Yes, the mag base antennas are designed to work when applied to a large conductive surface (such as a car roof or trunk). We test them internally on a 24 x 24 inch (609.6 x 609.6 mm) ground plane.
The size of the magnetic base is not sufficient to provide an adequate ground plane, particularly for the lower frequency band of the antenna elements. Using the antenna and base without sufficient ground plane will affect the radiation pattern and performance.