The plate things are junk, all the cop has to do is POINT THE LASER AT A DIFFERENT PART OF YOUR CAR! Also, thanks to the fact that the laser beam spreads, at 700 feet (the range that most cops train to use the gun at), the laser is wider than a license plate.
Active jammers are not just illegal in Ohio, they fall under FCC regulations concerning licensing of RF and microwave transmitters. You get caught with one, you could get a $10,000 fine and do time in the federal pen.