You've got the correct answer Jon. This becomes more important the higher in frequency you go, ie, the shorter the wavelength becomes. The ground plane of a roof or hood of the equipment becomes a larger and larger ground plane to the antenna the higher you go in frequency. At 100Mhz, the wavelength is 3 meters.

Without a ground plane, unbalanced vertical antennas are pretty pathetic antennas. As we used to say of LF/MF/HF vertical antennas, often installed with mediocre grounding systems: "A vertical does equally poorly in all directions." Well, at VHF/UHF frequencies and the antenna mounted at the edge of a roof, there's enough ground plane out in front of the antenna in the direction of the roof so that it now sucks less in that direction.