interdigital cavity filter synthesis

Hi,

I am trying to design an interdigital bandpass filter with round rods and tried two synthesis sofwares
1- An excel software in rfshop.com
2- intfil.exe
both of them are free on the internet

However, when i used the results of these sofwares in CST, the simulation results are very very bad. Is there a limit for the frequency range of these synthesis tools (can I design at 50GHz) or how shall i synthesize at high frequencies ? How can input coax model and tap point be modeled in CST? Also is there limitations for rod diameter, ground plane spacing and first rod to wall spacing ?

Can anyone suggest me any free synthesis software for this kind of filter and CST examples?

Thank you...

I am not sure how to adjust the tap position since when I used a higher value than the pormgrams suggested the result is better and also I cannot tune the screws exactly.
Does anyone know about this filter?

I am not sure whether you can design a filter at 50 GHz. The frequency looks too high.