Why Won’t Cell Phones Work in an Elevator?

Nicholas Jones | May 24, 2018

 

 

This question about cell phones and elevators comes from our email in-box, where it shows up from time to time. I’ll keep this post non-technical, so for all you engineers – no discussion here of bound electrons and free electrons. But anyone who is interested and doesn’t know why they would matter in this discussion, Google them.

Before we discuss elevators we first need to talk about how cell signals work.

How cell signals work

Cell signals are radio frequency waves, and RF waves are a form of electromagnetic energy. As we’ve posted before, there are only two causes of poor cell reception: (1) distance from the nearest cell tower, and; (2) obstacles between you and the tower that block, absorb, deflect or diffuse the signals so they can’t reach your location, or they arrive in a greatly weakened state.

Among the obstacles in (2) above are pretty much anything made by man. The materials used to build virtually any structure can block or weaken RF waves as they attempt to pass through, and metal and concrete are particular offenders here. Metal, as we know, can interfere with electromagnetic energy, as can concrete – which typically contains some metallic elements.

And the bad news is other building materials like shingles, masonry, wood, drywall, even glass (especially the energy efficient type coated with metal-oxide) – can likewise weaken or block RF waves.

That’s why when you’re at home, in the workplace or inside almost any building you can encounter cell phone reception problems. Building materials, especially the exterior shell of the structure, make it difficult for the RF waves carrying your cell signals to get inside.

It’s one of the ironies of modern life – we’re typically inside, but the strong cell signal is usually outside. You’ve probably noticed that cell phone reception is almost always better as soon as you step outside a building.

OK now we’re ready to go back to our elevator question.

Why cell phones won’t work in an elevator

Any building with an elevator has multiple floors, so it probably employs concrete and steel in its construction, or maybe block or brick. Regardless, you have a barrier to good cell phone reception – the RF signals first have to pass through the building’s exterior walls to reach the inside space.

Then, when you take the elevator, you are stepping inside a steel box. Yep, that’s what an elevator is.

Ever heard of a Faraday cage? It’s a purpose-built structure with metal (usually copper) walls, floor and ceiling. RF engineers use a Faraday cage (named for Michael Faraday, the 19th century English scientist who studied electromagnetism) when testing cell phones and other electronic devices.

The Faraday cage is useful for this testing because its metal construction completely blocks RF waves from exiting or entering the cage. Wilson Electronics has a Faraday cage for testing purposes, and we set it up for a product demonstration at the big CES electronics show in Las Vegas a few years back. You can watch a video about that Faraday cage here.

 

An elevator is a Faraday cage

So for our discussion here, an elevator is essentially a Faraday cage. A metal box. No RF signals in or out. In addition, it’s located inside another steel-reinforced concrete space – the elevator shaft. Which in turn is located inside the multi-story building we mentioned above.

So when you think about it, when you’re inside the elevator you have at least three layers of concrete and/or steel between your phone and the cell signal source. That’s three layers of construction blocking and weakening the RF signal.

Pretty easy to understand at this point why the cell signal typically can’t reach your phone when you’re in an elevator. And the truth is, even if the tower signal could get to your phone inside the elevator, it would be virtually impossible for the puny transmitter on your phone to send its signal back to the cell tower to complete the communications loop. There are just too many layers of concrete and steel for your signal to pass through.

The only reliable way to get cell signal to a phone inside an elevator is to employ some kind of cell signal boosting device with antennas in the elevator. And since building codes don’t require such booster systems and antennas, we will rarely find reliable cell signal in an elevator.

So now you know why your cell phone won’t work inside an elevator. Another mystery of modern life solved.

 

Want to learn more about indoor cell boosting systems like those mentioned in this post? Just click the button below.