Rectangular antenna design

Hello,

I am trying to make a rectangular antenna (50X50 cm2) as a transmitter side for wireless power system . so what solution frequency should i choose
and how to found a frequency at which return losses (S11) is minimum for my antenna.

Is it a planar rectangular antenna? Like a patch antenna?

It's also very odd that you're choosing the frequency of your antenna to fit your physical dimensions.....usually it's the other way around. And 50cm x 50cm? Really?

Your solution frequency should also be the frequency of minimum return loss. Analytically, you can find great results in Balanis. Numerically, softwares such as Agilent ADS or HFSS will help you immensely.

My antenna is single loop rectangular antenna of 50cm X 50cm. I am following an ieee paper in which they made an antenna of 50 cm X 50 cm and they got minimum return loss at 13.56 Mhz frequency. so i bulid an antenna in hfss but i am not getting any minima at any frequency.

Wow, that frequency is low :) I'm just not used to working with that band, sorry for my disbelief.

If you're not getting a response anywhere near where you expect, it's potentially a setup error. Can you describe your HFSS setup?