Current flow in a planar transmission line during a spurious mode

Question for the EM engineers for an aspiring EM engineer:

How does the total current flow in a planar transmission line when it has a spurious mode excited? Say for example, you have a CPW with lower ground plane that has various non-desired TE or TM spurious modes exist at certain frequencies. When operating outside these bounds, staying within a quassi-TEM mode, there will be a total current that flows in a closed loop on the top trace and returning on the lower ground plane directly under the trace. This I understand. But when a spurious mode is excited, like a TE or TM, how does the total current flow in a planar transmission line? Is this related to Surface Waves?

Thanks.

In any kind of a transmission line, any undesired mode (spurious or any other name) causes a loss, therefore the power source delivers more power than becomes available at line output, in the desired mode.
In RF and microwave region , measuring voltages and currents is technically not possible, we use power terms that we can measure by power meters. Again, each particular kind of transmission line utilizes a specific propagation mode, so the power source and power meter must be technically matched to it.
This is the reason that most test equipment is made with 50-Ohm coaxial connectors. A coaxial line carries the TEM propagation mode but at the upper frequency limit, spurious modes appear that cause line loss. Other than TEM mode transmission lines like waveguides, CPW etc. need to use transitions to the 50-Ohm coaxial line. Such transitions also introduce certain loss as due to their design, spurious modes are generated.

Thank you for the reply. It makes sense what you say. The thing I'm confused with is when hearing the perspective of a high-speed digital engineer focused on signal integrity. They have it ingrained in their minds that all current always flows in a closed loop and that the return current is on the ground plane and flows directly underneath the trace (if Microstrip ). They say that the total current leaving the driver will return in its entirety and everything flows in this tight closed loop.

I've seen on another message board where a so called signal integrity guru like Douglas Brooks, argue vehemently this point of how the current flows, and his "opponent" was a microwave engineer trying to explain that there are exceptions to this rule and he used examples like waveguides and transmission lines operated in a spurious mode. (I tried to find that dialogue but couldn't. It's on a signal integrity message board somewhere)

Is it true then that during a spurious mode, the total current does not flow in a tight loop? Some "gets lost" some how? During one of these modes, does this mean current can flow non transverse and simply dissipate and the result is power loss?

In coaxial line or a stripline, the dominant propagation mode is TEM. Voltage and current distribution along a homogeneous and matched line is uniform. The current may follow closed loops, mostly confined close to the "live" line on the ground plane. With growing frequency the possibility of spurious modes becomes higher and the current loops may extend farther. In a microstrip and CPW lines the propagation modes are "slant" with respect to line axis, causing loss due to consuming a part of the power on the line.

Microwave engineers tend to "see" various modes of propagation in various line types. Digital engineers are rather used to strip lines as such type is prevalent in typical PCBs. With higher clock frequency, complex PCBs support generation of spurious modes. Load mismatch is seen often, causing standing waves and an additional loss.

Analyzing a real situation becomes more difficult as the line pattern on a PCB rarely conforms with a good RF design. We will see more problems in near future due to it.

Thank you so much for the follow up. Ok, I think I get this. It makes sense and can explain why with spurious modes, those can also be seen as dips in the frequency domain response. What got me all interested was seeing a field pattern that a teacher got in a simulation and he said it was "spurious modes" and you can see a very interesting pattern to the surface currents or fields, I'm not sure yet what it was I was looking at but it was colors and it resembled a field plot like one might see in a waveguide. So I guess these are non transverse currents. I'm just finishing my BS and going to go for my MS and focus on electromagnetics. It's just a very interesting field. Thank you for taking the time to help out an aspiring future expert someday

I have recently seen several papers on visualizing field intensity in wave guiding structures. Try to google field intensity visualization. In my own experiments I used microwave noise which to the contrary is wideband. There is a lot of software promising nice colorful pictures but no authors of it reveal their equations or limitations. If you can trust it is a question. Experiments are better but require time and money.

Good luck to your future study!