Tunnel question

When travelling through the airport tunnel, why does am radio come in either crystal clear (more clear than normal) or completely white noise?

Reflection.

AM waves bounce off rock and concrete. If they come in at the correct angle to the tunnel mouth, though, they enter the tunnel and bounce around inside it. Otherwsie they don't get in at all. I suppose the shape and size of the mouth has something to do with it, too.

Thanks Nareed. That's a good start to the explanation I'm looking for. Riddle me this: Why does it change from day to day? The angle that the waves are being shot out changes day to day? I've noticed the weather does not seem to have an impact.

Some tunnels are fitted so that you can receive AM Radio inside them. The Liberty Tunnels a/k/a Liberty Tubes in Pittsburgh once had this feature. Look for what looks like some pipes hanging from the ceiling and running the length of the tunnel.

Here is one article on the practice.

Another from 1941!

Quote: Mooseton

When travelling through the airport tunnel, why does am radio come in either crystal clear (more clear than normal) or completely white noise?

Normally, you would expect AM to vanish long before FM in a tunnel. I don't know the age of the tunnel you are discussing, but for emergency purposes they may have installed "a leaky transmission line" which is just a single length of wire or flashing running near the ceiling. They would power this with a broad-band amplifier.

My guess is over the years the requirement for reliable AM transmission in the tunnel has lessened since it is not as important as it once was. The amplifier may not be 100% reliable or in operation all the time. Hence the changes from trip to trip.

I don't think reflection is the cause because AM radio signals are very long (between 200 and 600 meters) which is many times larger than the mouth of the tunnel. Do you notice a difference depending on the station being tuned in (i.e. top or bottome of the dial)?

Quote: pacomartin

Do you notice a difference depending on the station being tuned in (i.e. top or bottome of the dial)?

I haven't noticed a difference. Next time I go through I'll try and remember to test the ends of the am stations. I usually stay on 720 or 840 so I wouldn't have noticed anyway. And to clear any confusion I'm asking about the tunnel under the airport tarmac in Las Vegas.

Quote: Mooseton

I haven't noticed a difference. Next time I go through I'll try and remember to test the ends of the am stations. I usually stay on 720 or 840 so I wouldn't have noticed anyway. And to clear any confusion I'm asking about the tunnel under the airport tarmac in Las Vegas.

When reading this thread, I was imagining that very tunnel in my head!

Quote: RS

When reading this thread, I was imagining that very tunnel in my head!

Well, I would have had posted this at DT if I wasn't. :)

AM wave transmission can be affected by weather condition

Oh I definitely believe that too. One reason I brought this question up is the fact that our weather here is pretty consistent from day to day. Never mind the weak storm in the area this week, of course.

Quote: Mooseton

When travelling through the airport tunnel, why does am radio come in either crystal clear (more clear than normal) or completely white noise?

The tunnel as previously mentioned has an antenna (pipe) in it. Otherwise the signal vanishes. There are matters of propagation that do factor, but generally, don't count on AM RX in a tunnel. FM with a very short wavelengtth can enter a tunnel as generally the height and width are greater than the FM wavelength (about 3-4 meters). In long tunnels the FM can fade. Along this line of reasoning, the AM wavelength (170-550 meters) does not "resonate in the tunnel, it needs an antenna to carry the signal inside. HTH

But why the change from day to day? Same tunnel, same lane, same time usually, same weather usually. I'm honestly not trolling intentionally; just being curious.

Must be a propagation issue. And its possible if the tunnel is long, say 1200 feet (300 meters) that in some spot(s) a weak resonance does occur. The electrical wiring for the lights in the tunnel can act as an antenna. A bad ground might cause occasional reception at various places. Lotta intangibles here and generally not-well-understood things happen. I like to think of it as 20 variables that occasionally add up to reception.

Quote: Mooseton

But why the change from day to day? Same tunnel, same lane, same time usually, same weather usually. I'm honestly not trolling intentionally; just being curious.

When you're in a weak reception area, little things can make a difference between usable signal and no signal. Are there cars/trucks next to you or are you mostly unsurrounded, are the overhead lights in the tunnel on or off, what's going on above your head as you travel through the tunnel (and you can't usually know this), etc. The last time I was in Vegas, my cell phone got sporadic reception from my room (facing away from the strip, up about 20 floors) and where I stood in the room actually changed the number of bars. Also, I just upgraded my home WiFi and the orientation of the router determines whether I get signal out on my patio. Radio waves can be finicky.

If my memory serves,. doesn't the airport connector tunnel also have tiles inside? Would that not help with the reflection of the radio waves?

I don't have any experience here just what I would imagine would happen?