Hello there,
it is known that if we realize a filter (for instance a low pass filter) by using distributed elements (transmission lines), there will be some parasitic passbands (which moreover show a behaviour which seems "periodic").

For instance I did this simulation of a Butterworth filter of 5th order:

Note that it behaves as a perfect low pass filter only for few GHz after the cut-off frequency. What is the cause of this phenomenon?

 

This is a characteristic of this type of filter. The impedance of the elements will change based on wavelength.

 

It's called the "re-entrant mode". A transmission line that resonates at F will also resonate at 2F, 3F, 4F ... etc. And they aren't parasitic. You can minimize the effect of the re-entrance by using "defective" transmission lines.

 

It's called the "re-entrant mode". A transmission line that resonates at F will also resonate at 2F, 3F, 4F ... etc. And they aren't parasitic. You can minimize the effect of the re-entrance by using "defective" transmission lines.

Precisely, what do you mean with defective transmission lines?

 

Precisely, what do you mean with defective transmission lines?

Here's an article on them:

Designing Filters with Defects Can Optimize Results.pdf