How to characterize the attenuation of transmission line

[winglj]

transmission line attenuation

Hi, everyone. I have a question about characterize the attenuation of transmission line, such as waveguide or microstrip line.

For example, for the EM theory, the attenuation should be the real part of the complex propagation constant real(γ)=α. And it can be converted to dB by 20*log10(exp(α*z)), where z is the length of transmission line.

However, in the measurement or EM simulation, what it can be got is the S-parameters. So how can I get the attenuation from this data?

For example, I simulate an 1" microstrip line in HFSS. It shows that from the insertion loss is around -3.59 dB at 8GHz, but by using the formula 20*log10(exp(α*z)), the attenuation should be -0.19 dB/in. So which one is correct? Do I need to use advanced calibration to get the real attenuation from the insertion loss?

Thanks.

 

[Abhishekabs]

attenuation in transmission lines

I guess, you can get real values in HFSS.
when you create graph, check right hand side menu.
select real,imaginary,db etc.
you can get direct values. no need to use formula.

 

[parris]

attenuation transmission line

Dear Abhishekabs

You mean that we just need to get dB(real(S21))???

Cheers
Parris

 

[Abhishekabs]

s-parameter attenuation

see the image.
In this image you will find on right side there are many options such as re,img, dB etc.
According to my info yes you can get real(S21) values.

 

[a.mal]

You have to use multi-line calibration method. To do so you have to simulate two transmission lines with different length then derive the attenuation and propagation constants. However, in this method you should develop a code which is sometimes tricky.

 

[johnjoe]

you can use s21 only if you have a 50 Ohm transmission line and 50 ohm termination (i mean all impedances should be Z0), otherwise the attenuation includes also the reflection of port 1.