None of what what said is true.... Ground plane is a metal surface that is placed below the coil causing the signal to go up to atmosphere. Causing better receive and transmit.... A car roof is a great ground plane. Ground planes are ment for big truck mirror mounts and other mounts where the coil is placed up high away from a large flat surface
close but not quite right.
a GP does not "cause the signal to go up in the atmosphere", the radiation angle with or without a GP is the same.
A vehicle is not a ground plane for an HF antenna. Rather, it acts like a capacitor between the antenna, and the surface under the vehicle in question. That surface, whatever it is, forms the actual ground plane.
I don't think GPs have any effect on receive (except under certain multi-path conditions)