Electrical engineers: Why don't cell phones work underground?

[Rakehellion]

Why don't I get reception when I'm on the subway?

Okay, so there's about twenty feet of concrete and steel to block the signals, I get that, but why do I get reception at certain stations? Also, the subway in Philadelphia gets cell phone reception all the way through. What's different there?

Another question: Say you're five stories underground. Theoretically, would it be possible to build some sort of structure that allows signals to pass through so you can access your wireless data?

 

[Cerpin Taxt]

C.H.U.D.s

 

[Childs]

Subways probably use something like a cell phone signal repeater or access point to extend the range underground.

 

[Red Squirrel]

Radio waves is a whole science on it's own. The shape of the tunnel, the items in/around it can all have various effects. The signal could even be bouncing off the track and hitting you right but only in certain situations etc.

Here's what I always wondered, wifi and microwave ovens use the same frequency. Why is it that my wifi still works from inside the microwave but the microwaves from the oven don't come out when it's turned on? Why is it that if I wrap my cell phone in foil paper or put it inside a completely enclosed metal box, ground it, it still works? Why can't my phone fly in air plane mode?

 

[Grooveriding]

Because the subway can be annoying enough without it also being filled with the ramblings of everyone on their cell. They are discussing putting repeaters in here in Toronto and there is a lot of pushback. I still use the subway to get to work fairly often because it's faster than driving most of time with the traffic and I'd rather not hear it filled with cell phone chatter.

NO CELL PHONES ON SUBWAYS PLEASE.

 

[Red Squirrel]

Now the fast paced suit wearing briefcase holding business nerds can try to make themselves look even more important, with their bluetooth earpiece and cell phone.

 

[mmntech]

There's a few different factors at play here which largely depends on frequencies and depth. Lower frequencies can penetrate the earth easier than higher ones, but there's still a limit. Cellphones work in the high UHF and low microwave bands.

If the subway station is sub-surface, a UHF signal might be able to penetrate through the entrance. Microwaves are largely line of sight. You won't get a signal at all in a deep tube station.

To combat this issue, some transit operators are installing repeaters at certain stations.

To answer the OP's second question, yes, sort of. You'd need to use a low density, non-conductive material. Wood is one such material. That's assuming the cell tower's transmission is omni-directional. You might get enough bounce to get a signal.

 

[EliteRetard]

Radio waves is a whole science on it's own. The shape of the tunnel, the items in/around it can all have various effects. The signal could even be bouncing off the track and hitting you right but only in certain situations etc.

Here's what I always wondered, wifi and microwave ovens use the same frequency. Why is it that my wifi still works from inside the microwave but the microwaves from the oven don't come out when it's turned on? Why is it that if I wrap my cell phone in foil paper or put it inside a completely enclosed metal box, ground it, it still works? Why can't my phone fly in air plane mode?

What? You obviously got a defective unit...simply return it under warranty.

 

[EliteRetard]

My computer is running like shit right now...making it hard to use the internet.

 

[Blackjack200]

Theoretically, would it be possible to build some sort of structure that allows signals to pass through so you can access your wireless data?

Yeah, I guess you could tunnel down and open up a path for the signals to get through. Or, you know, you could just run a fucking cable.

It's like if you can't get wireless signal in a room in your house, you can either run some ethernet to it, or you can chop a hole in the wall.

Because the subway can be annoying enough without it also being filled with the ramblings of everyone on their cell. They are discussing putting repeaters in here in Toronto and there is a lot of pushback. I still use the subway to get to work fairly often because it's faster than driving most of time with the traffic and I'd rather not hear it filled with cell phone chatter.

NO CELL PHONES ON SUBWAYS PLEASE.

I don't know about Toronto, but in the NYC area, no-one really talks on their phone on the train, even when it's above ground. There are exceptions, and I've done it myself when it was urgent, but it's nice to be able to text and use wireless internet as opposed to just sitting there until you're above ground again.

 

[TecHNooB]

I hear some stations put a big cable through the tunnel. This makes the tunnel look like a coax cable, which can propagate waves straight down the tunnel instead of bouncing around and incurring losses like in a parallel plate waveguide.

 

[SlitheryDee]

Isn't this a similar question to "Why doesn't my 5g network reach the other side of my house"? Too far away from the signal source with too many things between you and it.

 

[z1ggy]

Radio waves is a whole science on it's own. The shape of the tunnel, the items in/around it can all have various effects. The signal could even be bouncing off the track and hitting you right but only in certain situations etc.

Here's what I always wondered, wifi and microwave ovens use the same frequency. Why is it that my wifi still works from inside the microwave but the microwaves from the oven don't come out when it's turned on? Why is it that if I wrap my cell phone in foil paper or put it inside a completely enclosed metal box, ground it, it still works? Why can't my phone fly in air plane mode?

There is a Faraday cage inside your microwave that blocks the microwave signals from exiting the device.

I also hear that if you put your phone on airplane mode, then throw it out the window, it does fly for a little while

 

[Scarpozzi]

I can't even get a signal inside most buildings around here. There's just too much material between you and the tower for the signal to penetrate without interference. It's like tuning an FM radio when the station is 100 miles away...

One of the datacenters I worked in was underground. We had a repeater. It wasn't great, but at least worked with 3G service to get some speed on data transfers.

 

[DrPizza]

I also hear that if you put your phone on airplane mode, then throw it out the window, it does fly for a little while

But won't the other passengers on the flight get upset if you open the window?