Once you've built the Receiver, your first stop will be The Radio Tower. This is a rig on the ocean with a fair amount to explore. Anchor your Raft when you arrive and jump on board.
Cuckoo Note (Room B-1)
Your first stop will be Room B-1 where you'll find a note for your Journal. The note is written by a man named Cuckoo and talks about another character named Owl and a missing character named Sparrow.
Once you've read the note (or listened to it with the voiceover), you can explore the rest of the Radio Tower. There's loot to be found in each of the rooms, but one room, in particular, has some important items.
Head Light Blueprint and Owl's Radio Notes (Room C-1)
Climb up the boxes and debris to the second level. One of the rooms will have the Blueprint for the Head Light and Owl's Radio Notes on a wall. Owl's Radio Notes are a bit of fluff about radio transmissions she heard.
Thoroughly explore the area and you'll find various loot. Then, look around for a staircase going up. It will take you to the rooftop of the building.
Cuckoo's Second Note (Room D-1)
From the C1 rooftop, you can walk along a vine-covered red beam to a small building that says "Took Boat Won't Be Back." A vine-covered staircase is nearby -- jump onto it to get even higher up the Radio Tower.
The top of this next staircase will put you next to a wind turbine and an air duct. Another vine-covered air duct is a short jump away -- hop over to it and search the area outside of D1 for loot. Once you're done, head into D1 and you'll see another note from Cuckoo on a counter talking about a reactor at Selene.
Exit Room D1 and jump up on some boxes -- it's time to reach the top of the Radio Tower.
The Top of the Radio Tower and a New Friend (Room E-1)
Carefully look around and you'll see a fairly simple jumping puzzle. This will lead you to the top of the Radio Tower and your first new character unlock: Tala. This top room will also have the Blueprint for the Recycler. Finally, make sure to grab the note that says "People?" with an arrow.
The "People?" note gives you the Code for the next area you're heading to, but we're not leaving just yet.
Killing the Shark and Diving Deep
If you recall, the first level of the Radio Tower was labeled "B-1" and the upper level was labeled "E-1." That begs the question -- what happened to A-1? The answer is, of course, that A-1 is underwater.
Before you dive in, you're going to want to kill the Shark. You really don't want to be trapped in an underwater building while you're trying to fight this thing off. Once it's dead, head into A-1 and grab all of the delicious loot you can.
Inputting the Radio Tower Code Into Your Receiver
Your final task for the Radio Tower area is to input the four-digit Radio Code on the "People?" note into your Receiver. This will give you the coordinates of the next place you're supposed to go.
A note on Radio Codes: they are randomized in a playthrough. You can always find it in your Journal (which you can open by pressing "T"). Unfortunately, you won't be able to find a Radio Code without seeing it yourself firsthand.
Input your newly-acquired Radio Code and move on to the next island, Vasagatan!