What are its effects?
The Sensor Jammer completely blocks any scanning done by enemy Sensor Sweeps and Motion Sensors that creates a “dead zone” area for nearby squadmates (50 meter radius).
Is it stackable with other Skills?
No. No other skill affects the Sensor Sweep. But Improved Stealth helps you stay hidden from the Motion Sensor.
Do you have to activate it for it to work?
You do not. The Jammer is a passive gear item and only needs to be present in your loadout inventory to work. You cannot turn it on or off.