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<CST> Waveguide losses accuracy ?

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Pushhead

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cst waveguide

Hello all,

I am trying to calculate losses of a waveguide using a 20 mm microstrip line on a grounded dielectric slab.
The formula (and some other microstrip calculators) show that overall losses should be ~ 5.65 dB/m (0.113 dB for 20mm line).

The Problem :
After simulating the waveguide i'm getting losses of ~3.1 dB/m, which disagrees with the m.s. calculators.

Can you help me understanding what's wrong ?
Tnx alot,
P.

Line specifications:
freq.=5GHz
Zo=50ohm
sub. height = 20*mil
Copper(line) thickness = 1.4*mil
Copper(line) width=44*mil
ε=3.48, tgδ=0.004
Copper cond. σ=5.8e7[S/m]
εeff~2.72
overall length = 20mm

BTW :
In my opinion the meshing is OK, i.e. even the thickness of the line is defined by 2 congruent mesh lines.

Tnx again,
P.
 

From ur theoretical calculation, 1meter transmission line is 5.65dB/m,
then 20mm length loss should be 5.65-log(50)= 3.95.

In CST simulation, did u take the metal loss (skin effect) into consideration?


Regards,
 

    P

    Pushhead

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Yes, I did :

Copper, cond = 5.8e7 [S/m].

Yet, the results disagree with the calculation, i.e. over 40% deviation.

Any suggestions ?

Tnx,
P.

BTW:
5.65dB/m×(0.02m)=0.113dB losses.
 

Pushhead said:
Yes, I did :

Copper, cond = 5.8e7 [S/m].

Yet, the results disagree with the calculation, i.e. over 40% deviation.

Any suggestions ?

Tnx,
P.

BTW:
5.65dB/m×(0.02m)=0.113dB losses.
Click to expand...

Yes, My blur mind :p your calculation about the loss is correct.
I still can not figure out why you get such discrepancy from simulation, I have done much more complex simulation using CST, very good simulation results.
I think the model set up may have probelms. You may check the ports set up, simulation boundary.

Regards,
 

    P

    Pushhead

    Points: 2
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Well, I think meshing and port definitions are OK.I have followed the guidelines of the only CST example dealing with lossy metal (lossy spiral inductor).

The big question still holds:

Can CST predict waveguide losses correctly(especially microstip), and what are the right settings for it?
** Maybe it should be brought up for discussion in the forum ?

Thank's for the effort ,
Regards,
P.
 

Pushhead said:
Well, I think meshing and port definitions are OK.I have followed the guidelines of the only CST example dealing with lossy metal (lossy spiral inductor).

The big question still holds:

Can CST predict waveguide losses correctly(especially microstip), and what are the right settings for it?
** Maybe it should be brought up for discussion in the forum ?

Thank's for the effort ,
Regards,
P.
Click to expand...
From my CST simulation and experiments results comparison, CST can do this job, even without include the metal loss into simulationi in order to speed up simulation. We have done ustrip and strip line simulations in much more complex EM enviroments.

About the right settings for it, you may contact with CST technical support or post the model here. Some CST users in this forum may help to check the models setup.

Regards,
 

    P

    Pushhead

    Points: 2
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I can say nothing about losses in a microstrip.
But I have experience with lossy circular waveguide with series overmoded cavities inside simulation at X-band freq.
I did calculation with HFSS and MWS both.
And the result of MWS was completely WRONG until the last version (5.1sp2)
Starting with v5.1sp2 the predicted loss of main mode is roughly same as in HFSS (~ +- 10 %) but the calculated losses of high order modes are still wrong.
The HFSS calculation was proved with experiment very well.
So be careful with MWS lossy media calculation especially with mulltimodes environment.
 

    P

    Pushhead

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Thanks for the replies,

The Goal:
Predicting microstrip losses correctly (dielectric and conduction) with CST.

I am attaching the model files (*.mod, *.par CST MWS v.5.0)
I hope you'll be able to help me.

Thanks again,
P.
 

Check the predicted loss comparison.

Regards,
 

    P

    Pushhead

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Your results are surprisingly similar to my calculations (Not to my CST simulation).
Did you use my model? What parameters did you set?

looking 4ward 2 your answer ASAP,
Tnx in advance,
Pushhead.
 

you can check the attached project file.

Regards,
 

    P

    Pushhead

    Points: 2
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