Difference Between S Parameters and Z Parameters in CST Microwave Studio and Network Analyzers

I'm currently studying Electronics engineering and as part of this course we sometimes have to use CST Microwave studio.
This is a program that simulates the electromagnetic behavior of circuits and other more complex conductors.
When we used this program, we had to design a patch antenna that would radiate high power at a specific frequency (so a narrowband antenna).
To do this, we had to sweep different values for the geometry of the antenna and plot this against the S parameters.
The geometry of the antenna which had the smallest S parameter was the most efficient option.

I did this project and got an A, however, to be honest I do not even know what an S parameter is.

Z parameters are also sometimes mentioned in my course, but not as much. I assume that these are the resistance, impedance, and capacitances of a system which make up the total impedance of the system, but i'm not sure.

And one last question: Does a network analyser measure the S or Z parameters of a system? I'm inclined to think that it is the Z parameters because we once used a network analyses and it gave the real and imaginary impedances, however on the wikipedia page for network analysers it says that they are mainly used to measure S parameters.

Thanks for reading!

How did you get an A then ?

I was surprised when I got it haha. I somehow managed to managed to blag my way through it. In the report I said that it was desired to minimise the S parameters, as this was mentioned in class, so I plotted the S parameters against different sweeps of dimensions of the antenna (e.g the width) and then chose the dimension that had the lowest S parameter. Although I did this without really knowing what an S parameter was.

I just got lucky I guess

hi, craiq Hickman *S* is a parameter of scattering radiation ...... i thing it so

Congrats on getting the A. It can be a difficult area to work in but can be very interesting.

S parameters, also known as scattering parameters are a useful design guide that are usually provided by manufacturers for high frequency transistors. Basically S parameters are a relationship between the Load impedance(ZL), the Source Impedance(Zs) and the Characteristic Impedance (Zo). With regards these Impedances the S parameters deal with the incident wave and reflected wave on a given input/output port in the form of the Reflection Coefficient. You've probably seen the following values below, but weren't sure exactly what they were. S12 and S21 are what you would design for greatest power and minimum reflected power through your transistor/device.

S11 = The Input Reflection Coefficient
S12 = The Reverse Transmission Coefficient
S21 = The Forward Transmission Coefficient
S22 = The Output Reflection Coefficient

Any given circuit can be described using S parameters, but also in Z parameters, Y parameters, H, T, ABCD parameters as well. Each one of these have their own specific advantages and disadvantages. S parameters are more popular as they are easier to deal with and easier to calculate and measure on a network analyzer. Z parameters focus on the impedance, Y parameters are the inverse that focus on the admittance. Both Z and Y parameters can be difficult to measure as they require an open and short circuit to measure their values which can damage a circuit at higher frequencies. H is hybrid of voltage and current, T transfer parameters, ABCD is the older transmission line parameters.

I hope that helps :)

I really recommend the book "RF Circuit Design" by Chris Bowick, it's a very useful book if you want to further your studies in RF design.

Thank you this was a very help full answer.

Yes S12 and S21 do look familiar, I think S12 was the value that was plotted against the dimension sweep. It was measured in dB, so the smaller the dB, the less power that is being reflected.

I just have 3 questions now:

.If the S parameter is measured in dB, what is it relative to? (ie. what is the reference power)

.When the reflected power is zero, (or at a minimum) is this when there is a full standing wave, so all the reflected power is "canceled out" by the transmitted power? (this comes from the theory that at an impedance mismatch you get an inverted reflected wave, this will bounce back and cause a standing wave or a partially standing wave)

.And my last question, how can you actually have a wave in electrical circuits. This has always confused me. Does the energy from the electrons (electrical potential energy) travel as a compressional wave like sound does?

Thanks

Hi again Craig,
It is good to see students trying to get right in to grasp the heart of a subject ! This is the road to new discoveries and scientific advances !

I have had similar difficulties to you in trying to get to a nuts-and-bolts understanding of subjects. I have felt on electronics courses that I was just using maths and formulas as mere tools without really understanding what I was doing. Yet I also managed a few quite good marks !

I would suggest googling to find radio frequency engineering forums, where there may be engineers with greater specific knowledge of the issues you are commendably so diligently pursuing.

The great scientists and engineers who make discoveries and inventions are those who earnestly question the current understanding of things, gather all the available knowlege and then either build on it or totally challenge it ! I can see you being on that road !