Find the hidden signal in one of the Rat Man’s dens
This can be done in Chapter 2, Test Chamber 6 where you can also do the Drop Box Achievement/Trophy. You need to bring a radio into a secret room in this level.
The first time you press the switch, you will be getting a lots of garbage. There will be a radio included in the garbage when you press the switch for the first time. Before doing this you need to make sure you have set your portals right so you would be able to the radio.
You can either place your portals set directly to go to the button or place them on these white walls.
Once you press the switch for the first time watch the garbage (turrets, a CD, spheres, and a radio) just leap around and once they are coming right at you, get the radio. Then place a portal to the wall on your right and another portal on the wall in this picture.
Go through the portal, don’t forget to bring the radio! Then create another portal as seen in this picture.
Pick up the radio, get on the Aerial Faith Plate, enjoy the ride and you will find be in the hidden room in the end.
Doing this will earn you 20G points for the Xbox 360 and a Bronze trophy for the PlayStation 3.