And the Fenris and Leviathan? 
Well the Fenris and Leviathan are FS1 era ships - refer to my first post above for why they have radar dishes in-universe
Doesn't the Colossus have Radar Dishes too?
To be fair, the colossus did take 2 decades to complete. Its going to be a mix of the best tech available for something at the time, various other doo-hickies that seemed like a good idea at the time but now have a new compact hight tech macguffin to replace it, and sudden inspirations...
I'll be perfectly honest, i dont think the Colossus was ready during FS2. I think she was rushed into service due to the shivan presence in/beyond G-Draconis and the tide of the NTF war turning against the GTVA. Once completed, she would require a shakedown cruise, then a Refit to bring her completly up to modern standards, followed by another shakedown to ensure that she works properly.
-I dub this problem of launching a ship before its ready "Enterprise Refit Syndrome" (ERS) after the problems that the USS Enterprise and Enterprise-A suffered from when they were rushed into service - See ST:TMP and STV:TFF to see what i Mean