Ground plane ? What is it ?

The "ground plane" is best described as a reference point for your antenna. It can "physically" be the body of your car and in many cases it is.
But electrically or in "radio speak" it is the electrical centre of your whip antenna, the physical whip forms half of the electromagnetic circuit, the
other half is (theoretically) produced below the "real" antenna, the ground plane between the two'

http://t0.gstatic.com/images?q=tbn:A...tYG02N4VCaPNaD

Remember, this is radio theory. You cant see it or measure it (the reflective part of the antenna that is) and it best explains how this stuff works.
I have studied and worked with this stuff for years and it is all FM (F.....g Magic)

The ground plane's mission is to turn your 1/4 wave whip into a half wave dipole so that more effective transmisson and reception is achieved.