Final Transmission Achievement
Find the hidden signal in one of the Rat Man's dens
This can be gained in Chapter 2 - Test Chamber 6.
When you make your way across to the button, press the button and GLaDOS will dump some garbage instead of a cube. The Garbage will come towards you (as long as you keep your two portals on the angled platforms). Stand in front of the garbage so it hits you. Shoot a portal to where you started and grab the Radio. Now look at where the Angle platforms are where you need to shoot the portals to get to the button you. If you look up will see that you can shoot your portal on the wall to the left of the angled platforms. Shoot a portal as far as you can to the right. Now shoot another portal on the Angled Platform CLOSEST to you. Use the Aerial Faith plates and you will be launched into a secret room. Go to the desk and place the radio on the desk. Stay in that room for a bit and the achievement will unlock.
You can see where the Secret room is if you stand beside the button and look at the angled platforms. Look to the right and up a little and you will see a broken window which leads to the secret room.