I am making a wheel encoder out of a Hall Effect sensor with magnets in a concentric circle on the wheel. The Hall effect sensor is on a 2-layer 9x5mm pcb. I usually fill in the layers by drawing a ground shape and then deleting the islands. My question is: should I not add the ground shape because the magnets will induce currents in it- making my circuit unstable? or to put it another way, with magnets passing the pcb, is the less copper the better?