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Cell phone works in elevator?!

by physwizard
Tags: cell, elevator, phone
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Apr11-13, 08:46 AM
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Very often the principles of faraday cage are extended to non electrostatic situations, i.e. situations involving time varying electric and magnetic fields. This means that the interiors of a metallic shell should be impervious to external time varying electric and magnetic fields.
However, I have found quite a few times that my cell phone has worked while in a metallic elevator, even though the signal strength becomes low. Can anyone explain this?
(When I wrap it in an aluminium foil however, I find that it is not able to receive calls.)
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Apr11-13, 08:56 AM
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My guess would be that it is not a perfect Faraday cage. If you look at this picture of a research centre on electricity located in Quebec, that big brown square building in the background was built as a Faraday cage, in order to conduct experiments in a controlled environment. Even though, there is one FM radio station that you can still listen to when inside.
Apr11-13, 01:11 PM
P: 153
Thank you for your response. I am really just trying to understand the general phenomenon.
Incidently, my cell phone works well if I put it in my microwave. Now my microwave door has a mesh with spacing of about 2 mm. Obviously, this doesn't shield out all EM waves as you can see the light inside the microwave. I've not really worked out all the math, but from various internet sources it appears that for a mesh to be effective as a faraday cage : mesh spacing << wavelength of waves it is trying to shield. 3 x 10^8/2 x 10-3 = 1.5 x 10^11 = 150 Ghz. So frequency of EM wave << 150 GHz for the wave to be effectively shielded. From wikipedia cell phones operate at roughly in the range of 800 MHz to 2.7Ghz so in theory at least the microwave body should have been able to shield the cell phone. Now either it means there is a flaw in my microwave (and a possible safety hazard) or there is something I am not understanding here. Incidently, the signal strength does go down from 4 bars to two bars when I put it in the microwave, so something is happening. However, the phone still works.
Also I have observed that sometimes in the interior of some buildings the cell phone doesnt work - absolutely no signal. I wonder why this happens? Is the building material capable of acting as a faraday cage?
Unfortunately, it appears that grad school trains us how to perform difficult calculations but doesn't really equip us to answer real life physics questions.

Apr11-13, 01:29 PM
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Cell phone works in elevator?!

RF is typically line of site however it can bounce off surfaces. If your cell phone has a signal above -90 dbm roughly it will function. Inside buildings typically the signal can penetrate as its mainly lumber and drywall. concrete and metal buildings pose a different problem in that RF doesn't penetrate those materials well. However RF signals can use surfaces to bounce and find connection paths that way. I've had to trace radio signal pathways to locate radio interferance. I used a uni-directional antenna or Yagi antenna, and a spectrum analyzer as I swept the directions buildings often confused the findings. Eventually by following the signal strength I was able to locate the signal source.

Another factor to consider is location of the cell phone towers, how close are the towers are you conducting your tests? How much overlap from other towers is there? Each cell phone company such as Koodo, Bell etc have their own towers or license usage from another comapnies tower.
The frequencies used by cell phones is not a dedicated frequency per phone rather it hunts for an available carrier wavelength each time it transmits. Its also not a sinusoidal wave in cell phones rather its a compression format called FSK or frequency shift key. which is a carrier wave being shifted to add more data per wave.
Just some things to consider for your testing of your phone

edit -90 dbm is 7.07 microvolts so it doesn't take a strong signal to work
Apr11-13, 02:04 PM
Mordred's Avatar
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Electromagnetic Field Penetration
The penetration depth of RF energy depends on the electromagnetic wave
frequency, the permeability of the tissue and the tissue's conductivity.

the depth of penetration in millimeters is inversely proportional to the square root of the frequency. Hence, lower frequencies have a higher penetration.

If you think about this and consider your microwave shielding is developed for microwave frequencies and not RF frequencies, it should be obvious why your phone will still work

also higher frequencies travel further than lower frequencies through the open air
Apr11-13, 05:39 PM
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Quote Quote by physwizard View Post
However, I have found quite a few times that my cell phone has worked while in a metallic elevator, even though the signal strength becomes low. Can anyone explain this?
An elevator isn't a very good Faraday cage. The frame is probably made of metal, but it's doubtful the entire elevator is really a metal box. While the box has to be structurally sound, lighter is better, since something will have to support the box, counterweights, and all the other equipment associated with the elevator.

The signal attenuation in elevators is actually a problem nowadays. You'd want devices accessing a wireless network to continue working (a hospital, for example). You can get around the problem by providing access points inside the elevator shaft itself (so at least you're closer to the source), but how much a particular elevator attenuates radio signals is probably at least considered when installing elevators in new buildings.

I wonder if there's much of a difference between old elevators and new elevators? (You'd think some attenuation problems would accidentally be corrected just by trying to keep the weight down.)

A common frequency for microwave ovens is 2.45 GHz. The mesh should filter out a lot of your cell phone frequencies. I think quite a few 4G phones would work, since they operate just above your microwave oven frequencies. (In other words, I'm not sure how precise your measurements were for the mesh.)
Apr11-13, 05:54 PM
Mordred's Avatar
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A lot of companies including the hospital in my area have installed cell phone boosters for the reasons you just described with elevators. Some cell phones use as low as 900 mhz and so far I've yet to work on one that goes above 1.6 ghz, nor have I seen any boosters of a higher range.
However that may also be due to location as each area typically gets assigned a set of licensed frequencies per area. The RF regulations in the US is further ahead than in Canada

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