A vehicle is not a
ground plane, but rather acts like a capacitor between the antenna and the surface under the vehicle which acts as the ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. If we wish to maximize the
system efficiency (the mobile station as a whole), we need to maximize the RF continuity of the vehicle, hence proper bonding. Obviously, proper antenna mounting, and placement are important too. Remember, it is the metal mass directly under the antenna, not what's along side, that counts. And a
ground strap
is not a replacement for proper mounting!
Bonding also minimizes the leakage of RFI into (ingress) and out of (egress) the various bolted on parts of the vehicle. The exhaust and tail pipes are good examples of RFI egress. Bonding horizontal surfaces (trunk lid, hood, etc.) will have a greater effect than bonding vertical ones like doors, and hatches. It is not uncommon to see a 20 to 30 dB drop in received noise levels once they're properly grounded. Bonding is especially important for
body-on-frame vehicles, like pickup trucks. In these cases, the bed and cabin should be bonded to the main frame as well as
between the bed and the cabin to prevent
ground loops.