8-Pole Round Rod Cavity Interdigital Filter HFSS Simulation

Hello everyone!

I have to design an 8-pole Round Rod Cavity Interdigital filter. Theoretically, it meets my required specs which are as follows:

-Fc= 1090 MHz
-Bandwidth= 270 MHz
-FBW= 24.77 %
-Insertion Loss= Less than 1 dB from 955-1225 MHz (270 MHz)
-Rejection > 45 dB @ 807.5 MHz and 1372.5 MHz

I took the physical dimensions from http://www.wa4dsy.net/cgi-bin/idbpf (you are welcome :) ) and the resulting dimensions are attached as a text file.
I am attaching my simulation in HFSS. Please see if I have missed anything or something I did wrong. Correct if you can, please. If any other simulator is preferred by you, kindly take some time to simulate it so that I may be confirmed about the design specs.

Note: Please use HFSS 13.0 to avoid any compatibility issues. If it opens without any problem, ignore this note :D

Thanks in advance to all who see and comment. :)

Peace.

dimensions.txt

reference fig.pdfIdFilt-1.rar

Do ask if you need more info.

The simulation has obviously too few points. This doesn't necessarily explain the deviations from expected filter curve, but higher frequency resultion is necessary to understand the simulation results.

I'm not a FEM expert but would expect too coarse meshing or errors in the structure definition causing the deviations.

Secondly, how do you know that the specified filter characteristic is feasible as 4th order? Stop band attenuation isn't included in the data entry and as far as I see from a general filter tool would require 6th to 8th order for the given ripple and pass band.