Greetings to all the antenna experts here!

I want to create horizontally polarized antenna, low profile, located above
large ground plane. And I need it to be omnidirectional (sort of) and have
sufficient gain in horizontal plane.

My frequency of interest is 900 MHz. The ground plane is a roof of a car.

I tried loops, from 50 to 300 mm diameter, radiation goes up if I mount it
above the ground plane.

Any ideas?

Thank you,

Andrey