Intuitively possible but mind-boggling as it is, I took another look at single-wire earth return and nown I understand the operating principles. Intuitively, this would work better with AC than with DC current, and indeed AC is quite practical for this for a number of reasons I will explain.
First of all, single-wire earth return uses an isolation transformer to keep the grid voltage loop out of the single-wire earth return voltage loop. There is also another such isolation transformer on the customer premises to keep their local power out of the single-wire earth return distribution current loop. So, right off the bat, we can say that isolation of current loops is important.
Some more intuition would lead you to believe that AC at a sufficiently high frequency is needed to be able to pump the charge out of the ground without having the signal want to short striaght to ground. Because if it did, you would have virtually no current flow through your distribution wire, and thus your electricity would travel nowhere.
Indeed, this is another important consideration. If the resistance of the ground soil is too high, you won’t be able to get an efficient distribution system. And, surprise surprise, what happens with single-wire earth return is that there is in fact technically a current loop that travels through the earth, finding its way from the distribution end-points right back to the source grounding steak. Wow, now intuitively, if that is what is happening, then you also do need high-voltage electricity to get any usable supply at all. Actually, this is a practical example of what is called a “phantom loop.”
So, there you go, the key operating principle of single-wire earth return has been disclosed and explained. You technically do need a full current loop, as is the case with any electrical transmission, but the trickery here is that part of the current loop, the earth return, runs through an implicit, undefined path in the ground. For this to work, your ground source must be sufficiently conductive of electricity, and, ultimately, it must be contiguously connected between the source and destination points. This has the obvious implication that you cannot supply electricity to “airborne” users who have only one wire connected and no conductive path to ground.
Now, here’s the important key point to myself. This means that for small electronics, like a key fob for public mass transit, cannot be constructed using truly only one wire for 1-wire communications via the “single-wire earth return” principle, because the voltage is too low and the ground insulation between the user and the mounted device is too high for there to be an “earth return” ground path. For current to flow in such a “suspended in the air” use, you need two wires. Otherwise, your only other alternative like “one-wire” is to electromagnetic radiation of the power and data signals, just like RFID. Yes, you can guide radio frequency through a single-conductor antenna, but the best radio frequency carrying cables always have two conductors, or one conductor and one ground. Yes, indeed, radio frequency is modulating electrical power and signals at really high frequency, so it is like ultra-high frequency AC.
20191123/https://en.wikipedia.org/wiki/Single-wire_earth_return
We’ve found practical ways to distribute data singles long distance through electromagnetic radiation, but power, those have long only been practical for short-distance transmission. Magnetic resonance was once long ago hyped as a solution for being able to send power over a longer distance as electromagnetic radiation, but in the intervening years, the technology has stagnated in research and development progress, no useful products made it to market, and it is pretty much all but forgotten from the public consciousness.
20191124/https://en.wikipedia.org/wiki/Resonant_inductive_coupling
Oh, okay, looks like the primary update in the research of this is as follows. People have successfully built commercialized realizations of this technique that can reach up to 3 meters, but the concept of reaching several kilometers with this technology has been shelved. 3 meters is good enough to wirelessly power a maglev train and to power equipment inside of a mid-sized closet, but most typical “small rooms” are going to be just a tad bit too big to power all electronics in the room solely from a single magnetic resonance node inside the room’s one central lighting fixture.
Also, more importantly, you’re not remotely going to get this technology rolled out in the home residential market on a large scale, that market moves too slowly and is relatively resistant to change.