 5
shahid

I guess you have to subtract the incoming field from the total scattered field. From the phase data, I can tell that what you observe is some sort of interference between the reflected signal and the incoming signal. There is a discussion on the forum about this issue .

Points: 2

#### sassyboy

##### Full Member level 1
Pushhead, irfan1 is right! The probes are measuring the interference between the incident and reflected signal. Moreover. this topic has been discussed before and was first initiated by irfan1 and the link is below. It should help.

Last edited by a moderator:

Points: 2

##### Full Member level 4
Thanks alot!

I think it works (have'nt checked it thoroughly). I have built a (1)
PBG model+probe, (2) a vacuum model+probe and (3) a PEC model+probe.

I've calculated the following with complex field values:

a. E-reflected,pbg=E(1)-E(2);
b. E-reflected,pec=E(3)-E(2);
c. Ref. coeff. angle with respect to PEC=arg(E-reflected,pbg)-arg(E-reflected,pec).

I'm still having those weird "jumps", any idea ?

regards,
P.

#### eckypting

##### Junior Member level 3
If you're only concerned with a normal incident plane wave, I would suggest using a waveguide port excitation instead of the plane wave excitation. Doing so will allow you to separate the incident and reflected fields, as well as deembed the port reference plane all the way to the surface of your structure so that you can observe the reflection phase for the EBG structure.

Cheers,
eckypting

Points: 2

#### irfan1

##### Full Member level 5
What would be the effect of using waveguide port along with the periodic boundary conditions on the plane of the waveguide? This seems a lot confusing and complicated to me. Any comments?

Points: 2

#### eckypting

##### Junior Member level 3
irfan1 said:
What would be the effect of using waveguide port along with the periodic boundary conditions on the plane of the waveguide? This seems a lot confusing and complicated to me. Any comments?

Actually, the periodic boundary condition is not required for this type of problem if a waveguide port is used and all that is desired is the reflection phase at normal incidence... For a vertically polarized waveguide mode, the E-plane boundaries (i.e. top & bottom of the "waveguide") should be defined as PEC, and the H-plane boundaries (left & right sidewalls) as PMC. This will create a uniform, plane-polarized incident wave on a unit cell of the surface.

The advantage of the waveguide port is that it will automatically compute S-parameters and deembed to a user-defined reference plane (e.g. the top surface of the unit cell). This cannot be done with the plane wave excitation mode of analysis.

In the end, I would contend that this approach is much simpler and far less of a headache than the plane wave analysis approach. It doesn't require any sort of post processing - unless you consider de-embedding a post processing step...but that is relatively trivial to set up.

Cheers,
eckypting

soroush1992

### soroush1992

Points: 2

#### irfan1

##### Full Member level 5
it is getting much clearer now. Still, I have the following question. Usually the lattice constant would be the half of the wavelength. This will restrict the dimensions of the waveguide port. Then one can not excite below a frequency range. Because one will hit the cutoff of the waveguide.Any solution?

#### eckypting

##### Junior Member level 3
irfan1 said:
it is getting much clearer now. Still, I have the following question. Usually the lattice constant would be the half of the wavelength. This will restrict the dimensions of the waveguide port. Then one can not excite below a frequency range. Because one will hit the cutoff of the waveguide.Any solution?

This is part of the reason for having the PMC (& not PEC) on the sidewalls, so that the waveguide appears as 2 parallel plates with no low-frequency cutoff...not to mention providing a uniform amplitude distrubution across the width of the cell. With PEC sidewalls, the waveguide will have a cutoff frequency, and the amplitude distribution will be sinusoidal.

Cheers,
eckypting

Points: 2

#### sergag

##### Member level 1
Sorry - I new user in MWS... Where I can find a manual on MWS?

#### eckypting

##### Junior Member level 3
CST Microwave Studio Manual - Getting Started

sergag said:
Sorry - I new user in MWS... Where I can find a manual on MWS?

I've uploaded the MWS Advanced Topics Manual recently in another topic here:

I will upload the Getting Started and Tutorials manuals shortly.

eckypting

This manual is primarily designed to enable a quick start to CST MICROWAVE
STUDIO®. It is not intended to be a complete reference guide to all the available
features but it will give you an overview of the key concepts. Understanding these
concepts will then allow you to learn how to use the software efficiently with the help of
the online documentation.
The main part of the manual is a so-called Quick Tour (Chapter 3) which will guide you
through the most important features of CST MICROWAVE STUDIO®. We strongly
encourage you to study this chapter carefully.

CHAPTER 1 — INTRODUCTION................................................................................................................ 3
Welcome.................................................................................................................................... 3
How to Get Started Quickly?...................................................................................................3
What is CST MICROWAVE STUDIO®? ..................................................................................3
Who Uses CST MICROWAVE STUDIO®?..............................................................................4
CST MICROWAVE STUDIO® Key Features............................................................................... 5
General ..................................................................................................................................5
Structure Modeling..................................................................................................................5
Transient Simulator.................................................................................................................5
Frequency Domain Simulator..................................................................................................6
Eigenmode Simulator..............................................................................................................7
Modal Analysis Simulator........................................................................................................7
Visualization and Secondary Result Calculation.....................................................................8
Result Export ..........................................................................................................................8
Automation.............................................................................................................................8
Document Conventions...........................................................................................................9
Contacting CST – Computer Simulation Technology................................................................ 10
Worldwide Distribution Agents ..............................................................................................10
Technical Support .................................................................................................................11
CHAPTER 2 — INSTALLATION............................................................................................................... 12
Installation Requirements.......................................................................................................... 12
Software Requirements.........................................................................................................12
Hardware Requirements .......................................................................................................12
Licensing Options ..................................................................................................................... 12
Installation Instructions ............................................................................................................. 13
CHAPTER 3 — QUICK TOUR .................................................................................................................. 15
Starting the software ................................................................................................................. 15
Overview of the User Interface’s Structure................................................................................ 16
Creating and Viewing Some Simple Structures ........................................................................ 17
Create a First “Brick”.............................................................................................................17
An Overview of the Basic Shapes Available .........................................................................20
Selecting Previously Defined Shapes, Grouping Shapes into Components and Assigning
Material Properties................................................................................................................20
Changing the View................................................................................................................24
Applying Geometric Transformations....................................................................................26
Combine Shapes by Using Boolean Operations ...................................................................28
Pick Points, Edges or Faces from Within the Model .............................................................30
Chamfer and Blend Edges....................................................................................................32
Local Coordinate Systems ....................................................................................................39
2 CST MICROWAVE STUDIO® 5 – Getting Started
The History List .....................................................................................................................42
The History Tree ...................................................................................................................44
Curve Creation......................................................................................................................46
Local Modifications ...............................................................................................................50
Trace Creation ......................................................................................................................52
The First Real World Application Example................................................................................ 54
The Structure ........................................................................................................................54
Start CST MICROWAVE STUDIO®.......................................................................................55
Open the Quick Start Guide..................................................................................................55
Define the Units ....................................................................................................................56
Define the Background Material............................................................................................57
Model the Structure...............................................................................................................57
Define Ports ..........................................................................................................................65
Define the Frequency Range ................................................................................................67
Define Boundary and Symmetry Conditions .........................................................................68
Visualize the Mesh................................................................................................................70
Start the Simulation...............................................................................................................72
Analyze the Port Modes........................................................................................................75
Analyze the S-Parameters ....................................................................................................77
Analyze the Electromagnetic Field at Various Frequencies ..................................................83
Parameterization of the Model and the Automatic Optimization of the Structure ..................88
Summary............................................................................................................................104
Which Solver to Use? ............................................................................................................. 105
Antenna Computations ........................................................................................................... 108
Simplifying Antenna Farfield Calculations...........................................................................111
Digital Calculations ................................................................................................................. 112
Frequency Domain Computations........................................................................................... 115
Eigenmode (Resonator) Computations................................................................................... 117
Modal Analysis Computations................................................................................................. 120
Discrete Ports ......................................................................................................................... 122
SPICE Network Model Extraction ........................................................................................... 123
Model Order Reduction Based Network Parameter Extraction ...........................................123
Transmission Line Based Network Parameter Extraction ...................................................127
CHAPTER 4 — FINDING FURTHER INFORMATION............................................................................. 131
The Quick Start Guide ............................................................................................................ 131
Tutorials ................................................................................................................................. 132
Examples ............................................................................................................................... 133
Online Reference Documentation ........................................................................................... 133
Referring to the Advanced Topics Manual .............................................................................. 133
Access Technical Support....................................................................................................... 133
Macro Language Documentation............................................................................................ 134
History of Changes ................................................................................................................. 134
APPENDIX A— LIST OF SHORTCUT KEYS......................................................................................... 135
General Shortcut Keys Available in Main Structure View........................................................ 135
Shortcut Keys Available in Edit Fields..................................................................................... 136

legenda

### legenda

Points: 2

#### shahid78

##### Full Member level 5
i am still confused with the waveguide port excitation to find the bandgap of the PBG
surfaces.
normally we select some surface and then assign waveguide port to it.
but where will be our waveguide port located in case of unit cell model of the structure.
i mean for probe fed patch antenna we assign our port to the coaxial cable which is at the back of the ground plane.

can anyone explain in detail how to do it with example posted on this forum.

thanks

#### qqqooo

##### Junior Member level 1

need more points??

I am so sorry why it needs so many points?

could you give it freely?

#### echo47

##### Full Member level 4
i am still confused with the waveguide port excitation to find the bandgap of the PBG
surfaces.
normally we select some surface and then assign waveguide port to it.
but where will be our waveguide port located in case of unit cell model of the structure.
i mean for probe fed patch antenna we assign our port to the coaxial cable which is at the back of the ground plane.

can anyone explain in detail how to do it with example posted on this forum.

Hi. I am glad to see that this conversation is still alive (started somewhere in 2005...)

Regarding your question, try refering to the beginning of this topic, especially to
eckypting's replies - which helped me eventually to implement it in CST.
Where to place the port? Well, I have placed it away from the unit cell in the direction of incidence and de-embedded the reference plane to the surface of the cell.

Good luck
P.

#### hamid2000

##### Junior Member level 3
Hi
I also want to simulate a 2D Photonic Crystal with CSt MWS 2006, but I can not reach Transmission spectrum and Band Gap region.
I get S12 always ZERO!!! I will be so thankfull to anybody helps me.

Regards

#### abdoeng

hamid2000,could pls upload ur cst file here

#### hamid2000

##### Junior Member level 3
I will try to load it..but it is some big(about 6MB !)

I think in another way....such as telling setting and geometry.

#### shahid78

##### Full Member level 5
i have tried your method i got results which seems to be correct but not in good agreement with published results. ( the resonant frequency that is frequency at which phase passes through 0 is less than what i read in journal papers)

also just to confirm we need to model only one structure which is the actual EBG single cell.

because we need to model 3 structures if we use plane wave excitation technique

thanks

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