Did you know that you could optimize an ultra-wideband filter with Filter Designer 3D in CST Studio Suite? Here is an example of a 14th order filter achieving a 60% fractional bandwidth.

Yes, we do use the Coupling Matrix synthesis and yes, it is only valid for narrowband implementation… However, thanks to advanced parasitic handling, the extracted CM will always guide the optimizer to an equiripple response. This is indeed no longer a 1-to-1 representation of the distributed model, but numerically it is a great surrogate for gradient-based optimization techniques.

Give it a try and let me know if you are able to do even better than this using FD3D.