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<blockquote data-quote="2old4this" data-source="post: 847" data-attributes="member: 174998"><p>Interesting discussion. The question (why does my mobile phone receive refleted signals?) is asked because you've assumed (based on satellite experience) that microwaves always need line-of-sight reception. But in fact that's not generally true, and satellite reception turns out to be a special case.</p><p></p><p>So what are the differences between satellite and mobile-phone signals?</p><p></p><p>For a start, satellite transmissions use higher frequencies than those used by mobile-phone networks. The latter are between 900 and 1800 MHz, while even the lowest (C-band) sat. transmissions are at twice that higher figure. And the Ku-band is much higher still (three times higher than C-band). </p><p>The higher the frequency of electro-magnetic radiation, the less susceptible it is to refraction ("creeping"). Visible light hardly creeps at all - if it did, you would constantly be able to see "round" objects. This is because visible light is VERY high frequency (about 43,000 times higher than ku-band!).</p><p>So that's part of the answer to the question.</p><p></p><p>The other thing to bear in mind is the relative field-strength of the signals. A mobile-phone cell is no more than 35km radius, so that the user is never further than 35km from the nearest transmitter. But the satellites are at 1000 times that distance. And since signal strength drops with distance according to an inverse-square law, that means what arrives at the earth's surface is very weak indeed.</p><p>So whereas a typical mobile-phone antenna only needs to achieve something like a 50-fold gain, a satellite dish/LNB has to amplify the signal by a factor of about 100,000! Such a feat can only be achieved by gathering as much coherent signal as possible and focussing it all to a single point. This means a large gathering surface, and the larger that is, the greater the accuracy with which it must be aligned to the source. Furthermore, once you've plumped for such a focussing system, you are committed to having clear line-of-sight. This is because although refracted or reflected signals may well be incident on the dish, they will be arriving from different angles and be "focussed" at a point not occupied by the LNB. </p><p></p><p>And one final point: reflection is never perfect. Any reflecting object will absorb some of the incident signal, and will also introduce distortion in the reflected wavefront. Refraction (creeping around corners) will also cause similar losses and distortion. The resulting signal arriving at the gathering device may be highly degraded. So although a reflected signal from a relatively powerful source such as the mobile phone or terrestrial TV transmitters might still be usable, it would never be so for the much weaker satellite signals.</p><p></p><p>See here for a frequency spectrum: http://www.adec.edu/tag/spectrum.html</p><p>Here's an interesting article on mobile phone technologies: http://www.iegmp.org.uk/IEGMP-4.pdf</p><p></p><p>2old</p></blockquote><p></p>
[QUOTE="2old4this, post: 847, member: 174998"] Interesting discussion. The question (why does my mobile phone receive refleted signals?) is asked because you've assumed (based on satellite experience) that microwaves always need line-of-sight reception. But in fact that's not generally true, and satellite reception turns out to be a special case. So what are the differences between satellite and mobile-phone signals? For a start, satellite transmissions use higher frequencies than those used by mobile-phone networks. The latter are between 900 and 1800 MHz, while even the lowest (C-band) sat. transmissions are at twice that higher figure. And the Ku-band is much higher still (three times higher than C-band). The higher the frequency of electro-magnetic radiation, the less susceptible it is to refraction ("creeping"). Visible light hardly creeps at all - if it did, you would constantly be able to see "round" objects. This is because visible light is VERY high frequency (about 43,000 times higher than ku-band!). So that's part of the answer to the question. The other thing to bear in mind is the relative field-strength of the signals. A mobile-phone cell is no more than 35km radius, so that the user is never further than 35km from the nearest transmitter. But the satellites are at 1000 times that distance. And since signal strength drops with distance according to an inverse-square law, that means what arrives at the earth's surface is very weak indeed. So whereas a typical mobile-phone antenna only needs to achieve something like a 50-fold gain, a satellite dish/LNB has to amplify the signal by a factor of about 100,000! Such a feat can only be achieved by gathering as much coherent signal as possible and focussing it all to a single point. This means a large gathering surface, and the larger that is, the greater the accuracy with which it must be aligned to the source. Furthermore, once you've plumped for such a focussing system, you are committed to having clear line-of-sight. This is because although refracted or reflected signals may well be incident on the dish, they will be arriving from different angles and be "focussed" at a point not occupied by the LNB. And one final point: reflection is never perfect. Any reflecting object will absorb some of the incident signal, and will also introduce distortion in the reflected wavefront. Refraction (creeping around corners) will also cause similar losses and distortion. The resulting signal arriving at the gathering device may be highly degraded. So although a reflected signal from a relatively powerful source such as the mobile phone or terrestrial TV transmitters might still be usable, it would never be so for the much weaker satellite signals. See here for a frequency spectrum: http://www.adec.edu/tag/spectrum.html Here's an interesting article on mobile phone technologies: http://www.iegmp.org.uk/IEGMP-4.pdf 2old [/QUOTE]
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