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Not to sound all tin-foil-hat-like but I'm learning about wireless signals and am trying to understand it better. So my question is:

When multiple wireless signals are transmitted at the same time on different frequencies, they end up getting superimposed or combined together to create the field that travels through the air. So if I had two signals at different frequencies whose amplitudes varied between 1 and 0, the combined signal in the air could have an amplitude greater than any signal by itself (In this case, the signal in the air could have an amplitude greater than 1).

Wouldn't that mean if you had a lot of normally harmless wireless devices (say cell phones or wifi routers) in a given area that you could create a region that is no longer safe since each small harmless signal has combined to create a higher powered dangerous one?



Update to add more context:

Outside of even standing waves, if I look at an image like the one linked below, I see points where the red, blue, and green waves are all positive and close to their peaks creating a combined amplitude of around 2.5 times that of any wave by itself. While I can certainly see some waves canceling out because they are negative when another is positive, with the hundreds or thousands of wireless devices saturating an area, wouldn't we still have some decent amount summing to create a much higher level of energy transmission? Or is it simply the power transmitted by these devices is just so low that even with say a thousand devices summing together, it's irrelevant?

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    \$\begingroup\$ If you buy into the idea that there is a hard "threshold of danger" then yes. But it's been well over a hundred years with no solid evidence for one - even in the days when transmitters used spark gaps... \$\endgroup\$ – Brian Drummond Jan 7 '16 at 22:31
  • \$\begingroup\$ As @BrianDrummond says its not impossible but in order to have an infinitly building wave you would have to have enough wireless devices that have the perfect frequency shift such that NONE of the wireless devices cancels out another one. \$\endgroup\$ – R. Johnson Jan 7 '16 at 22:38
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    \$\begingroup\$ I should add ... no solid evidence, other than the effect of nuclear EMP on electronics. You can even be fairly close to a lightning strike, which generates extremely high amplitude RF pulses, with no ill effect. (Directly in the path is another matter! :-) \$\endgroup\$ – Brian Drummond Jan 7 '16 at 22:47
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    \$\begingroup\$ The field strength falls as you move away from a transmitter, following the inverse square of the distance. This means that as you approach a transmitter the field strength becomes much greater the the combined field from many relatively distant ones. If you ever come across a dangerously high field strength it is likely to be due to close proximity to one high powered transmitter. \$\endgroup\$ – user1582568 Jan 7 '16 at 23:17
  • \$\begingroup\$ hhmm but there must be a threshold of danger no? I mean there must be a point where your body's own electric field is being distorted due to the influence of another electric field to the point where your heart doesn't beat properly, your cells incorrectly function. etc.. \$\endgroup\$ – John Jan 8 '16 at 0:40
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This is what happens when you have a standing wave combining with itself in a microwave to produce a "hot spot". However, that requires:

  • very powerful transmitter (800W microwave vs 2W cell phone or 0.25W wifi)
  • confined spaces into which all that power goes
  • stationary transmitter and reflectors
  • continuous transmission (both phones and wifi transmit in bursts)

So if you take a bunch of low power devices which deliberately try not to transmit at the same time (like cell phones and wifi), and put them in a not-radio-tight space where they're not exactly stationary, then you're never going to get a powerful constructive interference spot. I don't think you could make it happen even deliberately with micro-metre precision positioning of the devices.

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  • \$\begingroup\$ Outside of even standing waves, if I look at an image like the one linked below, I see points where the red, blue, and green waves are all positive and close to their peaks creating a combined amplitude of around 2.5 times that of any wave by itself. While I can certainly see some waves canceling out because they are negative when another is positive, with the hundreds or thousands of wireless devices saturating an area, wouldn't we still have some decent amount summing to create a much higher level of energy transmission? \$\endgroup\$ – John Jan 8 '16 at 0:44
  • \$\begingroup\$ Or is it simply the power transmitted by these devices is just so low that even with say a thousand devices summing together, it's irrelevant? link \$\endgroup\$ – John Jan 8 '16 at 0:44
  • \$\begingroup\$ It's the combination of low power + inverse square law + intermittent transmission. Even if you have a thousand devices, the physical distance between them reduces the signal. And both cell phones and wifi transmit only for very short times in de-synchronised bursts, so they wouldn't all transmit at once unless you somehow forced them to. \$\endgroup\$ – pjc50 Jan 8 '16 at 9:31

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