Air box radiation PIFA antenna

Hello every one,

I'm a student, i have designed a PIFA antenna with coaxial feeding.it work correctly but the problem
when i want to design the airBox for radiation pattern i have a bad result.i have tried to put the length of the box lambda/4 but i have the same problem.
Please it's urgent any body can help me to configure my box and thanks for advance.

pifaaa coaxial 4.rar