The cheapest countermeasure is just cheap decoys. If the enemy open up on a box placed on a forsaken hill somewhere then they are now exposing THEIR positions.
You could also have a droppable radio repeater in a box. With today's spread-spectrum and encryption technology, no way for OPFOR to tell if a particular box is a repeater or a decoy. Drop 25 of those suckers around a potential op zone, OPFOR can't run around policing up radio boxes and prep for a likely drone strike.