Error when simulating ridged conical horn in HFSS

Hi i have a problem with HFSS.I'm studying a ridged conical horn using HFSS.I have drawn if and I have drawn the box where I have put antenna.When I have started simulation an error message appears
" [warning] Solution Setup 'Setup1': Given the specified frequency and model dimensions, an extremely large mesh will be required to produce an accurate solution. Your model and/or frequency units may be set incorrectly"

I have used a bandwidth that goes from 2 GHz to 9Ghz.
How can solve the problem?
Please help me.I'd like to know where I could find such articles about ridged conical horn and its simulation with FEM
Thanks and excuse me for my english,but I'm italian

You can draw your model.then maybe some suggestion can be given.

this is my model

Added after 1 minutes:

Excuse me.How can I draw my model?

hi, sayan4
im getting the same warning as u...though my freq is 300GHz.Did u find out a solution to this problem...please suggest me