simple questions about the microstrip line

Hi all,
Can anyone tell me how to get the conductor and dielectric loss from S21 and S11? Thanks a lot!
pudding

Hi, Pudding:

If your S11 and s21 are for a uniform TLN, you can find the TLN parameters from them. You will be able to find hte loss per unit length. However, I don't think you can separate the conductor and dielectric losses. Regards.

Hi Jian, thank you for your reply. The S11 and S21 is for the microstrip line, and I don't need to seperate conductor and dielectric losses. I just want to get the losses from S11 and S21. Can I use this formular: loss=1-(S11^2+S21^2) ?

Hi, pudding:

Precisely, loss is defined as:

P_net = |a1*a1| + |a2*a2| - |b1*b1| - |b2*b2|

a1, b1 -- incoming and outgoing wave at port 1
a2, b2 -- incoming and outgoing wave at port 2

for a 2-port circuit.

P_ratio = 1 - |s11*s11| - |s21*s21| means that P_ratio is the ratio of loss and net input power IN THE FOLLOWING CONDITIONS:

1. Port 2 is terminated with Zc.
2. There is only input power at port 1 and there is no incident power at port 2. Your port 1 can be unmatched.

Depending upon you how excite your 2-port structuer, the P_ratio for the specific situation may vary. Regards.

Thanks! Jian. But I still don't quite understand for P-ratio to be accurate, why I need to match port 2. I just consider the energy conservation. Do you think it is true although there will be reflection on both port 1, port 2, I still can get the loss from the above equation for P-ration?

Hi, pudding;

Following are the comments:

For a 2-port structure, depending how we excite the port 1 and port 2, we will have different a1, b1, a2, b2, a1 is the incident wave at port1, b1 is the outgoing wave at port 1. a2 and b2 are similarly defined.

The incdent power at port 1 is: Pin1 = |a1*a1|
The outgoing power at port 1 is Pout1 = |b1*b1|
Similarly, we define the Pin2 = |a2*a2| and Pout2 = |b2*b2|

The total incident power is Pin = Pin1 + Pin2
The total outgoing power is Pout = Pout1 + Pout2

The net input power into it is; Pnet = = Pin - Pout = Pin1 + Pin2 - ( Pout1 + Pout2). Pnet is the loss inside your 2-port s-parameters.

Depending upon how you excite your 2-port, you may get different value for the Ratio = Pnet / Pin

For example, if you excite port 1 and terminate the port 2 with Zc of port 2, you get:

b1 = s11 * a1
b2 = s21 * a1
Pnet/Pin = 1 - |S11*S11| - |S21*S21|

If you termiante port 1 with Zc of port1 and excite port 2, you get:

b1 = s12 * a2
b2 = s22 * a2
Pnet/Pin = 1 - |S12*S12| - |S22*S22|

For most structuers, S12 = S21, however, |S11| = |S22| is not necessarily hold. Then, the loss in this structure will really be dependent upon how you excite the 2 ports of the structure.

Best regards,

hallo jian,
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Thank you !

Hi, Cuongtran:

I can't answer your 1st question because I am not familiar with HFSS.

For your 2nd question, I could not understand what you mean? Thanks!

Thanks mr Jian,
For the 1st, I mean, how could we calculate the permittivity and permeability from S11 (return loss of one structure) ?
For the 2nd, I am wrong, I am sorry !
Thks

Hi, cuongtran:

You can find the Ereff from S11 only. You need S11 and S12 for a uniform TLN. You convert the S-matrix into ABCD matrix and compare it to the ABCD matrix of a uniform line. You will be able to find the complex propagation constant and the Zc. Regards.