Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EM wave penetrates?

  1. Apr 23, 2007 #1
    I'm rather new here.
    I've got a question that's bothering me, and therefore decided to ask here.

    Why is it that in a lift/elevator, radio signal usually gets weakened?
    And, as far as i know, FM radio operates on the radio wave band, as well as cell phones. But why is it that mobile phones generally have acceptable reception in lifts/elevator whereas FM radio reception remains poor? Don't they operate at the same wavelength/freq?

    Has this something to do with the energy level stated by E=hf, or something?
  2. jcsd
  3. Apr 23, 2007 #2
    It has nothing to do with E=hf.
    Maybe that the reason why cell phones are more tolerant that FM radio is that in the cell phones the signal is digitally coded. And as you know digital communication works well, even is the signal is degraded, until it is so degraded that it stops working. The situation is the same with others digital communications as digital TV (broadcast). When analog reception is poor digital TV is still excellent. A similar case is with vinyl records and CDs: even new vinyl's where noisy and old CDs are not.
    Last edited: Apr 23, 2007
  4. Apr 23, 2007 #3
    I believe that high frequency EM radiation penetrates much deeper into matter because it is much more energetic (E=hf). Wouldn't that explain why cell phones work better than FM radios? FM radios are operated at ~80-100 Mhz whereas cell phones are operated at ~1800 Mhz.
  5. Apr 23, 2007 #4
    AM Radio waves have longer wavelengths (on the order of meters) than FM Radio wavelengths. This is the reason why when you pass under a bridge while driving and your radio is tuned on the AM dial the signal gets cut off while on the FM dial it doesn't. (AM waves can't penetrate through the bridge).
  6. Apr 23, 2007 #5
    Cabins of lifts/elevators are usually metallic.
    The penetration in metals is less for higher frequencies than for lower ones. It is called skin effect. See:
  7. Apr 23, 2007 #6
    This is a somewhat different problem. Bridges and tunnels behave as (bad) waveguides. The maximum wavelength that can penetrate a waveguide is twice the width of the waveguide/bridge/tunnel.
  8. Apr 23, 2007 #7

    Claude Bile

    User Avatar
    Science Advisor

    Have you considered that the signal from a mobile phone tower may simply be stronger to begin with than an FM broadcasting tower potentially tens of km away? (Also, I don't really agree with you anecdotal evidence, my phone loses signal in lifts with frustrating regularity :rolleyes: ).

  9. Apr 24, 2007 #8


    User Avatar
    Homework Helper

    First of all as Claude Bile has said.

    1. Mobile phones use digital encoding. FM radio = analog. Digital encoding allows for signal error checking, error correction, and pretty much "perfect" transmission as long as there is enough bandwidth.

    2. No, mobile phones DO NOT operate on the same frequencies as FM or AM radio. FM radio goes from 88MHz to 108 MHz. There are three GSM bands I know of that are in common use: 900MHz, 1800MHz, and 1900MHz. Not close at all.

    3. An elevator acts as a Faraday cage, because an elevator is pretty much a cage of metal, which fits the bill for being an electromagnetic shield. If it were a perfect shield, you would get absolutely no signal in the elevator as there is theoretically no electromagnetic field inside.

    The last say as to whether a particular frequency band is better than another depends on too many factors: the operating reception range, the presence of materials that absorb certain EM frequencies, the geometry of the landscape, and particularly in digital signals the type of modulation used.

    P.S. I usually don't have problems with my phone on the lifts I use ;D
    Last edited: Apr 24, 2007
  10. Apr 24, 2007 #9
    An elevator acts as a very poor Faraday cage. It has all that is needed to be a Faraday cage: inside you are surrounded by metal sheets. In a lift, the sheets are not welded together and do not form an equipotential surface. There are isolating "slots" between sheets. Even if in DC or low frequency two neighbor sheets are connected, this is not true in high frequency.

    EM field induces differences of potential in the sheets. Even between connected sheets there can be differences of potential if the connection point is at a distance comparable to wavelength.

    Two sheets with a difference of potential act as an antenna in the inside of the cabin, and re-emit the signal received on the other side.

    As wavelengths used in cellular phones are shorter than those used in FM, the lift cabin behaves as a worse Faraday cage for cellular phones than for FM receivers.

    Of course a lot of things depend on the power of the emitters. But you should not forget that the emitter whose power is very limited is that of the cellular phone itself and which must reach the receiver of the cell.
    Last edited: Apr 24, 2007
  11. Apr 25, 2007 #10

    Claude Bile

    User Avatar
    Science Advisor

    Maybe it's time for me to buy a new phone then :wink: .

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook