Hi Akash

In the macros area (under "construct" section) theres some macros that automatically model helixes. You can use it for a starting point of your structure.

helix structure are available there but i want to design folded helically corrugated waveguide as given bilow

You should be able to do this if you generate several helices and add them together.

by adding helices i can get shape with circular cross section, but i need 3-folded cross section, u can see the figure, the cross section it is 3-folded not circular.

You have added the helicis together to give one helix - you need three helices (each perhaps made of several others) with three times the pitch, 1/3 of a period out of step, to get the three-way symmetry in the first diagram.  Make your pitch 3 times a long and start three helices at 120 degree intervals.

i didn't get your design approach, would you send me CST file of your design please ? 

No.  Your shape has three humps round the perimeter.  This means there are also three humps per period along the length.  Imagine that the cross section has three separate wires.  This would mean that there were three separate helices interwound.  If you made each of the three wires elliptical so that they touched at the sides you would start to get something like the cross-section you want.

The surface of a helically corrugated waveguide is represented in a cylindrical coordinate system (r, phi, z) as follows:

r(phi, z) = r0 + l cos(mB *phi + kB z) .

Here r0 is mean radius of the waveguide, l is amplitude of the corrugation, mB and kB=2pi/d define the azimuthal number and axial component of the Bragg periodicity vector, respectively, and d is the corrugation period