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It is possible to design networking systems that only use two wires: one for data and one for a common ground. Examples include 1-wire and Pin&Play. These are called single wire systems because the requirement for an earth wire is implied too. But you can also get systems to extend home networks that use the home earth to connect network points, like Power Line Communication. How is this possible over just one wire?

== EDIT ==

From the answers (thanks!) I think I failed to word this question clearly. Let me try again.

Can you send data usefully over one wire, literally one wire? Radio is zero, 1-wire is two, but is it possible with one? "No, and here's why" or "Yes, here's how it is done in X" are the kind of answers I am hoping for.

(N.B. I'll also change the question title from "Single wire systems need two wires; so how does ethernet over ground work?" to "Can you send data usefully over one wire, literally one wire?")

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    \$\begingroup\$ What is the source of your belief that these are single wire systems? \$\endgroup\$ – Chris Stratton Apr 30 '13 at 16:43
  • \$\begingroup\$ I don't really know enough about these systems to provide an authoritative answer, but here are some interesting links: no wires, one wire, and a great question \$\endgroup\$ – Phil Frost Apr 30 '13 at 17:13
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    \$\begingroup\$ "Power line communication (PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers": that is, it uses the 2 power conduction wires, not the earth. \$\endgroup\$ – pjc50 Apr 30 '13 at 21:14
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    \$\begingroup\$ Do you count the earth as a wire? Also, how would you classify a rectangular square pipe used as a waveguide? \$\endgroup\$ – Chris Stratton May 1 '13 at 17:24
  • \$\begingroup\$ @ChrisStratton it need not be square ... and while a waveguide is just guided radiated energy it certainly meets the requirements of a single wire. I say write it up! \$\endgroup\$ – placeholder May 1 '13 at 19:20
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Over a very limited range - yes. You will need to have a return path that is supported by electric fields. The best way to look at this would be like a AC coupled circuit - coupled through a capacitor of which the capacitor is formed by some plate that the circuit is coupled to and another plate that is providing a return path.

We know that electric fields can couple over long-ish distances, some Anti-aircraft proximity fuzes from WWII used an e-field detection technique that would trigger the bomb because the shell and the aircraft would be carrying different levels of charge and thus e-field lines would form linking the two and thus change the capacitance in an internal circuit.

This in no way violates physics, it's best to view it as a capacitor that is so physically large that you can walk between the plates. However, the actual capacitance value would be very small.

uChip just released some technology that uses a similar effect that is called GestIC and they couple with E-Fields. Here they couple on both the top rail and the return path so it is a "Zero" wire solution. But it will also work if you ground one of the plates inside the remote device to one polarity of the plate in the "pad".

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EDIT - because of the reshaping of the question. my answer at the bottom isn't that relevant. To the edited question: -

"Can you send data usefully over one wire, literally one wire?"

My answer is "Not with any level of success". Reasoning - if you are sending signals you need a forward and a return path for the current. If the return path is tenuously connected (such as through the ground), any ground interference in the vicinity of the return path adds to the signal received and has a tendency to corrupt that signal. However, couldn't the ground be classed as a wire? Avoiding this issue, if there were a tenuous ground connection that is validated as not adding an extra wire, the answer could be "Yes" providing the transmitted signal was powerful enough to overcome natural and unnatural interference in the vicinity of the return path.

If the signal were RF and transmitted via "a single wire" such as an antenna you could argue the answer is "Yes" but would the receiving antenna be counted as a wire making the RF argument null and void? Ultrasonic signalling or optical signalling don't need wires to make a connection so they are invalidated in my answer.

Answer to question before revision: -

I use zero-wire systems quite a lot - these receive their power and transmit data using NO (repeat NO) interconnecting wires. Data transmission is a little under 1Mb per second and of course for a (decent) network that is dreadful. Ditto for powerline comms and 1-wire and Pin&Play (worst of the lot).

If you want a decent high-speed network then wide-bandwidth systems are required and twisted pair is needed to get any reasonable distance between offices. WiFi is good but this uses sophisticated modulation techniques and radio to achieve high-speed - the ones in your home haven't got a lot of range though.

Yes you can design networking systems that only use two wires but they would be significantly faster and less-prone to errors if they were a balanced pair. You can even phantom power equipment connected to them just like 1-wire (2 really) does.

Don't be hoodwinked by all the marketing up-front blurb

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  • \$\begingroup\$ Isn't a balanced pair only two wires? \$\endgroup\$ – Phil Frost Apr 30 '13 at 17:53
  • \$\begingroup\$ @DrFriedParts thanks for that Doc but who was the meany who down-voted me - c'mon now own up LOL \$\endgroup\$ – Andy aka Apr 30 '13 at 17:54
  • \$\begingroup\$ @PhilFrost it is two wires and so in fact is 1-wire!!! \$\endgroup\$ – Andy aka Apr 30 '13 at 17:55
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    \$\begingroup\$ I didn't downvote, but I didn't upvote either. Twisted pair isn't a requirement: coax can also work just as well (think of broadband internet by cable TV providers). There are also wireless systems that work well above 1Mb/s, and WiFi ranges can reach 20 km easily with an affordable antenna even under FCC part 15 rules; even more with licensed operation, higher power, and better antennas. I think the answer could do a better job of explaining the knowledge underlying these claims, and more directly addressing the question. \$\endgroup\$ – Phil Frost Apr 30 '13 at 19:08
  • \$\begingroup\$ @PhilFrost i don't disagree technically with what you say but I don't understand what you mean when you say "explaining the knowledge underlying these claims, and more directly addressing the question". \$\endgroup\$ – Andy aka Apr 30 '13 at 19:17
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Yes it is possible, but only really as a extension to radio if you don't want to consider systems that use a implied second conductor, like the earth.

Radio works, so you can use the single wire to greatly decrease the loss between transmitter and receiver. With radio propagating thru space, the power per area falls off with the square of the distance to the transmitter. The falloff with a wire is quite different, with modest distances yielding less loss. The loss in a wire is exponential due to ever-increasing resistance, so eventually radio actually wins over wire.

You can also have what appears to be a single wire system by implicitly using some nearby large conductor, like the earth, as the other "wire". Some old commercial power distribution systems worked this way, with the earth carrying the return currents. If you can send power this way, you can also send information this way. However, no such power systems are being built new, and I think all the old ones have been phased out and replaced with full out and back wires.

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  • \$\begingroup\$ Even "radio over one wire" requires the existence of some sort of common reference -- even if only capacitively coupled -- which is still equivalent to a second wire. And something like a waveguide really needs to be considered as two wires above its cutoff frequency -- the only condition under which it can transmit information. \$\endgroup\$ – Dave Tweed Jun 8 '17 at 22:27
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There is such a setup, called a Goubau line - but it actually requires the wire to be terminated with antennas, as it is more of an open-faced waveguide than an electrical circuit. Like a waveguide, it won't be DC coupled and won't be practical for operating frequencies below the shortwave RF range.

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