The key elements of worldwide wireless power transfer consist of:
4. system considerations
5. environmental impact
6. economic viability
Each of these will now be explained in more detail, along with their subgroups.
The source/transmitter, consisting of Tesla's Magnifying Transmitter is the most highly developed elements of the system, as evidenced by the standard terminology of "wireless power transmission". In this paper, the term "transfer" emphasizes the importance of other system elements as well. The Tesla Coil is remarkable efficient power processing element, and Corum and Corum have shown that Tesla's Colorado Springs Transmitter operated a power levels high by even modern standards, with peak average power levels some four orders or magnitude higher that those of the Stanford Linear Accelerator. 
Earth-Electrosphere/Ionoshpere Cavity with Dielectric Atmosphere
The path comprises the earth (ground) and the atmosphere. The ground is a good conductor at lower frequencies, conductivity decreasing with frequency due to the skin effect. The lower atmosphere is normally a good insulator. At higher altitudes the air becomes conductive due to ionization casued by cosmic rays. The conductive layer, termed the electrosphere,  provides an electrostatic shield and an equipotential surface due to its high conductivity relative to the ambient currents. Lord Kelvin, in 1860  originally postulated the existence of such a conductive layer based upon the fact that rarefied gases act as good conductors, and he thus postulated that this conductive layer together with the earth and intervening insulating atmosphere forms a capacitor. The potential of the electrosphere is about 300 KV. The ionosphere, located above the electrosphere, is caused by ionizing solar radiation, different ionospheric layers (D,E,F) being attributed to different components of the radiation. The ionosphere is that part of the earth's atmosphere which reflects radio waves [54,56] . The properties of the path are normally measured under conditions (voltage, current, frequency) quite different from those expected for wireless power transfer, and this should be considered before drawing conclusions on the suitability of the path for such purposes. Also, the effects of weather on conductivity and the effects of magnetic storms must be considered.
The spherical cavity between the ground and the ionosphere resonates at specific modes as predicted by Schumman [57,58] and discussed by Wait and Galejs. The transverse electric field mode (TE) is cutoff below 1.5 KHz, so for the ELF frequencies normally considered for wireless power transfer, the cavity will only support transverse magnetic TM waves, . The first seven Schumann resonances are naturally excited by lightning and this fact has been used to track lightning strikes around the globe. [61-67]. The polarization and ellipticity of the waves vary diurnally. Waves propagating in the cavity are attenuated with distance due to the finite conductiveness of the conductive and dialectric layers, and the attenuation increase exponentially with frequency, increasing from 0.25 dB/Mm at 10 Hz to 20 dB/Mm at 1 KHz. (compared with 1.15 dB/Mm for a conventional 200KV power line . Tesla has indicated that very little power is required to maintain a state of resonance in the cavity .
The key issue in wireless power transfer is how to couple power into and out of the cavity with minimal, or at least acceptable loss. Corum and Corum have indicated that Tesla more likely created the necessary current moments to excite the cavity by electrostatically charging an isolated capacitance at RF rates via a single electrode x-ray tube and then suddenly discharging this capacitance at a resonant frequency of the cavity [20-21]. They reported that the currents measured by Tesla would have been sufficient to generate relatively weak ELF global field strengths . Tesla noted that the discharge tended to pass upward away from ground, which he attributed to either electrostatic repulsion, or convection of the heated air. However, with such an electrically short tower, radiation into the cavity at cavity resonant frequencies would not be sufficiently efficient for technical or commercial viability. And while a resonating cavity would have purely reactive fields, and hence zero point radiation resistance together with non-stationary fields would be required for power transfer within the cavity. A radiative coupling approach appears to be infeasible for reasons stated above by Marincic.
A second method for coupling power into the cavity would be via direct conduction/displacement with the conductive surfaces of the waveguide, which appears to be Tesla's original concept dating back to 1892. Several mechanisms could be considered as follows: 1) Recall that, in 1900, he proposed using balloons at 30-35 thousand feet of elevation. Conceivably the power could be conducted to these via an ionization path, created by a single electrode x-ray tube driven by the transmitter. 2) The conducting path formed by ionizing radiation might be used to couple directly into the electro sphere without the elevated conductive sphere. 3) An approach might also be borrowed from those used in present ionospheric modifications experiments . 4) Perhaps with the extremely high operating voltages that Tesla had proposed, the displacement coupling with the atmospheric conduction path would be direct, as apparent from an artist's rendition of wireless power distribution from Tesla's Wardenclyffe facility , Tesla originally indicated that the atmosphere could be made conductive at lower elevations with either high voltage or high frequency so this should be studied further. . With such a direct coupling approach, the power transfer mechanism would then be a spherical "transmission line", rather than a spherical wave guide.
The ground currents in Tesla's Colorado Springs experiments were reported to have caused sparks within the ground, and to have shocked horses through their metal shoes within 1/2 mile from his transmitter. . As an aside, ground currents were separately exploited for communications during WW I, when conversations over the then prevalent single wire telephone systems were susceptible to enemy interception by differentially amplifying the signals extracted from two separate and displaced ground plates. The phenomenon of magnetospheric plasma whistler waves was first noticed with these receivers, but was not identified until later .
Power loss can occur in all elements of the path, which have finite conductivity: the ground, the dielectric lower atmosphere, and the conductive upper atmosphere. Elaborate and extensive ground planes are often constructed with antenna systems in order to minimize resistive power loss to the ground. Since the ground is an intrinsic conductive element, losses are inevitable, but can be reduced by operating at lower frequencies and/or establishing distributed area contacts at the transmitter and receiver sites. The poor conductivity of the Colorado Springs soil appears to have caused Tesla some difficulty . At Wardenclyffe, Tesla was planning to use saltwater filled with viaducts under the transmitter to establish a good ground connection. Similar to the ground, atmospheric losses can be reduced by operating at lower frequencies. This appears to conflict with Tesla's notion that gases conduct better at high frequencies, but could be explained by higher dialectric losses. One important feature to the wireless system is the possibility of storing power in the resonating fields within the earth-ionosphere cavity, however, the feasibility of doing this will be dependent upon the Q of the cavity and upon the relative amount of excess power being stored therein. As Tesla had indicated, the power losses are reduced with higher operating voltage since power would then be distributed at lower current levels. Precipitation can dramatically change the conductivity of the atmosphere, and the effects of this on power coupling need to be considered further.
The receiver is the least understood element of the system, and one that is most crucial to the system's success. For system using a radiative coupling mechanism, an antenna's efficiency and size both benefit from higher operating frequencies which, as noted above, increased the system's path losses. A transmission line approach would require conductive/displacement coupling into the electrosphere, which requires invention and development.
Tesla expressed confidence in being able to extract power for both individual and home use as well as for powering ground and air transportation vehicles, as illustrated in an artist's rendition . He indicated in patent 649,621: "Obviously the receiving coils, transformer, or other apparatus may be movable - at, for instance, when they are carried by a vessel floating in the air or by a ship at sea. In the former case the connection of one terminal of the receiving apparatus to the ground might not be permanent, but might be intermittently or inductively established without departing from the spirit of my invention. IT is to be noted here that the phenomenon here involved in the transmission of electrical energy is one of true conduction and is not to be confounded with the phenomenon of electrical radiation which have heretofore been observed and which from the very nature and mode of propagation would render practically impossible the transmission of any appreciable amount of energy to such distances as are of practical importance .
Tesla separately described the utilization of energy from ionized air, in connection with his description of the art of telautomatics; "Most generally I employed receiving circuits in the form of loops, including condensers, because the discharges of my high-tension transmitter, ionized the air in the hall so that even a very small aerial would draw electricity from the surrounding atmosphere for hours. Just to give an idea, I found for instance, that a bulb 12 inches in diameter, highly exhausted, and with one single terminal to which a short wire was attached, would deliver well on to one thousand successive flashes before all charge of the air in the laboratory was neutralized..." 
A wireless system would entail a multiplicity of transmitters and receivers each coupling into a common propagation and storage cavity, each requiring proper phasing and balance.
A wireless power system would expose the entire biosphere to ELF fields of varying intensity. The 78 Hz Seafarer/Sanguine/ELF submarine communication system provoked health concerns, as do high-tension power lines. The fields of wireless and wire-based power transmission systems need to be compared for equivalent power levels. There is much speculation about the adverse effects of magnetic fields on health. However, recent reports from PACE indicate that ELF energy at the lower Schumman resonance frequencies constitute a natural biological clock . The first four Schumman resonances frequencies are within the range of brain wave activity. The fundamental mode is coincident with the theta wave spectyrum, which ranges from 4 to 8 Hz, and is attributed to a normally unconscious state with enhanced mental energy and a high level of creativity.  The next three Schumma modes are coincident with the beta wave spectrum which ranges from 13 to 26 Hz, and is associated with the normal conscious state.
Operating at high voltages and surrounded by a glow discharge, the transmitter could be a source of pollutants, including ozone, NO and nitric acid, as reported by Tesla during his experiments and steps would have to be taken to mitigate any such hazards if they exist.
Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI)
The operating frequencies of a wireless system could be expected to be low enough so as to not interfer with present communications of electronic systems. The FCC does not make frequency allocations below 9Khz and Tesla had predicted the operating frequency to be below 20 Khz. Circuit interrupters in conventional Tesla coils could be expected to create a significant amount of wide-band EMI; however, modern transmitters could be expected to utilize more advanced switching devices which, together with shielding, could minimize radiated EMI/RFI. The glow discharge surrounding the high transmitter could also be a source of EMI/RFI.
Since the potential of the electrosphere is about 300 KV relative to the earth, and the wireless system as proposed by Tesla was designed to operate at 30-100MV, there is a significant potential for electrically disturbing the atmosphere. It is not know whether this would be beneficial or harmful. Vonnegut  has suggested that the destructive effects of tornadoes may result from atmospheric electrical effects; however, Wilkins  concluded from laboratory model vortex experiments that the electrical effects were the effect, rather than the cause, of tornadoes.
Given technical feasibility and safety, the wireless power transfer system must still be economically viable in order to succeed. Multiple transmitter could conceivably be phased to control the location of antinodes form which power could be extracted, however, this could be at best, a short term solution, unless wireless is constrained to a relatively few large scale facilities that will be expensive and technically difficult to construct. The worldwide regulation and control of wireless power distribution will be difficult if physically constrained to operate at selected resonant frequencies.
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