system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you lose your radio signal(FM) when you've driven a fair distance into a tunnel, but you don't regain it until you're completely out of it?

Title.
gpt
It depends on the direction the FM signal is coming from. Think of it like a beam of light (actually, radio signals technically *are* light!) where solid structures casts shadows.

If the light is shining towards the entrance of a tunnel, then the inside of the tunnel is still lit up near the entrance. At the exit, the shadow of the mountain or building still blocks light past the tunnel exit. That is why different places have different signal strengths.

It depends on where the FM transmitter is and where the blocking objects are.

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**EDIT:**  
This is one explanation, but if you find that it doesn't matter which side of the tunnel you enter/exit, then this next explanation may be what's happening...

If you think of the FM signal as having a particular address (the frequency), then you can only read the signal if you know exactly where it is, and you actually have to know much more accurately than the number you put into the radio. Luckily, your FM radio can find the exact address once you give it the ballpark number and is able to stay locked onto that signal.

The hard part is finding it in the first place, since once you see it, you can follow it even if the signal becomes faint. However, your FM radio can't find it if it's too faint to begin with. So on both sides of a tunnel, the signal is weaker, but you can still hear it as you go in since you've locked on to it. When you come out, your radio needs to wait for the signal to be bright enough to pinpoint and lock on to it again.

Other answers explain this in more detail: it's called a phase-locked loop.