2NO, if you are talking about the "rubber ducky" antennas that come on the portable PowerFlarm, they do not want to be mounted on metal (they don't need a ground plane).

For the center-fed dipole antennas that now come with the "core" (previously called brick). Again, no ground plane needed and they should be mounted as clear of metal (or conductive material like carbon fiber) as possible. Metal (or CF) is especially bad if nearby and in the same plane as the radials (not quite so bad if the metal is at right angles to the antenna radial, though it may still block RF if close. Also, be sure to keep the coaxial cable running straight away from the antenna for a few inches - - don't run it alongside the radial.

bumper