monoploe antenna

Hi friends,
I am working on attached article, about monopole antenna, dimensions of ths structure is 9cm*9cm, at frequency 1-15 GHz, that need very large box to simulate it, with box 40*40*30 cm I cannot give good resulte, bad result! can help me every body?

thanks
also i attach my simulation

between 1-15 ghz isnt it too much? and the box should be λ/4 far away from the radiation direction

Thanks, you mean my mode is wrong?