The radar jammer is best used in conjunction with the Active Camouflage power-up. When it is deployed, the enemy will be distracted enough that the player may be able to score a quick melee kill, or an assassination.
This type of equipment is both useful and detrimental to the user, allies, and opponents, since everyone's motion trackers are affected. The radar jammer should only be used if the player is in a position where he or she does not need to use their motion tracker to score a kill.
Watch out for opponents throwing radar jammers at you. If this happens, nothing annoys them more than shooting it immediately and making it explode in their faces. This is also useful in the Campaign against Brutes. If shot soon enough after having been deployed, it can damage or even kill the Brute dumb enough to deploy it.
In CTF, using this together with the Flare can be useful. Have one player throw in the radar jammer, then have another throw in a Flare while looking in the direction of the Flag so you know which way to go. By the time your opponents can see again, they will notice their flag is gone. If flares are unavailable, use an Active Camouflage instead.
Make sure you have your enemy in sight before you deploy the radar jammer; if you do not, the radar jammer will work against you as well, leaving you vulnerable.
It is especially useful when you are the sniper on your team. If you need to switch positions, just drop it and run.