It's not just an antenna, it's also a receiver and has a bunch of electrical things going on. You need to check with a handheld compass and the GPS turned on to be sure. (as per the manual)
I would be working on the 2' rule of thumb at least. You may get away with less but why risk it. They do emit noise that can effect VHF coms, so avoid long runs of coax and the GPS cables together.