how to buildan omnidirectional antenna
I am also doubt about whether there exists omni-directional circular-polarization antenna, not matter in what forms.
Since I reviews all the antennas I know for circular polarization, such as microstrip antenna, helix antenna and so on, all of them are only broadside or endfire, but not omni-directional.
I am not sure whether the antenna arrays can generate the pattern with omni-directional circular polarization, but I think even it would, at some angle the axial ratio should be not acceptable.