These structures can usually be found in bases behind enemy lines. They prevent any enemies from using radio calls within a 5 block (175m) area. Use a brick of C4 or several shots from explosive weaponry to destroy these and free up your support radio. Destruction gives an instant XP and RP reward to a single player.
Unlike AA emplacements, Radio Jammers will respawn after several minutes. The only way to get them to permanently end their blockage is to capture the facility they are based at.