Dual Band DRA Antenna in HFSS

I need help .. I need to design a dual band antenna with a dielectric using HFSS .. i tried and tried and just can't get the dimensions right .. I used a DRA cut in half with a radius of 5 mm and height of 17.85 mm . I got these numbers using some formula i found in a book that my teacher gave me ... The goal is to have a dual band antenna between 880-960 Mhz and 1710-1880. The results are way way off ... what should i do ? what should i change ? if i change the dimensions nothing much happens .. In my results i got very high frequencies 10 14 Ghz .. i'm out of ideas . :( help !

hi
for DRA antenna u should read paper of ahmed kishk in university of Mississippi who which in this antenna or read handbook of volakis a chapter which written by ahmed kishk

actually i'm looking for answers from someone who is familiar with HFSS program and could give me some pointers ! Thanks anyway

Need to post project for troubleshoot. Sounds like a feed location issue or a material property issue.
Have Fun

i'm using a 12 permittivity dielectric because that's what the book said .. also used a 36 and 89 perm just for the sake of it .. nothing improved .. the feeder , well i tried putting it on the center (more or less) .. also no improvement .. my bet result was a dual band at 11 Ghz and 14 Ghz .. my professor laughed :))

The location of feed is a big deal...you have to match the field distribution of the mode you are trying to excite. In your case, your dual band is really just inclusive of the second harmonic of the radiator. If you post your project, and reference your inputs, I will take a look at it.

Later.

here it is .. radius 7 mm , height 4.45 mm , r/h=1.57 ratio ... the feeder was calculated as being d1=3mm , d2=0.85 mm with a ratio of d1/d2=3.49 .. could you please tell me what is the best dielectric that i should use where should the feeder be situated and what parameters should i change to achieve the desired dual band frequencies 800 Mhz , 1.8 Ghz ( or at least the best posibil result that gets close to what i want ) .. thanks for your help , i really appreciate it