The FAA has an Advisory Circular regarding distance to stay away from a radar disk. It is the greater distance of 1.) where the radar beam forms away from the antenna and 2.) Where the radiation level reaches 10 mW per square cm.
Honeywell publishes a safe distance of 14 ft away from the center of the disc for their state of the art RDR-4000 series radar, to comply with this circular AC20-68B.
Even in test mode a distance of 3.5 ft is required.