Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: realmswalker on 04/05/2007 01:28:24
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The way crystal radios work is they have an antenna that gets hid by radio waves in the atmosphere and send electricity down the wire to a grounding point. Before it reachers there one of the frequenceys is seperated out and powers a little speaker to hear whats going on.
If theres enough energy to power a speaker in one frequency, why not set up large grids of antennas each taking every frequency of radio waves and use them to charge a battery or what now?
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The best you can achieve out of a crystal set would power only the smallest of speakers (in fact, normally only a headset, not a free standing speaker).
In fact, if you simply tuned the receiver to 50Hz (60Hz for the USA), you would get far more power simply by picking up signals from the national grid. It has been demonstrated that if you happen to be standing under a power pylon, you can even receive enough power to weekly light up a florescent tube. But all of this pales into insignificance with the energy you can obtain from a solar array using the same amount of space (at least during the hours of daylight). That is not to say that you will get an enormous amount of energy from such a small solar array, but it would still be considerably more that the energy from a resonant circuit trying to extract radio wave energy from the air.
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I have heard stories about people living near BBC broadcasting stations putting up big aerials in their loft and getting free power. I think there was even someone prosecuted for stealing electricity or something similar.
I guess the advantage of a resonant circuit picking up e-m energy from the air would be that the components are basically just wire and possibly a capacitor or two which are very cheap. I guess the problem with picking it up under power lines would be that you would actually have to be very close as the field will drop off quickly.
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I have heard stories about people living near BBC broadcasting stations putting up big aerials in their loft and getting free power. I think there was even someone prosecuted for stealing electricity or something similar.
I guess the advantage of a resonant circuit picking up e-m energy from the air would be that the components are basically just wire and possibly a capacitor or two which are very cheap. I guess the problem with picking it up under power lines would be that you would actually have to be very close as the field will drop off quickly.
I would have thought it would still have been quite limited amounts of power, otherwise we might have been hearing all sorts of scare stories about the field strengths these people were exposed to (bear in mind the scare stories we get about the field strengths from the few watts of power from mobile phone transmitters, after the power has been attenuated by tens of metres of air). Yes, broadcast radio stations can measure their output even up to the megawatt range (although most are only a small fraction of that), but they are usually situated well away from houses, and my the time the first house is reached by the signal, I would have expected the watts per square metre to be quite small. It is I suppose also true that some of the military radar installations can throw off some substantial amounts of power (albeit, only short pulses, so the average energy remains small).
The point is that any energy you are taking out of the local atmosphere is also energy that is running through your body all of the time that you are in the vicinity, and there are limits to how much energy you do want your body exposed to (or that are likely to be allowed under normal circumstances).
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Put the two stories together...
The last prosecution for this sort of thing was, I believe, someone who had a garden shed under a 400kV transmission line and had it lit for free by flourescent tubes! What you're actually doing is 'stealing' reactive power (VARs) from the grid - there is a loss of power from the lines (though admittedly very small).
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Put the two stories together...
The last prosecution for this sort of thing was, I believe, someone who had a garden shed under a 400kV transmission line and had it lit for free by flourescent tubes! What you're actually doing is 'stealing' reactive power (VARs) from the grid - there is a loss of power from the lines (though admittedly very small).
So what your saying is even though their is leakage through the atmosphere from the power lines the effect of using that leakage causes more to be leaked to fill the vacuum cuased by using whats already leaked so to speak .
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Yes. You're changing the environment in which the power lines normally operate. At the other end of teh line it would appear that the line had become more 'lossy'.
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Yes you are effectively building a transformer.
The power lines carry a changing electric current and so will produce a changing magnetic field.
If you put a loop of wire near it this will induce a changing voltage and therefore current in the loop - just like a generator.
This current in the loop will produce it's own magnetic field.
This will induce voltages in the power line which will act to reduce the current flowing in it. Effectively increasing its resistance, and therefore the energy losses. So you are stealing energy from the power company.
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Both 50Hz mains electricity and Radio transmissions involve electromagnetic waves and the same Physics applies to both; there are parallels between both phenomena - the difference is in the detail.
The wavelength at 50Hz (UK mains frequency) waves is 6,000km and the transfer of power from one wire to another nearby wire is pretty much all by magnetic fields. Only a tiny fraction of the mains power is actually radiated out into space, because of the effective size of antenna (the supply cable), compared with a wavelength - just as well, for the generating company; most of it gets to the customer.
Actually, DC transmission is even more efficient - but inconvenient because you can't use transformers.
To steal power from the Grid,
Yes you are effectively building a transformer.
as Daveshorts says.
To transfer much power through a transformer, you need a lot of 'coupling' between the two parts (the primary and the secondary coils). In a 'normal' transformer, you use a lot of iron (a core) to wrap your coils around. This maximises the changing magnetic field, concentrates it through the coils and stops the field (power) from 'leaking' out into space (not the 'atmosphere'). If you have no iron, you need to be very close to (i.e. just under) the cables and you need a big loop to intercept a significant amount of the field around them.
Effectively increasing its resistance, and therefore the energy losses.
It is a weird concept to picture an actual resistance appearing in the cables, due to the power being stolen but the sums show that it is, effectively, there. You can measure it.
With radio frequency power, the situation is only different because the frequency is much higher
For radio waves around 1Mhz, the wavelength is about 300m and and, of course, much shorter for even higher frequencies. In these cases, the transfer will involve both electric and magnetic fields. The antenna is 'tuned' so that the maximum possible amount of power is radiated.
This weird 'resistance' turns up again here. Once your antenna is 'tuned' your transmitter just 'sees' a resistance across its output terminals and it is due to the 'radiation resistance'. The fact that power is radiating out into space shows itself as a volts times current (the power) and a volts divided by current (a resistance). Empty space has a resistance of 377Ω!! How about that?
You can say, roughly, that the power is radiated over a hemisphere (for a medium wave system). Once you are just a km away, the power is spread out over a huge area so the 'density' of power flow will be quite small. At several km it is really small (the inverse square law). You need a big effective area over which you can collect this energy. For microwaves, the area of the 'dish' is obvious; for medium waves, the effective power-gathering area of a wire is less obvious but it will only intercept a tiny fraction of the total power flowing outwards. The way that a wire antenna interacts with a passing wave is difficult to understand but it relies on choosing its size so that it resonates, in step with the wanted signal. At resonance, there is a lot more current induced in the wires and these currents modify the passing fields in such a way as to suck power into the cable to the receiver.
The actual power getting into your receiver is a matter of microwatts, in most cases. Even with thousands and thousands of people looking and listening, most of the radiated power (kilowatts) is lost into space. What a waste, I hear you cry.
However - distribution by cables over distances of more than a few tens of km is, actually, more lossy (due to copper resistance , leakage through the braid and heating up the plastic insulation). For wireless transmission, the inverse square law is a pain when you are close up (the signal drops very quickly over the first km or so) but, from 1000km to 2000km, the signal to the antenna only goes down by a factor of 1/4; it almost goes on for ever! Cable is also very expensive except in towns.
Cable has its own advantages, though.