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Why do ultra high frequency radio waves (ex: 2.4GHz or 5GHz) have limited range ?Is it to do with absorption ?If so how do these waves get absorbed when the ELF waves are not ,and on the other side of the spectrum cosmic rays and UV rays have high penetrating power but are they also limited in range ?

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    $\begingroup$ The interaction of photons with matter is highly dependent on the frequency of the photon, and the allowed transitions of the matter. There should be no expectation that all photons will behave in the same way across many orders of magnitude in photon energy. $\endgroup$
    – Jon Custer
    Commented Jul 12, 2016 at 20:07

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Range of electromagnetic radiation is a function of several factors:

  1. Inverse square law: if you can consider your source to be a point (good approximation when you are many times further than the size of the source), then intensity (energy per unit area) drops with the inverse square of the distance. In the setting of a home WiFi, that makes the signal many times weaker "upstairs" vs "right next to the router".
  2. Attenuation: particular frequencies will interact with matter, and be attenuated. The attenuation depends on the material and the frequency
  3. Reflection: when there is a change in refractive index (dielectric properties) of the transmitting medium (e.g. a wall), a large fraction of the signal will be reflected.
  4. Diffraction: when there is an obstacle between the source and the receiver, waves can "bend around" these obstacles. The Fresnel diffraction equations involve, among other things, the wavelength: basically, if there is a large additional phase difference due to the "longer path", there will be less diffracted signal. The shorter the wavelength, the greater the additional path length due to a certain extra path length.
  5. Attenuation length: for certain materials, the attenuation is often expressed as "attenuation per wavelength"; when the wavelength is shorter, this means you will get more total attenuation for a given path length (because more wavelengths).

These things have been extensively measured and characterized - for example, see this paper on propagation losses in building materials. This shows there is typically 1 dB additional loss for 5 GHz vs 2.4 GHz signals in building materials - but in both cases the thickness of these materials will be one or several wavelengths; by contrast, longer wavelength signals (e.g. FM radio) will have much less attenuation per unit length (because the same wall represents less thickness "in units of wavelength"). By contrast, for light and such, a wall represents "many wavelengths" - making it essentially completely non-transparent.

Note also that there are specific frequencies where attenuation of EM radiation can be particularly strong due to resonance effects; but as far as I know (and the Gizmodo article below seems to support this) there is not a particularly strong resonance for water at 2.45 GHz. There is, however, a strong dielectric absorption due to the fact that the polar water molecules respond slowly to the rapidly changing electric field. This forms the basis of this earlier answer of mine about microwave interactions with water and ice.


A few other helpful links:
Gizmodo explanation why "everything is 2.4 GHz - talking about the unlicensed spectrum that grew around the use of microwave ovens at 2.45 GHz
"Propagation losses through common building materials"

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