circular polarization conversion
The concept is easier to understand than explain by text.
I'll try.
1) The ensemble of radiatiatin wave energy is always the sum of two "orthogonal" polarization. We may speach of H&V or LHCP&RHCP.
2) A polarizer is a reciprocal device. If fed by Circular then a Linear come out and vice-versa.
3) A circular polarization is the vectorial sum of two linear one. For example just add the V with the H delayed by 90°.
4) Many practical modes exist to do this. The dielectric slab, since is slower than the air, will delay the linear polarization that flow into it (for example the V), the ortogonal one (for example H) don't flow into the slab, so it will be no delayed. So at the output You'll have a circular polarization.
5) apply point 2) so if you feed with a circular polarization , you'll have a linear one that is what you want.
6) Of course it works into a limitated bandwidth, tipically 10 to 20%.
7) depending on what do you want, you may collect only one polarization or both , in the latter case you need an OMT after the polarizator.