ground plane requirement

[thuor]

This antenna requires a small metallic ground plane which can be anything metallic even a tin can if you like. The largest wavelength of the RF signal (at 800mhz) is about 37cm - we need a ground plane of about 9cm in diameter ~3.6inches.

[desi2009]

what is the ground plane requirement for a magnet mount antenna for use in-building. In my vehicles its fine coz i can mount on the roof of my car but what to do in building???