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I'm interested in the estimation of the distance an electromagnetic signal can be transmitted. Background is that I would like to understand how materials absorb wifi-signals. For example, the most common bands are 2.4 GHz and 5 GHz and what I know is that the 5 GHz band is more suitable because more receiver and transmitters can participate but 5 GHz is not as far as 2.4 GHz. Hence, I've read that, for example, Intel has a technology with a 60 GHz and it allows only a stable distance within about 10 meters.

For sure I'm aware of the different interaction processes of photons (compton, rayleigh and so on) but I cannot apply it to the above situation. All I notice is that the only dependent parameter seems to be frequency which is connected to energy but this can't be it alone. Or?

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All electromagnetic waves propagate through 3-dimensional space with an intensity that scales as 1/r^2 where r is the distance between source and receiver. This is true regardless of frequency, in the absence of matter. This means that the intensity of the transmitted signal never goes all the way to zero, even as the distance through which it travels goes to infinity.

But because the universe is filled with radio noise of varying frequencies traveling in random directions, there is a background noise floor into which a weak radio wave will eventually disappear and become unreadable. A similar noise floor also exists within the components of the receiver, and therefore the received radio signal must be above this noise floor as well, if it is to survive passage through the receiver's circuitry.

This means that the radio signal must be transmitted with sufficient power to remain far enough above the system noise floor to be unambiguously detected at its intended destination. If we wish to minimize the amount of power required to accomplish this, it becomes necessary to choose a transmitting frequency at which the power content or "height" of the noise floor is minimized. In general, this means going to higher frequencies.

In addition, if we wish to have the radio transmitter and receiver conveniently small in size, the dimensions of the antenna must also be minimized. Since the required size of the antenna for best performance is directly proportional to the wavelength of the radio signal, higher frequencies are favored because they support the use of conveniently small antennas.

The amount of intelligible information that a radio signal must carry per second is limited by the bandwidth which that signal is allowed to occupy at a given transmitting frequency in the radio spectrum. High information rates require broad bandwidth, which reduces the number of transmitters that can populate a given slice of the radio spectrum. The bandwidth required to support a given information rate goes down as the transmitting frequency goes up which means that high frequency signals are used for high bit rate applications.

All of this means that wifi is done at frequencies in the gigahertz range, but this also means that any metal object of similar size to the antenna in the vicinity of the transmitter will itself behave as an antenna, and will reflect, refract, or absorb radio signals of that wavelength, and thereby interfere with the establishment of a useable link between the receiver and the transmitter.

Furthermore, at those extremely high frequencies, incident radio waves experience absorption via dielectric losses in materials like water, which means for example that the water content of materials through which the radio signal must penetrate will affect the strength of the received signal. This is the operating principle of the microwave oven, where the absorption of the radio wave's energy content is used to heat food.

Now it so happens that the ~2.4GHz frequency band used in cell phone and some wifi applications is the same as that used in many microwave ovens and the reason you don't get cooked by your wireless server or your cell phone is that the strength of the wifi signals is many orders of magnitude smaller than that produced by your microwave oven- but you can see that this effect places a practical limit on how much power a wifi system is allowed to use. In this case, better radio link performance would mean moving the link frequency down to a lower frequency, which reduces bandwidth.

And that means playing with link frequencies in the frequency range of ~tens of GHz, where the useability of the frequency is set by how the radio spectrum is allocated and where the common molecular resonances lie. For example, you would want to stay clear of frequencies used by radar sets and those set aside for ham radio operators to play with, lest your video streaming activities be jammed by passing airplanes or the random chatter of radio nerds in your neighborhood.

If you need more detail than this, the specific benefits versus detriments of operation at any specific frequency in the ~GHz band might best be obtained off the ham radio stack exchange.

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  • $\begingroup$ Thanks a lot for the detailed answer! You perfectly described a lot even around so I think I got a good feeling for the situation. I will have a look at the radio site but nevertheless I would like to ask something: Why is, for example, the 60 GHz band only stable within 10 meters? I imply that a lower frequency would reach more far? And the frequency itself (2.4 GHz or 5 GHz) doesn't tell anything about the bandwidth itself, right? $\endgroup$
    – Ben
    Aug 8 '18 at 11:34
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    $\begingroup$ Ben, I don't know why, but you could post this to the amateur radio stack exchange. plenty of experts over there who might know! $\endgroup$ Aug 8 '18 at 17:24
  • $\begingroup$ Microwave ovens have nothing to do with resonance with water. Water is simply a lossy dielectric: a higher frequency increases the dielectric loss. physics.stackexchange.com/questions/169173/… $\endgroup$
    – Phil Frost
    Aug 16 '18 at 12:21
  • $\begingroup$ will edit my response. $\endgroup$ Aug 16 '18 at 16:01

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