Once you´re inside his active radar envelope, you´re lighted up lika the proverbial christmastree
You can sneak in, if your ships are smaller than the resolution of his radar
Maximum Sensor Range x (Target Ship Size / Sensor Resolution)^2
Say, your flying 5.000t DDGs (size 100), and your enemy has a radar with a range of 90 mkm which is optimized to spot 10.000t CRs (size 200). He will only spot you at a range of 22.5 million klicks.
This is, what FACs makes actually so much fun. Most of your enemies won´t spot ´em, till the missiles start hitting (of course, this also applies to YOU when the enemy is fielding FACs and against a human/realisticly played NPR should at most work once or twice)