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In my sophomore year of high school, my P.E. teachers kept on complaining about how phones didn't have a network connection in our gym, regardless of model, service provider, etc. A couple of feet outside the gym, cellular reception was crystal clear, however, as you moved your phone towards the gym wall, it rapidly diminished in strength. A couple of months after taking AP Physics C, my mind randomly drifted back to this event, and it came to me that this is what one would expect to happen if the gym were a giant Faraday cage. Is this even possible or likely considering usage of fairly standard building materials and structural design? Here's an image of the building (it's the one in the foreground) if that helps.

Here's an image of the building (it's the one in the foreground) if that helps

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    $\begingroup$ One can build a rather good Faraday cage out of chicken wire, if needed. I have seen several of these in research labs that are building low noise electronics (e.g. for high energy physics). Could a building be built with standard materials and greatly reduce signal strength for cell reception? Absolutely, although most of the time the problem is probably not with the buildings but with the local cellular provider's network. $\endgroup$ – CuriousOne Oct 31 '15 at 20:36
  • $\begingroup$ Just as an example for a building that is a true Faraday cage. The image shows an atomic clock inside a facility of the PTB, whose walls are completely shielded with copper. de.wikipedia.org/wiki/Atomuhr#/media/File:Atomuhr-CS2.jpg $\endgroup$ – magnetometer Nov 1 '15 at 10:04
  • $\begingroup$ Another example of building that has to be (partial) Faraday cage: any building housing synchrocyclotron - otherwise the building would prevent anyone near from listening to the terrestial radio... $\endgroup$ – Jakub Narębski Nov 1 '15 at 19:38
  • $\begingroup$ Yes, it is definitely possible. Whatever country you live in, you probably have some sort of national intelligence collection department/ministry/agency/etc - guaranteed that among their various assets will be a secure facility that is electromagnetically shielded exactly like a Faraday cage, precisely for the purpose of keeping any transmissions from getting in or out. $\endgroup$ – J... Nov 2 '15 at 1:15
  • $\begingroup$ The wire mesh that's being discussed, especially that used to reinforce windows... doesn't it have to be grounded to function as a Faraday Cage? $\endgroup$ – Tony Ennis Nov 2 '15 at 2:56
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In general the answer is "yes it is possible" - but in your case the answer is "that is not a Faraday cage".

Radio waves are (partially) reflected by any discontinuity in dielectric constant of the medium they propagate through. The ones that propagate (through walls etc) will also experience attenuation.

A faraday cage is a continuous conducting structure with no openings that are "large compared to the wavelengths of interest". Your building has windows that are much larger than that. The wavelength of a cell phone signal (typical frequency 1800 or 1900 MHz so around 15 cm) is small compared to windows and signal would penetrate - meaning that it is not a Faraday cage.

On the other hand walls do provide significant attenuation depending on the material - and waves that have to diffract through the window would also be much weaker when they got to you. If the gym was sufficiently far from the nearest cell tower it is easy to get a "dead spot" in reception.

For reference, according to this link a concrete wall provides 10 to 15 dB of attenuation - which may be enough to drop the signal from "OK" to "not OK", depending on the signal strength outside.

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    $\begingroup$ I see. Would the interference of the reflected waves with the incoming waves account for the decrease in reception near the walls outside walls of the building? $\endgroup$ – lorentzfactor Oct 31 '15 at 21:29
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    $\begingroup$ There is stucco wire and expanded metal lath. $\endgroup$ – Keith McClary Oct 31 '15 at 21:30
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    $\begingroup$ Interference of reflected wave will result in standing waves near the wall and possible dead spots - but these would be only a few cm wide (peak to peak distance would be half a wavelength or 7-8 cm) $\endgroup$ – Floris Oct 31 '15 at 22:26
  • $\begingroup$ So the corollary from your answer seems to be that if the link required only signal transmission from the phone, this would probably work OK (since the walls don't absorb much EM radiation, it eventually escapes through windows). In reality the signal must also be received, and it has to enter relatively small windows to be received by the phone. Because of small size of the windows, the signal mostly reflects away, so its amplitude inside the building is small, thus the link doesn't work. Is it right? $\endgroup$ – Ruslan Nov 1 '15 at 8:24
  • $\begingroup$ @Ruslan not quite. The signal attenuation is the same in both directions: this is a fundamental theorem called reciprocity - a result of symmetry in Nature. This is why a cell phone can look at the strength of the received signal to determine how much power to use for transmission (which in turn is why batterers drain more quickly when you are in an area with poor signal) $\endgroup$ – Floris Nov 1 '15 at 15:18
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Just to add to what Floris has said. It is frequent (in the UK) that institutional settings would have toughened glass in windows, particularly in bathrooms, gyms etc. that would have the form of a wire mesh (of order 1cm grid) embedded in the glass. That would do a particularly good job of blocking phone signals that would otherwise penetrate the glass.

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    $\begingroup$ Good point. I believe this is the case for the windows on my school's doors, but I don't think that it's true of the big windows on the gym. I'll be sure to check when I see it again, though. $\endgroup$ – lorentzfactor Oct 31 '15 at 21:24
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    $\begingroup$ Good point Rob. Such wire stops a window from shattering (with fragments flying everywhere) if impacted with e.g. a ball (quite likely in a gym). The holes in the mesh are typically on the order of a cm - and as such would provide a better shielding. $\endgroup$ – Floris Oct 31 '15 at 22:24
  • $\begingroup$ Are those meshes usually grounded, though? If not, then they can only shield wavelengths much smaller than the window. So, yeah, probably works for mobile phones, but wouldn't really qualify as a Faraday cage. $\endgroup$ – leftaroundabout Nov 1 '15 at 9:38
  • $\begingroup$ @leftaroundabout Indeed if the mesh is smaller than the wavelength (which is the case here I think), then they reflect, or attenuate, the microwaves efficiently. No grounding is required. $\endgroup$ – Rob Jeffries Nov 1 '15 at 10:44
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    $\begingroup$ Not only the windows, but using a wire mesh as an reinforcing inner curtain is a common technique in plaster and concrete block work -- both of which are rather common in the school gyms I grew up seeing in Texas. I don't know if the buildings I saw were rod, mesh, wrap or combination reinforced -- but there is a strong possibility that the blockwork I saw in gyms would look like a relatively fine mesh web of metal if everything else were removed. @leftaroundabout In this case, the mesh would be interlinked and almost certainly grounded, though only incidentally. $\endgroup$ – zxq9 Nov 1 '15 at 12:07

protected by ACuriousMind May 8 '17 at 19:43

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