The reason is that a current must loop. We must always inject current between two poles, for example between a big capacity above the ground, and the ground.
When charges move, they influence in their environment other charges that move also. There is a global redistribution of charges that tends to cancel the electric field. At high frequencies, this current reduces to a vibration of charges and atomic dipoles about their rest position. In vacuum (or air), the displacement current is due to the influence of the field onto the electrons/positrons pairs of the ZPF. In any case, the wire being a real wire or the earth, there is never transmission of current through a "single wire" alone.
When the ground disturbance causes a greater potential difference between the terminal and the ground current flows into the terminal capacity as a result, which I think causes displacement currents to the atmosphere and the Earth, then when the ground disturbance causes a lesser potential difference between the terminal and the ground the current flows back the other way because the capacity of the terminal discharges through the coil to the ground. There is a loop. The receiver is tuned to the period of the disturbance (vibration) or a harmonic of it.
That's why a single wire transmission line won't work well, the terminal cannot displace current to it well because of it's capacity and physical form. The capacity of the Earth is immense and the ground surrounds the transmitter.
With a single wire transmission line the receiver terminal would still displace current to the Earth. And the terminal would still discharge through the coil.
The transmitter and receiver are separate systems, the transmitter can disturb the ground by itself. Then when a receiver is connected to the disturbed ground the effect is it that the receiver transformer is operated in reverse manner to the transmitter, rather than disturb the ground it takes the disturbance out of the ground. Simple. The transmitter and receiver are not connected except by the Earth.
You need to consider the transmitter would be disturbing the ground even with no receiver in place. This is a big point, the receiver could operate regardless of what is disturbing the ground, it's just that the transmitter does it best. The transmitter does not transmit as such to the very distant receiver. It disturbs the ground to resonate, the receiver then is disturbed by the ground. It's really very simple, and easy to visualize.
The way you explain it is like without the receiver the ground would not be disturbed, that is not reality in my opinion.
My record of transmission using a terminal capacity which was a wire at a height of 12 mtr, powered by 1 KV at 2 Khz, was only 50 mtr! And I don't even speak about energy recovering but of signal detection. Only EM waves propagate and so can be detected at long distances. Ordinary near fields remains linked to their source, with the field lines looping just around.
Of course a wire will radiate, the terminal needs to be a sphere or an improved terminal as described in this patent.
http://www.google.com/patents?vid=1119732You would need a lot more than 1 kV to cause a disturbance to a distance and the frequency should be a harmonic of the Planet. The transmitter should be operating with a continuous sine wave.
There is only one transformer in the patent because that is all that is needed to disturb the Earth, the receiver just need to tap the Earth disturbance it need not be a big thing, there could be many
small ones or small ones and bigger ones it would not matter. The transmitter works alone to disturb the Earth.
http://www.google.com/patents?vid=1119732The terminal is designed to hold the charge, a piece of wire can't do that because of it's tiny radius, the curvature is too great to allow an accumulation of charge, with a wire the charge leaks off.
Cheers
EDITED: Here is a rough drawing of the way I see it. The ground disturbance is produced by the transmitter which is designed not to radiate,
the terminal holds charge and is charged and discharged by displacement currents to the earth, ultimately caused by the exciting circuit (primary).
The receiver terminal is charged and discharged by displacement currents to the Earth as well but the cause of the current is the potential difference
between the terminal and the ground caused by the constantly varying potential of the ground disturbance. The output is via the "output" coil coupled to the receivers
oscillating coil. There is no voodoo about it. A non magnifying receiver would also work to utilize the potential difference but without magnification the potential is very low
and so not much current can be forced through the load. The phase of the receiver compared to the transmitter would depend on the distance to the receiver from the transmitter and the wavelength of the disturbance.
I neglected to draw the displacement current path on the "non" magnifying receiver. And the other lines I drew to represent the displacement currents are not meant to be an accurate depiction of the actual paths.
The drawing is not to scale in any way either, the transmitters/receivers would not be that big in relation the the planet.
The real beauty comes when travelling (walking) waves and/or beat frequencies are used.
And without an extremely good ground connection the current will face significant resistance in the surface before being able to reach the more conducting deeper layers of the Earth,
and so the energy would be dissipated quickly. The receiving apparatus should also have an extremely good grounding to bypass this higher resistance layer.
Tesla's Magnifying Transmitter
.