You don't put them where a gren could hit them, of course. The vulnerability is one of the tradeoffs.
Or, just spend 600 res, which is less than a decent tank. If you put this in games are going to depend on which team drops 20 radars at the start to get a massive lead on research, and then it's going to depend on which team can blow up enemy radars faster than their engies can replace them, so the whole game is going to be about farming points by blowing up COMPLETELY POINTLESS FUCKING RADARS...
If you let a grenadier get to your own dumb radar cluster, it's your fault. Having 600 res gives teams a lot of freedom. They could get a head start on research, they could build a new, expensive barracks or buy a decent tank. It's an interesting gambit as much as APC spawning.
if you surround the enemy comm with the 20 radar, it's extra win cause now you always know where he is!