How to study an object at 10MHz by simulation software!!!!!!

Hello, my friends. Right now i have got an problem. I will study an object with a size of 20cm*20cm*20cm. But the frequency is 10MHz. If I use Ansoft Hfss, by the quarter-wave theory, the radiation boudary would be very huge and the problem may not be solved. What others softwares can i use, or codes? How can i prove that the results are accurate enough?

the problem mentioned will be held in any other simulator software since the wavelength is about the dimentions. then I think for the best accuracy you should write a new code by defining the boundary condition and other equation for example in matlab.
by the way the discrete analysis in hfss is the best if you have to use hfss.

The discrete analysis is not vital for i just care about one frequency point, not a band. Can you give me some advice?