The bottom line is, it doesn't matter how long of a shaft I can get, I will never obtain / utilize the full potential of the Predator by leaving it installed on the bedrail because the base of the antenna will always be blocked by the cab.When I have tried comparing mobile antennas I have found the differences minuscule and hard to detect, but that was using the same mounting location.
I was one way or the other on what might work better for you in this case, a lot of it depending on directionality of the rail mount more than anything.
5’ is my magic number with antenna length...gains above that length seem very small, and losses below it seem large.
The Predator is in fact a "center loaded" antenna and as Jessie James Dallas has stated, the compete antenna from top to bottom is a radiating element.
I have learned several valuable lessons about radios and antennas from the folks on this forum. Unfortunately I have no use for the Predator unless I install it with the base of the antenna at or above the roof line. I can kiss the $94.95 + $11.80 (shipping) goodbye!