I just noticed the transmission jammer descriptions say visible hostiles:
Quote
Blocks local transmissions from visible hostiles within a range of 8, making it impossible for them to share information about your current position. Also prevents calls for reinforcements, and suppresses alarm traps.
Do they not prevent transmissions from hostiles outside your field of view even if they are within the radius?
Correct, they perform as described. You can also see the exact effect area in the animation when you toggle it.