Hi Sherry,
1. The extra piece of metals were in the layout when I got the file. I think it corresponds to the actual shape of the fabricated board. There are two pieces of ground/copper stick out in the lateral direction so that the SMA connectors won't short all metal layers in the board.
Ok, makes sense.
So now the question is: do we want to model in Momentum the same excitation that you have measured, or the line only (excluding feed effects)?
I assume that you want to model the measured case, and then the lower ground plane must
not be connected with a (-) pin because there is no such connection in your measurement.
I have placed my (-) (+) (-) ports at the outer edge, because this seems to be closer to your measured scenario.
With this, I simulated two cases: with finite (drawn) ground below, and with infinite ground. (Pictures below can't be compared directly because frequency and scale is different, but you see what I modelled...)
I think the best way to understand the results is the Smith chart here. You can see that the infinite ground results show decent match, whereas the finite ground moves towards the inductive region.
The difference can be understood as extra series inductance for the finite ground case, or less shunt capacitance for the finite ground case, or a combination of that. If we think about the extra notches at the ground pin location, this makes physical sense: more series L and less shunt C if we remove the ground below these notches. To me, this effect makes sense and looks real.
In the attached workspace, I have included a schematic to play around with these parasitic L and C.
You can see that the S11 zeros (frequency) now match up decently with your measured values, but the absolute value is off. If you play around with the parasitic L and C, you will notice that very minor changes also increase/decrease the S11 magnitude. I suspect that we have some parasitic effect there in the SMA connection/soldering in measurement that we haven't exactly modelled in Momentum. Minor parasitics matter here, as we look at rather good matching levels.
3,4 Thanks for your insightful suggestion, I left everything as default because I didn't know what should I touch.
For best accuracy, I recommend these two changes to the mesh options:
5) Regarding the default ground, I understand it's causing problem and I can do an easy fix in this case, but I have some question for another design I've encountered. So I had a 4 layer board, and none of the layers is a solid ground plane, so I realized I have to use the theoretical infinity ground. Then I worry about the accuracy of my result. In this case, is there any other way I can setup the port to avoid using the theoretical ground?
The individual S-parameters (port referenced to ground at infinity) will be invalid then, but you will get useful results as soon as you assign another physical pin as the explicit ground reference. This can be done on the port settings, and you had already use that option. If you had already done/tried that in your "strange results" case, I would suspect that you mixed up pin numbers and assigned wrong reference pins (physically far away from the signal pin).
Archived ADS2014 workspace is attached.