Problem with HFSS and Ring Loaded TE11 to HE11 mode converte

Hi.

I'm having trouble replicating the results for a Ring Loaded TE11 to HE11 Mode Converter proposed in "TE1l to HE1l Cylindrical Waveguide Mode
Converters Using Ring-Loaded Slots" in HFSS. Since I'm new to HFSS, I would wagger this is a software related problem (I've checked my calculations for the design 3 times)

Long story short, the ring loaded slots are supposed to convert from the TE11 to the HE11 mode. The theoretical performance is as follows:


And this is what I get:


As you can see, it's a very good return loss at 30 GHz, but at higher frequencies it's very bad, unlike the theoretical graph above.

I designed my mode converter for k_0 a_0 = 2.9 at 30 GHz and the rest of the parameters according to the aforementioned paper.

Here is a cross sectional view of my mode converter followed by a corrugated waveguide designed to propagate the HE11 mode.


Here's the setup I used to measure return loss (the same setup used in the paper). Two of the same MCs followed by a corrugated waveguide.


Any help figuring out why my HFSS results don't match the ones in the paper will be greatly appreciated. My HFSS Design is attached below.