I am routing a flexible PCB which connects a panel-mount ethernet connector (without integrated magnetics) to a rigid PCB which contains the ethernet magnetics. I plan to route the differential pairs as edge-coupled microstrips (100ohm differential impedance). The reference plane that I'm routing the traces over is connected to Bob Smith termination as shown in the diagram.
I have some doubts over whether my edge coupled microstrip will give me the expected differential impedance. I can use field solvers to determine the necessary trace width/space for my stack-up. The results I get show that most of the coupling is to the reference plane beneath the traces (
Z0) rather than directly between traces (
Zcoupling). But because my connector has no "ground" pin to connect to the reference plane, I don't understand how any return currents can possibly flow through this plane? Would this make my calculated differential impedance invalid?