Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 12/01/2008 22:12:38
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I have a pair of cordless RF Headphones that receive a signal from the base station.
It means I can listen to the radio or TV whilst in the attic or in the loo !
The same can of course be said for other types of receivers like Radios and Televisions be them digital or not.
So how does the signal get through the wall ?
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Walls aren't made of conductors (in most houses), and so are transparent to radio signals.
If you had strips of metal (or some conductor) in your wall, things would get a little screwed up (the same idea as chaff, I believe). You can observe this effect by running an R/C car down a school hallway with lockers on each side and losing the signal very quickly as the car gets further away.
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As kalayzor says, radio signals are effected by conductors, although it also depends on the frequency of radio signal (the longer wavelength signals will be less effected by small pieces of metal).
It is a little like asking why light passes through glass. Each part of the spectrum of electromagnetic radiation is effected by different materials.
Steal bars in reinforced concrete can certainly block radio signals, but they are probably acting more like a Faraday cage than chaff (chaff, or what used to be known as 'window' by the British in WWII) are generally cut to a length to resonate to the specific radio frequency they are trying to reflect, so making a short piece of aluminium look like it the reflection of a massive aircraft. They are not really optimised to totally block the radio signal, but rather to return a confusing reflection.
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Materials vary in terms of dielectric loss. Radio Frequency energy will be dissipated in all non-conductive materials (like food in a microwave oven) but some more than others. Glass, PTFE, Polythene, ceramics are low loss. PVC for instance is much lossier. Polythene is used within coax cable because it is low loss. The outer shealth is usually PVC where it doesn't matter.
Loss increases with frequency.. FM radio (100 MHz) will pass through brick walls with little loss but wifi (2.4 Ghz..2400 MHz, 24 times higher) will be reduced between 4 to 10 times in power terms (6 - 10 dB). Passing through two single walls or a cavity wall and it will suffer a further 4 to 10 times = 16 to 100 times...depending on brick quality and moisture content.
I found that only 3 sheets of wet paper was enough to stop a 12GHz signal when placed over an LNB (thing at the focus) of a satellite dish.
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I would think that for reinforced concrete the steel will block (by reflection) low to VHF frequencies. For higher frequencies the concrete itself becomes more and more significant as the frequency is increased (loss by absoption) and the steel becomes less significant because the gaps between the steel are a greater fraction or indeed a multiple of a wavelength.
If a mesh has gaps between the conductors of more than half a wavelength then it becomes more or less transparent to the signal.
To make an effective parabolic dish with mesh the gaps need to be not much more than 1/10 of a wavelength.
Vertical bars of steel will block a vertically polarised signal if the bars are close enough together but pass a horizontally polarised one...similar to polarised sun glasses.
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I just want to thank you all for your kind answers.
Kalayzor (and welcome to the site, George and Pumblechook.
I appreciate very much the time and effort in helping me understand.
thank you all