This is been discussed many times, but I will add some additional experience to it. We are running a challenger MT765 with Trimble RTK Autosteer. We mounted the antenna directly above the pivot point per Trimble's instructions and we were generally pleased with the performance. But, at times the tractor did seem slow to "come on line". This week I moved the antenna forward on the cab (actually, about 4" in front of the cab). This seems to have greatly improved the line acquisition. I would strongly suggest a front of the cab antenna position on any tracked tractor or front wheel steer tractor. The antenna will pick up the vehicle heading faster this way. I have no experience with articulated steer models and will make no recommendations there.