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1 wire power transmission?

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Patrick

Sat Jun 02 2012, 02:56AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Mattski wrote ...

randommscience117 wrote ...

Sudden realization: No frequency will only travel on the inner-most part of the wire, therefore it would always 'short circuit'. Oops.
New question: Can DC and AC be transmitted on the same wire under any circumstances?

Absolutely, it's just an AC signal with a DC offset. A bias tee allows you to separate them so that you can inject the DC without affecting the AC. Note that in the Wikipedia diagram the return path for both the AC and DC current is not drawn, but it must be there. I think some security cameras can be powered over their coax this way.

Yes ive seen some radio type equipment run off coax power, while still sending signal.

Dr. Dark Current

Sat Jun 02 2012, 08:25AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

One wire power transmission can be done, but requires high frequency AC and ground planes on both sides (not necesarilly connected to ground/earth).

Pinky's Brain

Sat Jun 02 2012, 08:46AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
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The trick is that you're always capacitively connected to ground. AFAICS the capacitance of the wire is going to be a bit killer if it's long though ... you essentially get one big capacitive voltage divider. I think you would you need a lot of capacitance at both ends to reduce attenuation. Big metal balls?

Dr. Slack

Sat Jun 02 2012, 09:34AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

"one wire" power transmission never happens, there's always a return.

At DC or power-line frequencies, that return will be the ground. You can improve the ground return conductivity by connecting each end to the house ground, or by wetting the ground around where your big metal spike is driven into. Unfortunately, if there's a band of impervious rock between the send and the receive ends, then you will end up with a high resistance ground return whatever you do at the ends.

You can mitigate poor ground conductivity by using a very high voltage for transmission. If you had a pole pig or a MOT at each end, that would make the best use of a very high ground impedance. It would also make the best use of a pair of thin wires, without needing to use the ground.

Neet Studio

Sat Jun 02 2012, 12:23PM

Registered Member #4037 Joined: Fri Jul 29 2011, 03:13PM
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Posts: 86

I am not a RF guy, so please correct me if I am wrong.

At some frequencies when your wire becomes an efficient antenna (i.e. at some suitable wavelengths), it might be possible to transmit power by "single wire". The transmitter and receiver just happens to share an antenna on opposite ends.

Both ends needs to attached to some grounds but not necessarily connected together.

Pinky's Brain

Sat Jun 02 2012, 12:28PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Dr. Slack wrote ...

"one wire" power transmission never happens, there's always a return.

That's a semantic issue, you can obviously use a wire as a wave guide for RF power transmission ... whether it can still be said to transmit the power is a question of definition. Most of the energy of the wave is certainly within the wire, that's why it's a wave guide.

Steve Conner

Sat Jun 02 2012, 02:35PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No it isn't. The power flow in an EM wave is E x H, and E = 0 inside a conductor. Therefore electrical power doesn't travel in wires at all, rather in the space between them.

A related idea is the Goubau line, which is something like an inside-out waveguide.

klugesmith

Sat Jun 02 2012, 04:18PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dr. Slack wrote ...
"one wire" power transmission never happens, there's always a return.

At DC or power-line frequencies, that return will be the ground. You can improve the ground return conductivity by connecting each end to the house ground, or by wetting the ground around where your big metal spike is driven into.

Agreed. Though not what OP had in mind, one-wire power transmission (with ground return) is practical even today in some rural areas. The cost of transformers is offset by eliminating many, many km of conducting wire. Wikipedia has a good write-up:

There are commerical products to enhance the conductance of earth connections. Here's one:

Dr. Slack wrote ...
Unfortunately, if there's a band of impervious rock between the send and the receive ends, then you will end up with a high resistance ground return whatever you do at the ends. ...

That's put to use in earth conductivity profiling (or even tomography), a well-developed tool in geophysical exploration. At its simplest, a four point R measurement at the surface becomes sensitive to deeper strata as we increase the probe separation.

Pinky's Brain

Sat Jun 02 2012, 04:29PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

wrote ...

No it isn't. The power flow in an EM wave is E x H, and E = 0 inside a conductor.

Only in a superconductor ... within a conductor it falls off exponentially (assuming AC current).

Steve Conner

Sat Jun 02 2012, 06:09PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's technically true, but the quantity E x H within the conductor just corresponds to I2R losses. The useful power flow from source to load still happens outside of the conductors.

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