Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: labview1958 on 14/05/2009 10:03:38
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Can a radio signal be blocked by a mountain. Can radio waves cast a shadow when it hits like a light wave. Is there a signal in the shadow region.
Mod edit - formatted the subject as a question. Please do this to help keep the forum tidy.
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Waves are always diffracted when they pass an obstruction. If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver. UHF Television signals, being the same sort of wavelength as your dimensions, will spread out after passing you; after they've gone past by a metre or so, it is as if you hadn't been there. Those signals, when they come to a mountain, will be blocked very effectively - beacuse it's so big. But a long wave radio signal (1500m wavelength) will diffract round a mountain.
Microwaves will be blocked by your body but may find their way around a screwdriver.
Diffraction always happens when any waves pass through gaps and around obstacles; it's just a matter of degree.
see http://en.wikipedia.org/wiki/Diffraction (http://en.wikipedia.org/wiki/Diffraction)
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"If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver."
In a ripple tank experiment small obstruction give a more pronounced diffraction pattern than a large obstruction. It seems that the experiment contradicts the above statement.
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Marconi had difficulty obtaining backing for his foolhardy scheme to transmit low frequency radio signals from Poldhu to Newfoundland but he played a hunch and built a high power transmitter and by a combination of diffraction and ionospheric reflection the signals got thru and ushered in the modern era of telecommunications.
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"If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver."
In a ripple tank experiment small obstruction give a more pronounced diffraction pattern than a large obstruction. It seems that the experiment contradicts the above statement.
No, it's precisely in agreement. Long wavelength / small object gives a fuzzy edge and vice versa. 'Sharp' means the light doesn't 'bend' a lot.
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If a transmiter is placed on one side of the mountain , is there a blind spot on the other side.
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Yes - unless you have a Long Wave transmitter and there isn't a cheap way to make a good Long Wave antenna to transmit with.
Actually, on the whole, it is good that radio waves are usually blocked by terrain. It makes it much easier to plan a broadcast network because it limits the service areas of transmitters - avoiding interference between them.
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Is it possible that a ling wave cause space to bend around a small obstacle. Thus the Long wave appears to bend around a small obstacle. Thus a long wave has a tremendous energy that causes space to bend.
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The only tremendous energy you get out is tremendous energy that you put in, and it takes energy as dense as matter to bend space which is nothing compared to the energy usage of an antannae, so no I don't think so.
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Is it possible that a ling wave cause space to bend around a small obstacle. Thus the Long wave appears to bend around a small obstacle. Thus a long wave has a tremendous energy that causes space to bend.
Long wave equals low frequency equals LOW photon energy, surely?
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I think the cause of confusion is the high power that longwave radio stations find it necessary to feed into their antennas, I believe Moscow runs about 500KW while the Americans working at 16KHz for submarine communication where antenna efficiency is very low put out several MW.
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Can diffraction be explained as the incoming wave bending space around an obstacle. Thus as to appear that it bends around an obstacle.
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I think the cause of confusion is the high power that longwave radio stations find it necessary to feed into their antennas, I believe Moscow runs about 500KW while the Americans working at 16KHz for submarine communication where antenna efficiency is very low put out several MW.
And the reason they need high transmitted power is that they are used for long range communications. The inverse square law applies -so high power is needed. Also, an ideal transmitting antenna would be, for example, a quarter wavelength monopole over a large perfectly conducting plate. This would involve a mast 400m high. Too expensive, so they use a shorter structure, sometimes a T shaped radiator (with wires stretched between two much shorter masts at the BBC Droitwich R4 transmitter) and a cheaper system of wires in the ground, This can be 'tuned' or matched to get the best performance but the inherent resistance of the system (ground conductivity etc) means that a lot of the transmitter power is just wasted.
Submarines trail immensely long wire antennae (kms, I believe) behind them but, as they are in a conducting medium (seawater), a lot of the power is absorbed before it gets into the 'aether'. There is also the problem that it is horizontally polarised.
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Can diffraction be explained as the incoming wave bending space around an obstacle. Thus as to appear that it bends around an obstacle.
If it "bent space" as you imply, then wouldn't we all see the effect and wouldn't aeroplanes have to fly a different course? What's wrong with the current explanation?
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I believe that space can be bent by wave obstacle interaction. The presence of an obstacle has an effect on the wave direction. How exactly to explain the diffraction pattern observed.
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The space in a given region is 'experienced' independently by many observers at once - for instance, Electromagnetic waves of all frequencies can pass through the same region of space at once without affecting each other (this is called superposition and applies for any linear medium). So if it were as you say, they would affect each other; superposition would not happen. How can you insist that things are that way round?
Where does you approach take us?
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mountain doesn't block radio signals,but there will be disturbance in signal, and the energy is lost.....
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mountain doesn't block radio signals,
Tell that to people in Wales who can't get TV where they live!
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The high layers of the atmosphere do not always act as a linear medium powerful transmissions can cause heating of the ionosphere and produce cross modulation
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True, but the mountain is not actually failing to block such signals. The ones that 'get through' are just not going in the direction of the mountain. Using a mirror to see round someone would not be used as an argument to say that were not blocking out the light. (Which is why I didn't introduce ionospheric propagation into the topic.)
It's not the heating that causes the crossmodulation - it's a 'coherent' effect, involving fields and vectors. Heating may be a secondary result of the effect.
This thread is starting to fragment, I fear.
The actual answer to the initial section of the thread is that diffraction ideas give the right answer and the theory is pretty consistent with most other ideas. If it ain't broke, don't try to mend it - especially if you are new to the subject. "Bending Space" is a red herring.
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Inside a mountain there's no radio reception. This is because the mountain has blocked the radio signal.
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There is still a finite skin depth.
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If a transmiter is placed on one side of the mountain , is there a blind spot on the other side.
In general, yes.
UHF (television, 500-800MHz, ~50cm wavelength) and mobile phone 900MHz/1800MHz/2100MHz 30cm/15cm/12cm are really blocked and are very much line-of-sight with respect to mountains.
With a powerful broadcast VHF transmitter you may get a weak signal spill into the 'shadow' of the mountain to some extent, especially if you're some way from the shadowing mountain.
Mediumwave and longwave radio signals (wavelengths of 250m to 1.5km) tend to get around British mountains reasonably successfully.
If you and your friend stood on opposite sides of a mountain with VHF/UHF walkie-talkies even of quite substantial power, you would be very unlikely to have a workable communications channel.
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"Mediumwave and longwave radio signals (wavelengths of 250m to 1.5km) tend to get around British mountains reasonably successfully"
Energy of wave = hc/λ thus a long wave has a less energy. This less energy causes the space around the mountain to bend? The wave appears to bend (diffract) around the mountain. A short wave with more energy is not affected by the mountain thus no bending of space occurs.
To test my assumption lets sent a short and long wave together towards a mountain. The long wave will bend (diffract). The short wave will follow in the foot steps of the long wave. It also should diffract. Possible?
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Energy of wave = hc/λ thus a long wave has a less energy. This less energy causes the space around the mountain to bend? The wave appears to bend (diffract) around the mountain. A short wave with more energy is not affected by the mountain thus no bending of space occurs.
To test my assumption lets sent a short and long wave together towards a mountain. The long wave will bend (diffract). The short wave will follow in the foot steps of the long wave. It also should diffract. Possible?
What you are suggesting, in effect, is that we could 'steer' the short-wavelength wave using the long-wavelength wave.
This is precisely what sophiecentaur was refuting when he said
The space in a given region is 'experienced' independently by many observers at once - for instance, Electromagnetic waves of all frequencies can pass through the same region of space at once without affecting each other (this is called superposition and applies for any linear medium). So if it were as you say, they would affect each other; superposition would not happen. How can you insist that things are that way round?
Where does you approach take us?
Where your approach would lead us would be that the radiowaves we intentionally send for an experiment might get bent all over the place by very low frequency military submarine communications which happen to be in the air at the time. They'd also get flung about by the 50Hz fields from the electricity distribution grid. And the waves you were intentionally generating would disturb higher-frequency TV and mobile-phone signals... and even visible light would fail to travel in straight lines - being deflected fairly randomly by all our radio transmissions in the air at the time.
This isn't how it is.
In free space (and for most practical purposes in our atmosphere - minor exceptions in special cases in the ionosphere excluded) the principle of superposition applies. That is, waves of different frequencies can be sent through the same space and not affect each other in any way.
Existing explanations in terms of diffraction explain the results of waves of different wavelengths getting around (or not getting around) mountains quite satisfactorily. Your proposed theory flies in the face of evidence and raises obvious absurdities when you think through the logical conclusions.
Scientifically, you don't get anywhere clinging to pet theories which don't match reality. [;)]
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Energy of wave = hc/λ . Thus a long wave has more energy. Imagine the long wave as a stream of particles. This stream of particles are bend as they move near a mountain. On the other hand a short wave. the stream of particles are less affected by the mountains gravity. This is my alternative explanation to the standard diffraction model.
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Many folks think Marconi did not cross the Atlantic by radio in 1901.
The frequency and time of day don't add up.
Either he was mistaken or faked it.
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Of course mountains block radio signals. It is difficult getting distant FM radio and TV here in this valley.
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How's your Maths, labview? The energy is proportional to ONE OVER LAMBDA. That means as lambda increases, the energy decreases!!
That, of course, refers to the photon energy, which is not relevant for Radio Waves, which will never 'penetrate' a mountain. They just have to go round the edges (diffraction).