# compute propagation constant by MATLAB and HFSS

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#### ntr2010

##### Newbie level 2
I built a microstrip line in HFSS (lumped port, driven modal) and get the S parameters, I use MATLAB to convert S parameters to ABCD parameters. Then use formula to caculate characteristic and propagation constant:
Zc=sqrt(B/C)
gamma=acosh(A)/L.

However, the phase constant beta is always smaller than I expect. The TL is 26um wide, substrate dielectric constant is 4, substrate thickness is 15um, TL length is 1mm. I expect the phase constant bigger than 4000 @120GHz. However I only got less then 2000. I tried perfect conductor sheet and solid conductor with finite conductivity in HFSS but results are almost the same.

On the other hand, when I use waveport and set the port size in consistent with HFSS tutorial (port height about 6-10*h,port width 10*TL width), I got the characteristic impedance and propagation constant different from computation results through the fromula.

I believe there is some problem in my computation process. Here is my MATLAB code:

s_11 = 0.0354+0.005*j;
s_12= -0.2296+0.9724*j;
s_21 = -0.2296+0.9724*j;
s_22 = 0.033+0.0115*j; %S parameters got from HFSS
s_params = [s_11 s_12; s_21 s_22];
z0 = 50; % lumped port impedance is 50 ohms
abcd_params = s2abcd(s_params,z0); %convert S parameters to ABCD parameters
A=abcd_params(1,1)
B=abcd_params(1,2)
C=abcd_params(2,1)
D=abcd_params(2,2)
Zc=sqrt(B/C) %computer characteristic impedance
gamma=acosh(A)/1e-3 %compute propagation constant

Thanks a lot!

#### Attachments

• HFSS file.zip
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You can get the propgation constant directly from HFSS by using waveport

Hi,

I have your same problem. Did you get any solution?

Thanks
Leo

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