Facing distortions at high frequencies.

Hello,
I have designed a coaxial cable on HFSS and added a reflector(rectangular block) in the middle of the cable. When I am plotting S11 parameter I can see the estimated(as in theory) reflection coefficient till 13 GHz then there is some sort of distortion noise happening at higher frequencies(till 50 GHz) which is not possible theoritically. For any model I design I can see distortions(noise) at frequencies greater than 13 Ghz.
I am kind of new to HFSS, so I might have messed up some parameter. Please help.
Thanks You in advance.

How could you understand that "distortion/noise" by observing S-parameters ?
You have probably seen a kind of irregularity on s-parameters, haven't you ?

Yes, exactly irregularities is the right word to describe it. I am uploading the plot. Just check. Thank You very much.

Can S11 be 0 dB @ Low Frequencies if that structure is a coaxial cable so that both ends are terminated by Zo ?
Check your design..

I think, it's useless to guess about the effects that show up in the s11 plot before a clear description of the actual test setup has been given. Apparently the "reflector" is either a short (s11,dc = -1) or open (s11,dc = +1), but the geometry details are important to understand the behavior at higher frequencies.

@BigBoss Yes its a coaxial cable and both the ends are terminated by Z0. I am uploading some images of my design. I can even upload my whole project if any one can say me how to.
@FvM It was just the default test setup like the solution frequency was 10 GHz, the delta S was 0.02 and the number of passes were 20, rest all the properties were kept as default. The reflector is a short circuit.
You can see in the images I just inserted a copper mirror into the inner core of the coaxial cable so the short circuit can take place.

The "reflector" isn't blocking the transmission line completely. The short has a certain inductance, it's expectable that higher frequency components are passing the short and the reflection factor drops.

I'm not sure what brings up the s11 ripple. Either the meshing is too coarse or other modes than TM0 are excited.

@FvM The reflection factor drop was expected but why the ripples? That is an issue. So what exact changes do you think can stop the ripples?

1. Block the coax cross section completely
2. Check if waveguide modes can be excited due to too large coax diameter
3. Check meshing

"distortion noise" is not a valid engineering term. there is no distortion, which woud be clipping or harmonic related modification of the input signal.

It seems like you have unexplainable variations. Have you considered that they might be circular waveguide modes of propagation super imposed on the TEM coaxial mode, and these waveguide modes only happen at the highest frequencies.

alternatively maybe the meshing HFSS is using is set to a too big value, and the simulation stops converging at higher enough frequency.

@FvM I cannot block the cross section completely as whole signal will get reflected completely and no signal will be transmitted which is not desired in my experiment. I tried all the mesh properties and they are fine.
As you all say the issue is the other waveguide modes are getting excited at higher frequencies that's true, because when I changed the size of my cable I can see this error: "Port 2 supports an additional propagating or slowly decaying mode whose attenuation is xxxxxx and propagation constant is xxxxxx' . Now can you please say if there's a solution for this issue?
thank you all.