Tuning my Predator 10k

[Fogdog]

Having trouble tuning my new Predator 10k SW. I mounted it on the aluminum headache rack of my truck, and can only get the swr down to 1.6.1. The Firestick it replaced was at 1.3.1. I may have cut the whip a few inches to short trying to tune it yesterday. Total length is 57''.

Does the whip when tuned have to be bottomed out in the lower shaft cavity? Right now mine is above the bottom a few inches supported by the set screw. I am waiting for my new coax cable to arrive, and I haven't done any additional grounding of the rack yet. That Firestick made contacts almost every day across the country. I couldn't get out yesterday. I know the conditions haven't been very good on the west coast for a few days.

Any help would be appreciated. Here's some pictures.