On Monday, March 3, 2014 10:51:14 AM UTC-6, wrote:
That the metal under a mobile whip is
acting a lot more like a radial system, or even the ground itself,
than the other half of a physical dipole.

It depends on where it is connected to the vehicle. If connected near the center, the vehicle acts like two opposing radials and radiates mostly omnidirectional. If connected on the rear bumper, it can act like one radial. On my S10 with a rear bumper-mounted 17m hamstick, my signal was quite directional beaming toward the front of the vehicle. I would sit in the parking lot at Intel and aim my S10 mobile antenna system. I often gained an S-unit by aligning the vehicle with my contact. The fact that the S10 was a quarter wavelength long on 17m probably helped.
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73, Cecil, w5dxp.com