Currently I am (non-engineer) trying to understand the origin of measured scattering parameters with a vector network analyzer and a 2 port system. The second port is terminated with a matched load to force a2=0. I am observing the amplitudes of |S11| and |S21| to verify the optimal matching, and to exclude standing waves.
|S21|= b2/a1 |S11|= a1/b1
I expect that with a1=100% of incident wave nearly everything is transmitted ~100% to the output port b2. So I would assume as best and maximum value for |S21| =1, rather smaller lets assume b2 being 80% |S21|= 80/100=0.8. This gives me a range from 1~0 for |S21|. The closer the value is to 1, the better is my transmission.
Now lets observe |S11| in case of a perfect match there should not by any reflected wave, a perfect match is leading to b1~0. Which brings me to be best measurable value of ~100 for |S11|. Since nothing is ideal lets assume b1=10%. Which shows me that |S11| can take values from 100~0. The higher the value the better is my transmission line matched.
In case of a standing wave I expect my |S11| being very good compared to other frequencies, however if |S21| is showing a poor transmission at that frequency with a value <1, it indicates that a standing wave probably is existing.
Am I right with my estimations so far?
Can I see more out of the |S21| and |S11| measurement than standing waves?
In reality I measure negative values for both parameters. |S21| is nearly showing a flat line close to -1 and |S11| has an average value of -15dB with a dip going down to -30 dB @1.5 GHz.
If I argue the following is that correct? I would like to say that this dip -30 dB @1.5 GHz is a very good matched frequency and not a standing wave, since I still see a |S21|-value ~-1 dB. A standing wave would show a value much "smaller" so something which is closer to "zero".
I am aware of the following conversion
Insertion loss = -20*Log|S21| in dB Return loss = -20*Log|S11| in dB
But I don't know what these formulas tell me. With |S21|=0.1 it gives an insertion loss of 20. Does this mean that 20% are not transmitted to b2?
With |S11|=15 it gives a return loss of -23. Does this mean that 23% are reflected or that 77% are reflected?
Maybe I am completely wrong, thank you for taking a look! =)