Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: acsinuk on 23/04/2012 12:34:11
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In my physics book of fundamental constants I have an item that states that the impedance of free space is 376 ohms.
If we placed 2 probes 1 metre apart in space at right angles to the solar wind what voltage would appear? If the probes were in line with the wind; what then?
Has anyone checked it out yet or is there a website to look up?
CliveS
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The solar wind isn't free space.
There's nowhere like enough information there to answer the question
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If you eat bean burritos in the sunshine, do you get solar wind?
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Seriously though, don't antennas have an impedance of 360 Ω?
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Seriously though, don't antennas have an impedance of 360 Ω?
I don't think so. Their impedance will be based on a lot of complicated stuff to do with wavelength and the like.
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This all originates from transmission line theory in classical radio engineering. To achieve the maximum possible efficiency in the transmission of power from a source to a receiver it must be matched. this means that the impedance of the source must be similar to the impedance of the receiver. it is measured in ohms the unit of resistance but if the sources and receivers are pure resistance only half the power can be transferred and half is lost in the source. So for most uses reactive (inductive and capacitative) sources work most efficiently and resistance (loss) is minimised. The reactive devices that do this are called transformers. There are many different sorts depending on the frequency and power being used most of us are familiar with inductive transformers used with mains electricity to step down from the high mains voltage to a lower one.
To transmit radio signals you need an antenna that matches the impedance of the transmitter to free space which has the impedance of 376 ohms which comes directly out of electromagnetic theory. If this is not matched some of the power is reflected back into the transmitter which can have disastrous results.
One other important fact a good transmitter antenna is a good receiver strict reciprocity applies if it works well one way ir works well the other.
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I'm no expert on the subject SS, but I believe the transfer of power from an antenna into space is a bit more complex than that.
I found an interesting paper on the subject recently. I'll try to dig it up.
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I agree it is an extremely complex subject and I have been involved with the design and use of all sorts of antennas from LF up to millimetre waves and beyond . These can be omnidirectional or narrow beam and all sorts of things in between. The same processes also extend right up into optical frequencies and beyond where quantum effects can be important. Spectrum lines are in fact tuned circuits or cavity resonators You are using the "geometry" of the atom (or nucleus in the case of Mossbauer effect) to create the resonant reactive elements to produce the effect required. However there are several critical features relating to all of them and this energy transfer process is one of them that was being discussed here.
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Seriously though, don't antennas have an impedance of 360 Ω?
The old flat cables with two wires side by side had 300 Ω impedance. The newer coaxial cables have 75 Ω impedance.
Back to the 1-meter antenna in the solar wind: A current on one side of the antenna induces a certain voltage; an equal current on the opposite side cancels that voltage. If the solar wind is uniform, it will not induce a voltage in the antenna.
The value of impedance of free space is exactly (149,896,229 pi)/1,250,000 Ω ≈ 376.73031 Ω. It is exact because it is derived from the definitions of meter, second, speed of light, coulomb and ampere.
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Here is the article I mentioned. It's quite easy reading.
http://majr.com/docs/Understanding_Electromagnetic_Fields_And_Antenna.pdf (http://majr.com/docs/Understanding_Electromagnetic_Fields_And_Antenna.pdf)
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That is quite a good article for basic principles but it does not deal with the directional pattern of the radiated energy and the use of more complex metal structures with reflectors and parasitic elements like Yagis (tv Antennas). or broadband antennas like log periodic structures.
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That is quite a good article for basic principles but it does not deal with the directional pattern of the radiated energy and the use of more complex metal structures with reflectors and parasitic elements like Yagis (tv Antennas). or broadband antennas like log periodic structures.
It certainly doesn't, but I thought it might provide some insight into the impedance question.
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Thanks SS and Gee
Ron Schmitts EM with no maths article figure 7 depiction of an electric field surrounding a wire carrying a current is meaningful as it shows that electricity is 3D with a current induced magnetic area travelling in phase with a voltage that is at right angles to the current loop.
Further note that Wikipedia has now redefined electric field as a 3D electricity field with some good B/H curve graphics which attempts to show the field, as viewed from the x,y and z axis.
Those of us that believed that electricity was just charged particles moving through a conductor need to rethink of electricity rather as a 3D volume of magnetic flux
NOTE This magnetic flux can be supported even in outer space which means that space itself is electrified and not empty.
CliveS