RD418HD mounting info

I have a RD418HD mounted directly to a hard top with no mount. My customer would like to mount a LED light bar in front of the dome. The question I have, what is the maximum height of equipment mounted in front of the dome without interfering with the radar beam. When I have opened earlier dome designs the array seamed to be in the top have of the dome leaving the lower half free to mount equipment i.e. light bars or horns. Is there a drawing available that shows this information.
The Light bar that he is looking at measures 4" high.

Thanks
George