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A Comparison of the Tesla and Marconi Low-Frequency Wireless Systems 

What is the difference between present day low-frequency wireless transmitters and Tesla's system?  When used as a wireless transmitter how does the performance/efficiency and characteristics of an electrical oscillator change when working with a sphere capacitor antenna, rather than working with a conventional monopole antenna?

In answering this question, three basic forms of wireless telecommunications antennas or launching structures are considered.  All three consist of an electric dipole excited with an identical impedance matched high voltage radio frequency power supply.  The first is the Hertz antenna, a vertical 1/2-wave dipole antenna, center fed, positioned 1/2 wavelength above the ground.  Needless to say, this is not a practical configuration at low frequencies.  Next is the Marconi antenna, a vertical 1/4-wave monopole antenna element with its lower end at ground level.  A vertical conductor with no loading coil and no capacitive top loading is assumed.  It is fed at its base by the standard RF power supply plus an appropriate matching section, with the opposing terminal grounded.  Third is the Tesla launching structure, a vertical high aspect-ratio 1/4-wave helical resonator with large capacitive top loading and small overall height as compared to the electrical 1/4 wavelength.  The 1/4-wave resonator is base fed by the standard RF power supply plus an appropriate matching section, with the opposing terminal grounded.  In the second and third cases a ground connection is used, and this must be constructed in such a way as to introduce the least possible resistance to ground.

Comparing these three configurations it is assumed the Hertz antenna, a physical embodiment of an electric dipole in free space, approaches an ideal source of electromagnetic radiation emitted in the form of space waves.  These space waves can reach the receiver either by ground-wave propagation or by reflection from the ionosphere, known as sky-wave propagation.  Sky-wave propagation will not be discussed here.  

The Marconi antenna is a modified 1/2-wave Hertz antenna.  It is adapted to the real-world conditions encountered in the construction of low frequency transmitters.  These adaptations are imposed by the wavelength involved and the resulting physical dimensions required of the antenna.  The dipole antenna is modified in that its lower half, 1/4 wavelength long, exists only as a mirror image of its upper counterpart.  The resulting 1/4-wave vertical monopole antenna takes advantage of the fact that at low frequencies the ground acts as a mirror for the radiated energy.  The ground reflects a large amount of the energy that is radiated downward from the antenna mounted over it.  In the physical construction of the ground connection is important to have as high a conductivity as possible.   The object is to provide the best possible reflecting surface for the downward radiated energy from the antenna.  The ground consists of a number of bare conductors arranged radially and connected, 1/2 wavelength long, buried a short distance beneath the earth's surface.  In practice these conductors may act as part of the reflecting surface as well as making the connection to ground itself.   An alternative type of ground is the counterpoise.   It is a wire structure erected a short distance above the ground, and insulated from the ground.  The counterpoise operates by virtue of its capacitance to the ground.  Not unlike the Hertz antenna, the Marconi antenna is a source electromagnetic radiation in the form of space waves.  Typically, these waves, that is to say the ground waves, take a direct or reflected path from the transmitter to the receiver.  They may also be guided by the earth's surface as a ground-hugging Norton surface wave.  The direct-wave component of the ground wave is limited only by the distance to the horizon from the transmitter plus a small distance added by atmospheric diffraction around the curvature of the earth.  The ground-reflected component is the portion of the radiated wave that reaches the receiving antenna after being reflected from the Earth's surface.  Prevailing wave propagation theory teaches that the surface-wave component is wholly the result of electrical currents induced in the ground by refraction of a portion of the reflected-wave component.  Upon reflection from the Earth's surface the reflected wave undergoes a 180deg phase reversal.  When both transmitting and receiving antennas are on, or close to, the ground, and the distance between them approaches the above-described limit, the direct and reflected components tend to cancel out, and the resulting distant field intensity is principally that of the surface wave.  Because part of its energy is absorbed by the ground, the electrical intensity of the surface wave is attenuated at a much greater rate than inversely as the distance.  It is the conductivity of the underlying terrain that determines the attenuation of the surface-wave field intensity as a function of distance.  The ground currents of a vertically polarized surface wave do not short-circuit a given electric field but rather serve to restore part of the used energy to the following field.  The better the conducting surface, the more energy returned and the less energy absorbed.  [Antennas and Radio Propagation, TM 11-666, Dept. of the Army, Feb. 1953, pp. 17-23.]

Of course the Tesla launching structure is also part of an electric dipole, consisting of the elevated capacitance, the helical resonator plus connections, and the Earth itself.  The above-ground portion is not intended as a source of electromagnetic radiation, rather, it is designed to minimize the production of electromagnetic radiation.  [The working of the structure's helical resonator may be associated with a transverse magnetic wave. [Corum and Corum] and with an interaction with the Earth's magnetic field [Papadopoulos.]  The principle that the ground acts as a mirror, which reflects electromagnetic energy radiated downward by the antenna mounted over it, is not applicable.  In operation, the Tesla launching structure induces ground currents in the earth along with an associated surface wave (this may be similar to the Zenneck surface wave) which propagate the transmitted energy.  At the Wardenclyffe facility the ground connection consisted of a 300-foot long vertical pipe driven downward from the bottom of a 120-foot deep shaft, placing the maximum depth of the installation beneath the earth's surface at 420 feet.  A conducting path is also establish through the rarified upper level atmosphere between the transmitting and receiving stations elevated high voltage terminals, leading to the name "air-ground system."  Tesla clearly stated that his system used conduction and that energy escapes from the system in the form of electromagnetic radiation.  The conducting media are the earth below and the atmosphere above 5 miles elevation.  While the region from 5 miles up to the ionosphere is not an ohmic conductor, the density or pressure is sufficiently reduced to so that, according to Tesla—s theory, the atmosphere—s insulating properties can be easily impaired allowing an electric current to flow.  His theory further suggests that the conducting region is developed through the process of atmospheric ionization, shifting the effected portions thereof to a plasma state.  A magnetic field is developed by each plant—s helical resonator, meaning that an embedded magnetic field is also involved.  The atmosphere below 5 miles is also viewed as a propagating medium for a portion of the above ground circuit, and being an insulating medium, electrostatic induction or so called —displacement currents— would be involved rather than true electrical conduction.  Tesla felt that with a sufficiently high electrical potential on the elevated terminal the practical limitation imposed upon its height could be overcome.  He anticipated that a highly energetic transmitter, as was intended at Wardenclyffe, would charge the elevated terminal to the point where the atmosphere around and above it would break down and become ionized, (see U.S. Patent No. 645,576, —System of Transmission of Electrical Energy—) leading to a flow of true conduction currents between the two terminals through the troposphere path connection. 

Assuming individually optimized RF power supplies and grounding systems, the only other difference between the Marconi antenna and the above-ground portion of the Tesla launching structure is in the geometry.  Using a frequency of, say, 25 kHz, a idealized quarter-wave Marconi-type antenna would consist of a vertical conductor extending about 9,750 feet above the earth's surface.  A Tesla-type launching structure for the same frequency would be much shorter, the bottom third or so consisting of a helical resonator followed by a relatively large conducting cylinder connected to a spherical or torriod-shaped terminal of large surface area.

The problem is to characterize the performance of these two different structures in response to the application of the rapidly varying alternating current.  In the first case, antenna theory indicates that with proper coupling between the transmission line and the antenna, the structure will be an efficient radiator of electromagnetic energy.  Because the velocity of the electric current in the conductor is finite, it takes some time for the applied charge to build up on the antenna.  The electric field follows the charges moving along the monopole antenna and the lines tend to spread out toward the position they would occupy under static charge conditions.  During the next quarter cycle, the monopole is discharged and some of the field lines break away to form closed loops.  Energy continues to propagate out into space as long as there is excitation.   The implication here is that energy is irretrievably lost from the monopole.  This lost energy exists in the form of electromagnetic radiation, that is to say, radio waves.  Along with the field energy lost or radiated by the monopole, a certain fraction of the energy returns to the vertical conductor during each RF cycle.  Consequently, it might be said that the fields near the antenna represent both energy storage and radiation components, with the storage component falling off as the distance increases. [The Radio Amateurs Handbook, ARRL, 1978, Chapter 21 -- Radiation and Antennas, p. 588]

In the case of the Tesla-type launching structure it appears the delay effect responsible for the dissipating radiation of energy, such as manifested with a quarter-wave monopole, is reduced and the stored-energy component of the electric field is increased.  While the amount of time expended to charge the structure remains the same as with the Marconi antenna, the overall distance between the bottom feed-point and the structure's upper extremity is much smaller.  For example, if the structure were to be 500 feet in height, when compared with the monopole the greatest overall distance the wave disturbance or a point of charge could move would be in the order of 5 percent.  The field throughout space would follow the charge movements more efficiently.  This implies that once the polarity of the RF source reversed a greater proportion of the energy in the field would return to the transmitting element and electromagnetic radiation would be suppressed.  Much of the RF energy, which in the case of the Marconi antenna is dissipated in the form of electromagnetic radiation, is physically retained within the oscillating system.    

A considerable expenditure might be expected for the complete antenna structure of a conventional long wave wireless facility—if it is to perform efficiently.  Installing the grounding system would not be much of a problem.  Somewhat the opposite would be the case in the construction of a proper magnifying transmitter.  Here is a little of what Tesla had to say about this:

"You see the underground work is one of the most expensive parts of the tower. In this system that I have invented it is necessary for the machine to get a grip of the earth, otherwise it cannot shake the earth.  It has to have a grip on the earth so that the whole of this globe can quiver, and to do that it is necessary to carry out a very expensive construction. I had in fact invented special machines. . . ."

In conclusion, it appears fundamentally identical electrical oscillators consisting of an RF power supply, an elevated conductor and a robust ground connection can be configured in ways which are conducive to the production of two different types of surface wave.  It is proposed that low frequency wireless communications can be accomplished by the production of either electromagnetic radiation in the form of space wave induced ground currents and an accompanying electromagnetic wave called the Norton surface wave, or the generation of a pulsed magnetic field and production of ground currents flowing between the transmitter and the receiver resulting in an accompanying trapped electromagnetic surface-wave bearing a resemblance to the Zenneck surface wave.  Once again to quote Tesla,

"From my circuit you can get either electromagnetic waves, 90 percent of electromagnetic waves if you like, and 10 percent in the current energy that passes through the earth. Or, you can reverse the process and get 10 percent of the energy in electromagnetic waves and 90 percent in energy of the current that passes through the earth.

"It is just like this: I have invented a knife.  The knife can cut with the sharp edge.  I tell the man who applies my invention, you must cut with the sharp edge.  I know perfectly well you can cut butter with the blunt edge, but my knife is not intended for this.  You must not make the antenna give off 90 percent in electromagnetic and 10 percent in current waves, because the electromagnetic waves are lost by the time you are a few arcs around the planet, while the current travels to the uttermost distance of the globe and can be recovered.

"This view, by the way, is now confirmed.  Note, for instance, the mathematical treatise of Sommerfeld, ["Propagation of Waves in Wireless Telegraphy," Arnold N. Sommerfeld, Ann. Phys. (Leipzig), 28, 1909, pp. 665-737.] who shows that my theory is correct, that I was right in my explanations of the phenomena, and that the profession was completely misled.  This is the reason why these followers of mine in high frequency currents have made a mistake.  They wanted to make high frequency alternators of 200,000 cycles with the idea that they would produce electromagnetic waves, 90 percent in electromagnetic waves and the rest in current energy.  I only used low alternations, and I produced 90 percent in current energy and only 10 percent in electromagnetic waves, which are wasted, and that is why I got my results."  (Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony and Transmission of Power)

Revised: 05/07/2004 

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