When thinking about how the WiFi signal propagates through a household, can I use the following thought experiment?

Assume absolute darkness. Place a strong lightbulb where the WiFi access point is. The illumination that reaches various places in the house is approximately proportional to the strength of the WiFi signal in that place.

How precise is this mental image? I know that the radio waves can penetrate some objects / walls that the light cannot. Is this at least somewhat representative?

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    Since you know that walls stop light but not WiFi (they do attenuate it), you have already shown that your image is not correct. – hdhondt May 18 '16 at 11:32
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    I know it's not precise - I would like to get an estimate on how incorrect it is. – vektor May 18 '16 at 11:34
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    Technically, radio waves and visible light are just different frequencies of the same thing: electromagnetic radiation transmitted by photons. – Wayne Conrad May 18 '16 at 13:18
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    @Wayne Conrad: Exactly -I think a more accurate model would be if the walls were transparent. The wifi signal like light would propagate uniformly in all directions and decrease in intensity by the square of unit distance. Depending on the walls are actually made of for example -wood is rf transparent, concrete with rebars would have varying degrees of transparency depending on rebar spacing. – resident_heretic May 19 '16 at 5:17
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    As for the propagation, without obstacles like walls &c., WiFi and light behave in absolutely the same way. Both propagate at the speed of light and both present power at a given location proportional to 1/r². Walls &c. are "partially transparent" to WiFi, like tinted glass is for light. The differences show in aspects where the wavelength matters, i.e. refraction and diffraction, in which the angle of redirection is different and hence, WiFi does not propagate in exactly the same direction as light would after the obstacle. – JimmyB May 19 '16 at 9:52
up vote 41 down vote accepted

It's more like the walls were semi-transparent glass, if you want to imagine it as light (and even then, you neglect diffraction effects). It would actually be better to imagine it as sound!

But this seems to be exactly what you're looking for:

http://arstechnica.com/gadgets/2014/08/mapping-wi-fi-dead-zones-with-physics-and-gifs/

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    That article doesn't reflect the updated simulations where the walls absorb some of the WiFi signal. If you go to the original source, you'll find this image for the absorbing case, where the absorption mostly eliminates the standing wave pattern and the signal strength behaves much more like you'd intuitively expect. – user2357112 May 18 '16 at 20:49
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    This image is simulated, not measured (!). Not to detract from his work, but he has posted three quite different versions of this image, accounting for different effects (absorbtion, etc). Without experimental verification, the most I get from this is: 'wifi obeys wave propagation'. – anon01 May 20 '16 at 1:02
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    We know that it's correct up to the effects included in the simulation. As we're talking about how to imagine wi-fi propagation, this is spot on: I assume OP already knows it obeys wave propagation, what's needed is a visual example with different effects taken into account, to create a mental image. In reality, walls have different properties anyway, depending on the materials used. Any example which successfully depicts diffraction, reflection, (absorption) and falloff with distance, would do the trick here. – orion May 20 '16 at 5:38
  • Thanks @orion for a great answer - accepted, but I decided to give a bounty to p_h to offset the upvotes his (hers?) answer deserves :) – vektor May 23 '16 at 7:14

It's hard to access how 'accurate' an analogy is (i.e. how is this being quantified?). But, I think, there is a simple - better analogy:

WiFi is more like sound in a house. The transmitter is a speaker. If its a good, loud speaker, you will still easily be able to hear it in the next room - through a wall. A few walls inbetween and it gets very faint. Depending on the materials in the walls, opening and/or closing doors may or may not make much of a difference. And there may be odd corners or directions where the sound gets louder, or gets extra soft.

  • Well its not like sound in that a metallic mirror on the wall will cast a shadow where you will get very weak signal. It doesn't flow round the edges. To me its more like ripples on a pool but in 3d. – JamesRyan May 18 '16 at 12:51
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    It's somehow ironic that the best analogy out of "light and sound" is the one that best matches observable properties but not the one that's literally the same mechanism. – Lightness Races in Orbit May 19 '16 at 11:44

I think your mental picture is pretty close to accurate, as long as you bear a few things in mind:

First, the wavelength of the wireless signals are much longer than visible light. At 2.4GHz, the wavelength is 12.5cm. Just imagine that the waves are about half a foot long (if you have 5GHz wireless, the waves are half as long). So you can get some phenomenon you don't really see with normal light but you've probably perceived with sound; dead zones where the signal is really weak or totally nonexistent.

Second, the opaqueness/transparentness of a material is dependent on the wavelength of the EM waves. The common dielectric structural elements of the house (drywall, furniture, etc) would bend the light to varying degrees (as a function of dielectric constant), as well as absorb/scatter the "light". Steel ibeams and any other metal elements would appear like really shiny surfaces, especially if they are significant fractions of the wavelength; these objects would cast "shadows" and create a lot of scattering and dead zones. A half-foot long wire matching the polarization of your wireless antenna would reflect brilliantly, but some small staples scattered on your desk would only scatter the light a bit.

So imagine a semi-transparent house with all the metal parts being jet-black until any incident radiation hits it, with most of the light bouncing off (less shiny if it's full of iron, more shiny if it's gold/silver). Then imagine pockets of extra brightness and extra darkness as a result of the interference patterns that will inevitably appear. If you switch out your handy 2.4Ghz "lightbulb" isotropic antenna with your 5GHz one, the interference patterns will be different sizes/locations. You'd also have to adjust the way you see everything; the transparent-ness of everything in the house will be different at a different frequency, with some things getting hazier and other things being more clear.

Hardware hacker CNLohr did a nice time lapse collection of signal strength mapping a 4 foot square in his house, and then a 3d cube with the help of a CNC router table. I saw it on hackaday, his project is here: https://hackaday.io/project/4329-wifi-power-mapping

And he links to a cool video here: https://www.youtube.com/watch?v=aqqEYz38ens

It pretty much verifies the arstechnica map from @orion, giving proof that sometimes your notebook has great reception on one part of a desk but move it an inch and the wifi signal can mysteriously vanish. It's like a waveform but bounces in two planes in 3d space causing good spots where the signal can get to the antenna and others where the signals are absorbed or reflected out of the way back to the receiver.

protected by Qmechanic May 20 '16 at 8:55

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