We know low freqs. (800 Mhz) are preferred for rural area for they are travelling great distances. So, why do em waves (of mobile phones) with lower frequencies travel greater distances than em waves with higher frequencies, even though the latter have higher energy? Is this related to Rayleigh scattering like blue light from sun scatters more and attenuates in the atmosphere?
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$\begingroup$ Scattering is a good point but not at atoms of the atmosphere as in the blue sky example. Urban areas have a high concentration of metallic perturbations. That is more likely to be the problem. $\endgroup$– mikuszefskiFeb 21, 2017 at 13:14
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2 Answers
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Free-space path loss (FSPL) occurs in a vacuum or air with inverse f² loss due to the antenna aperture, effective area, or receiving cross section area shrinking with frequency.
At any point, a beam of radio waves has an irradiance or power flux density, PFD, which depends on $ \frac{1}{\lambda ^2} $ for inverse area in $\frac{1}{m^2}$.
Side notes
This aperture effect occurs on both the transmitting and receiving antenna and becomes part of the Friis Path Loss calculations simply as a frequency dependent loss with gains defined for each antenna.
Satellite dishes can overcome some of this with larger area but accuracy errors of the parabolic shape diminishes the returns on gain with multiple wavelengths.
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$\begingroup$ So, i understand distance decreases the power distribution per surface area but does not understand the frequency relation or Friis Path Loss part. Antenna sizes are proportional to wavelengths or freqs so what makes the difference? $\endgroup$ Feb 21, 2017 at 15:18
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$\begingroup$ The smaller wavelength has a smaller aperture and hence smaller flux density per unit area. $ f=c/\lambda $ Friis Loss you can look up is how system losses are computed simply including FSPL $\endgroup$ Feb 21, 2017 at 15:53
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$\begingroup$ So actually the main reason is not any of those. It is that attenuation beyond the $1/r^2$ terms, given the same antenna sizes, is heavily dependent on additional absorption, scattering and other electromagnetic effects from the terrain, the vegetation and the man-made structures. And all of those tend to block the higher frequencies and allows the lower frequencies through because it'll go around objects - diffraction- much more. That is why lower freqs like 700 MHz can be seen without a direct line of light, whereas 3 GHz or 5 GHz needs direct LOS. See any wireless communications book. $\endgroup$– Bob BeeFeb 22, 2017 at 5:21
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$\begingroup$ None of the things you mentioned are related to FSPL, they are additional losses $\endgroup$ Feb 22, 2017 at 7:12
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1$\begingroup$ They are the real reason lower frequencies are better for longer range propagation ion REALLY terrestrial environments. The FSPL is very irrelevant in those environments. And the simplistic calculations with FSPL are too trivial for physics to spend time on. if you want to ask this question do it where it might belong, in an engineering or EE site. I've done both physics and EE and it's embarrassing. $\endgroup$– Bob BeeFeb 23, 2017 at 5:07
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Low frequencies waves suffer less from diffraction, by geographical features, implying they undergo less distortion from the cell mast to your phone.
A further reason is that low frequencies are less attenuated and absorbed over long distances than higher frequency waves.
Whalesong is a natural example of a similar phenomenon, as well as the fact that communication with submerged military submarines utilises low frequencies, the disadvantage here being a slow rate of information exchange.