Help me to design circular polarizer for conical horn antenna

Hello,
Does anybody help me? I need to design circular polarizer for conical horn antenna. It should be dielectric slab. Does anybody know where I can get any info about this? Do exists any formula for it? It's very important.
Thanks

Rather than a formula ... a concept.
The path of E field propagating along the slab should be 90° longer than the path of the E filed propagating orthogonally to the slab.

Of course, the slab should be low loss and should be tapered.

Hi,
Yes, this is the fact what I know, but I need to calculate the length of the slab.
There is the other topic in this forum, where I'm looking for help to find out the length of the slab by simulating this in CST. But If I will use analytic method is the method below correct?

Here is:
1) I know the length of the wave in waveguide, so the phase velocity in waveguide is known too.
2) If the permitivity of the slab is "e1", calculate the velocity in this dielectric (not in waveguide) v1 = c/(sqrt(e1))
3) From this I know the wavelength in waveguide which is filled by the dielectric with e1
4) So, I know the wavelength in the slab
5) From this, I know the phase variation between the wave propagating along the slab, and the wave propagating orthogonally to the slab

- is it true, or am I false?

I have here pat.

pl