low frequency anteena using cst

I wish to design an antenna using CST with resonant frequency as low as possibly preferably in khz(10-50 khz). Can anyone help me please .

The ARRL antenna book has the answers you need. You can read on line or go to the Public Library.
Question: Why so low in frequency? Check the regulations and do not exceed the power limits.

Here's an antenna design for 77kHz:
http://www.ptb.de/cms/en/fachabteilu...ime/dcf77.html

The link u hav sent nowhere gives an idea about designing .... if i am wrong plz help me but i jzz could not find what i needed

The link was to show that many different antenna designs exist, and full size antennas for this frequency are impossible to build. What we see als low frequency "antenna" in RFID and wireless power transfer is usually inductive coupling between coils. To get help, you need to specify much more clearly what you are trying to do, what type of "antenna" you want to build and how much of that design work you have already finished yourself.

I actually want to harvest energy from normal 50 hz supply available at home, so i wish to design a rectenna in cst, but my first challenge is a low freq antenna .

And i wish to design a rectangular microstrip patch antenna using CST software.

I fear your latest posts are further confusing your intentions.
In terms of RF engineering, an antenna is a device with non-zero coupling to far-field. Inductive couplers are designed to receive or generate a magnetic near-field. They are used in a kHz to multiple 10 MHz range to transmit data or power wirelessly. They can be described as AC magnetical devices, you don't need the capabilities of a full featured EM solver like CST (but you can use it nevertheless).
The 50 Hz power that can be harvested in a home enviroment will be hardly above a μW level, I fear the project is doomed to failure. You may be able to receive higher mW levels under an overhead power cable.
Microstrip patch antennas have dimensions comparable to wavelength, a realistic application range starts at several 100 MHz.

You can't build a patch antenna for 50Hz.
Wavelength at 50Hz is 6000km.
Dimension of the patch antenna is 1/2 wavelength = 3000km.

in dat case what is the lowest practically possible antenna design dat i can make using CST

You can design other antennas than patch, e.g. loop antenna.
It's still unclear what you want to achieve.