Essentially, what this is saying is that the port "window" you have defined has multiple solutions for modes that can propagate on your transmission line within the frequency range you specified. I don't know what your input transmission line looks like (microstrip, perhaps?) but its analogous to the way multiple modes can propagate in a rectangular waveguide. We don't normally think about it with QTEM lines like microstrip, but they can become overmoded too, if the frequency is high enough. Generally, you want the height of the substrate and the width of the microstrip to be less than lambda/8 in the substrate medium at the highest frequency to avoid overmoding.

With CST, it can also happen if your port window is just too big for the upper frequency limit -- it tends to find plane wave solutions that propagate outside and above the substrate. A common rule of thumb is that the port window should be 5 times taller than the substrate thickness and 10 times wider than the strip width to make sure you capture all the fields in the dominant mode. But that's just a guideline only, you may have to make it larger or smaller, depending on the particulars (and that might be especially tricky with a wideband design such as yours).

Does it have any effect on the result? Not always, but it certainly can. As matter of good practice I would always check the port mode solution results to make sure it gets the right dominant mode that you expect (look at both the line impedance and the 2D field pattern).

It can also effect convergence of the simulation, since energy put into the model in one mode may convert to another and become trapped if not accounted for at the port. If you just can't get rid of the extra modes, usually you have to increase the number of modes in the port definition, so that all are accounted for, and then tell the simulator to only stimulate with the dominant (or desired) mode under setup solver. It may also be a good idea to ensure that "Absorb unconsidered mode fields" is activated under the Solver Specials --> Waveguide tab.

Hope this is helpful.

Matt

Hello Shaik,

Some higher order propagating modes at port "xxx" are not considered in time domain calculation. This possibly leads to an inaccurate energybalance. Consider to increase the number of modes or decrease the upper frequency limit. For further detail check this link:

https://fr.scribd.com/doc/149560928/CST-STUDIO-SUITE-Help-pdf

Best regards

Hi,

This can happen, when the surface or edge where we define port, is not proper to support the mode (higher). So, in this case, define one more structure which has very very less thickness compared to your outer structure or wall and assign it as vacuum. Thereafter, assign port for the surface where really you want your port, considering with second structure.

Thanks,

This might be due to higher order modes propagating through the port you have defined, you may increase the no of modes (and even polarisation angle ) in port settings.

This will help you.

Look at the CST solver and re-define your K value by reducing the K value to satisfy the propagating modes, because the thickness of the port in consideration play an important role to the mode of propagation.