Radiation Efficiency improvement for miniaturized slot antenna

Hi,

I am working on a project to design the slot antenna with size 20*20mm. I have design antenna which resonates at 2.4GHz with good return loss -15Db and has a wide bandwidth of 500Mhz. But the problem is the efficiency. It's efficiency is around 40%.
Could you please different methods for to enhance the radiation efficiency.

I hve attached the cst file.

Thanks for the help.

Wow ! 20 X 20 mm, that's not a slot its a hole. According to my limited theory, slot says it all, good conductivity surrounding the narrow slot. Has you aerial a very wide bandwidth?, do you need it?
Frank

Low radiation efficiency results from losses (dissipation). In simulation, you can selectively make only the conductors lossless, and then make only the dielectric lossless, to find the critical parameter.

Just in case I would ask if the OP is actually measuring radiation efficiency or the measured "efficiency" might include mismatch losses.