Low-loss Antenna System With Counterpoise Insulated From Earth

Abstract

An improved antenna system for transmitting and/or receiving electromagnetic waves, having a counterpoise insulated from the earth and in which the counterpoise is connected to one terminal of a transmitter or receiver through an inductance. The inductance is tuned in conjunction with the tuning of a conventional loading inductance coil so as to maximize the field strength radiated by the antenna into the far field. The power lost in the earth or other antenna supporting surface is unusually small, so that radiated power is large.

Description

This invention relates to an apparatus and method for radiating (or receiving) electromagnetic waves from an antenna. It is suitable both for antennas having mechanical dimensions that are small compared with a free space wavelength of the wave to be radiated and for those having dimensions that are large.

Electrically small antennas, for which the invention is especially suitable, often employ an umbrella-like system of conductors called a top hat connected to the top of a main vertical radiating element and extending outward or else outward and downward from the top center of the antenna toward a supporting surface which is often the earth or a surface of a vehicle. The top hat is a capacitive top structure that is employed in order that a standing wave of AC current flowing in the antenna will not be zero at the top center of the antenna but rather will have some larger value at the top center and will be zero only at the outer periphery or ends of the top hat. The current in the main vertical element is therefore larger than it would be without a top hat, and it radiates energy better.

It is also common practice to employ a counterpoise which is a plate or system of wires near the base of the main vertical element of an antenna and extending outward to a sufficiently great radius that the capacitance between the top hat and the earth or other supporting surface is as small as practical compared with the capacitance between the top hat and the counterpoise. The top hat and the counterpoise form two plates of a capacitor. The purpose of the counterpoise is to provide a surface on which the displacement current that originates on a charge on the top hat can terminate near the earth without having to terminate on the earth itself which often has conductivity inferior to that of the counterpoise and is therefore susceptible to power losses when electron current flows in it. Such power losses reduce the efficiency of the antenna so that part of the power output of the transmitter is wasted in supplying the losses and only the remainder is used for launching an electromagnetic wave into the intended propagating medium such as the air or free space. The losses that are reduced by the present invention are those that occur in portions of the earth that lie under and near the counterpoise.

The strength of the radiated electromagnetic wave from an electrically small antenna depends upon and increases with the current flowing in the main vertical element of the antenna. For this reason such antenna systems ordinarily employ an inductance coil called a loading coil connected in series with the transmitter and the main vertical element of the antenna having a valve appropriate to cause electrical resonance with a self-capacitance of the antenna. When the resulting series resonant circuit, comprising the loading coil and the capacitance from the vertical radiating element and top hat to counterpoise, is tuned so as to be electrically resonant at the frequency of excitation of the antenna, a greater alternating current flows in the antenna and in particular in the main vertical element thereof, whose current determines the field strength of radiated electromagnetic wave.

Many electrically large antennas do not require the top hat, and in those cases a distributed capacitance of the main vertical element is sufficient to resonate the antenna. Counterpoises are used by both electrically small and electrically large antennas, and the present invention is applicable to both but especially to the electrically small types.

In some systems of the prior art and in some versions of the present system, counterpoises are constructed of wires near the earth radiating outward from the center of the antenna and spaced sufficiently closely to each other as comapred with their height above the earth so that they effectively collect most of the displacement current (sometimes called electric field flux) that exists in the space between the top hat and the counterpoise and between the vertical radiating element and the counterpoise. Any displacement current that terminates on the earth instead of on the counterpoise ordinarily causes losses in the earth due to a flow of electron current that is induced in the earth by the alternations of displacement current.

It is not economical to increase beyond a certain number the number of counterpoise wires or, if a grid is employed, the closeness of the grid spacing, in order further to increase the share of displacement current that is captured by the counterpoise as compared with that captured by the earth beneath it, because of the additional cost of the extra counterpoise wires. Neither is it economical to increase the share of displacement current that the counterpoise takes by raising the counterpoise very high above the earth so as to change the ratio of (a) spacing between grid wires at any particular radius to (b) the distance from the grid wires to the earth, because raising the counterpoise shortens the effective portion of the main vertical element of the antenna and thereby reduces the radiated energy for any particular fixed value of antenna current. Design compromises are ordinarily made between the height of the counterpoise, the angular spacing between counterpoise wires, and the radius to which the counterpoise extends, in order sufficiently to conceal electrically the earth from the top hat by means of the counterpoise, so that the earth will not have excessive power losses.

The present invention reduces power losses caused by currents induced in the earth, by reducing the currents induced in the earth under the counterpoise.

An advantage of the present invention is that a counterpoise can have its wires somewhat wider spaced than in antennas of the prior art or somewhat closer to the ground and have the same degree of efficiency as systems of the prior art. Alternatively, for a given spacing betweeen wires and a given height of counterpoise above the ground, the antenna system of this invention is somewhat more efficient than those of the prior art and, therefore, radiates greater field strength for a given transmitter power than do prior systems.

Accordingly, it is a principal object of the present invention to provide a new and improved antenna structure which is capable of adjustment to reduce losses in the earth, or other supporting structure such as a vehicle, so as to increase the efficiency of the antenna system for radiating and receiving electromagnetic waves.

Another object of this invention is to provide a new and improved method of connecting and adjusting an antenna system so as to reduce wasted power and thereby to increase radiated field strength or receiving gain of the antenna.

Still another object of the present invention is to provide an antenna system which is convenient for use in various and differing settings, such as irregular earth surfaces and earth surfaces having very poor electrical conductivity as well as those with good conductivity, so that the antenna system can be portable and yet be efficient.

Still another object of the present invention is to maximize by means of the invented apparatus the ratio of the power transferred from an antenna to a desired radiation transmission medium and the power transferred from the same antenna to an undesired neighboring medium such as the earth.

Still another object is to minimize the losses in the earth or mounting vehicle or ship or other such mounting surface of an antenna used either for transmitting or for receiving electromagnetic waves.

A further object is to provide an apparatus and a method for isolating the effects of an antenna from its harmful environment such as a lossy earth despite the unavoidable existence of a significant partial capacitance to the harmful environment from components of the antenna structure.

Another object of the present invention is to provide an antenna system in which an impedance is connected in series with the counterpoise and adjusted so as to reduce the earth losses.

A further object of the present invention is to teach the use of an impedance to buffer a transmitter from an earth ground for the purpose of minimizing loss.

Still another object of the present system is to provide an antenna whose counterpoise structure is small and yet sufficient for achieving a required degree of efficiency of operation, hence, a counterpoise system that is economical to construct.

Although the drawings show only a transmitting application of the present invention, the objects of the invention include both transmitting and receiving applications. In receiving applications, the object of the tuning of the counterpoise network is preservation of available signal-to-noise ratio; reduction of the antenna losses is sought in a receiving application in order to maintain a good signal-to-noise ratio at the receiver's input terminals.

Other objects and features of the invention will become more apparent upon a consideration of the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is an elevational view of an electrically small embodiment of the inverted antenna and its ancillary equipment. FIG. 1 also shows some capacitance symbols which represent the partial capacitances from top hat to counterpoise, from counterpoise to earth, and from top hat to earth.

FIG. 2 is a simplified plan view showing only one of four quadrants of an embodiment of the present invention which is, as to this view, the same as the prior art. The top hat conductors, their insulators and the counterpoise conductors and their insulators are shown in FIG. 2.

FIG. 3 is a schematic electrical circuit diagram depicting the antenna system in an equivalent circuit form similar to an open-circuited artificial transmission line for purposes of explaining the operation of the invention.

FIG. 4 is a phasor diagram which illustrates the great amount of current flowing in a lossy earth which results from even a small phase imbalance.

A transmitting application will be described; receiving applications are similar by reciprocity.

An improved electrically small antenna system 10 constructed in accordance with the present invention is illustrated in FIG. 1. The particular embodiment shown is a top-loaded 25 foot high monopole antenna with 12 umbrella-like top hat conductors and 24 insulated counterpoise conductors which is portable and supported by an inflatable mast. The principal working component of the illustrated antenna is a main vertical element 12 in which an alternating current flows. The current causes radiation from the antenna structure 10 into the surrounding air, free space, water, or other medium of radiation transmission. The main vertical element is driven by a transmitter 14 having a first terminal 15, through a series-connected loading inductance coil 16. At the top of the main vertical element 12 are connected a plurality of top hat conductors 18 which collectively resemble the stays of an umbrella.

The top hat electrical conductors 18 terminate in insulators 20 at their outer ends and are supported from that point onward to the periphery of the antenna by non-conducting nylon support guys 22. The purpose of the non-conducting support guys is to hold the conducting top hat wires in place without interfering with radiation of energy from the main vertical element to regions outside the antenna.

A counterpoise structure 24 comprising radial electrically conductive wires is located below the top hat conductors and extends outward beyond the radius at which the top hat insulators 20 are located. All of the counterpoise conductors 24 are connected to each other near the center of the antenna structure. The counterpoise structure is not, however, connected at its center to the main vertical element 12, but is connected through a counterpoise variable inductor 26 to a second terminal 27 of the transmitter 14. At its outer end, each counterpoise conductor terminates on an insulator 29 which supports it. Thus, the counterpoise is electrically insulated from the earth 32 and is conductively connected to other elements of the system only through the counterpoise variable inductor 26.

The angular relationship of top hat conductors to each other and to the counterpoise conductors 24 can be seen in FIG. 2 which is a plan view of the antenna system. In the embodiment depicted here, there are twice as many counterpoise conductors 24 as there are top hat conductors 18; top hat conductors are located at 30.degree. intervals of the full circle and counterpoise conductors are located at 15.degree. intervals. It should be noted in FIG. 2 that the portions of the counterpoise conductors 24 which are closest to the ground, namely, those portions at the greater radii, are the portions which are farthest spaced from adjacent counterpoise conductors. This is one of the problems inherent in an antenna system of this type which is alleviated by the present invention. The wide spacing between counterpoise conductors at their outer ends together with the close spacing of the counterpoise conductors to the earth causes displacement current which flows from the top hat conductors downward to tend to go to the earth at some places between counterpoise conductors instead of terminating on the counterpoise conductors, which would be much more desirable from the standpoint of minimizing losses. In FIG. 2 it may be seen that the insulators 20 form the boundaries between electrically conductive top hat lines 18 and electrically non-conductive nylon support guys 22.

The antenna top hat structure 18 is resonated by adjusting the loading coil 16. This is a reactance tuning network which comprises a tapped coil in series with a continuously adjustable variable inductor, such as a variometer, for fine turning between discrete tap positions.

A second reactance network 26 is also tuned, so as to resonate with the counterpoise system 24. Both of the reactance tuning networks are adjusted so as to minimize the power losses and maximize the useful radiation from the antenna.

A distributed partial capacitance exists between the wires of the top hat 18 and the wires of the counterpoise 24. This distributed capacitance 28 is shown in FIG. 1 where it is represented as a lumped capacitance for simplicity of discussion. Another partial capacitance 30 exists from the top hat conductors 18 to the earth 32, capacitance 30 ordinarily being smaller in value than capacitance 28. A third distributed capacitance of importance in this system is the capacitance from the counterpoise wires 18 to the earth 32, this capacitance being shown in FIG. 1 for simplicity as a lumped capacitance 34.

The value of the loading inductance coil 16 is approximately such as to resonante at the frequency of excitation with the capacitance 28. The capacitance 30 could be small by comparison with capacitance 28, but capacitance 30 is nevertheless significant for other reasons as will be seen below. The inductance of the counterpoise variable inductor 26 is preferably of such value as to resonate at the electrical frequency of excitation with capacitance 34 existing from counterpoise wires 24 to the earth 32.

Resistor 36 is a static drain which serves to connect one terminal of the transmitter to earth ground.

When the antenna is used as a transmitting antenna, a transmitter drives current through inductor 16 and up the main vertical element 12 to the top hat 18. Conduction current flows out along the top hat conductors 18 but does not flow beyond the terminating insulators 20. From that point displacement current flows downward.

Almost all of the displacement current that flows from the top hat wires 18 and the top of the main vertical element 12 through distributed capacitance 28 terminates on the counterpoise wires 24. The return path for this current is electron conduction through the counterpoise wires 24 to the counterpoise inductor 26 and through it to the second terminal 27 of the transmitter 14.

When the loading coil 16 is properly adjusted, loading coil 16 resonates with the capacitance 28 and a great current flows in the series circuit comprising those two elements and the counterpoise 24 and the variable inductor 26. A reactive voltage drop is created across tuning coil 26 by the flow of current through coil 26. This determines the potential of the counterpoise with respect to terminal 27 of the transmitter. It is believed that when the system is properly adjusted, the potential of the counterpoise is such that displacement current flowing from the counterpoise 24 through capacitance 34 to earth is equal in magnitude but opposite in sign to the displacement current flowing from top hat 18 through capacitance 30 to earth. Consequently, capacitor 34 supplies the current demanded by the capacitor 30 and very little current need flow through the lossy earth under the counterpoise.

The large current flowing in the main vertical element 12 causes electromagnetic radiation, which travels radially away from the main vertical elements, continues through the annular opening between the top hat 18 and the counterpoise 24 below it, and propagates outward from the antenna as a whole. For the antenna described, the radiation is uniform in all azimuthal directions and is vertically polarized, having its electric field vector vertical and its magnetic field vector horizontal.

A procedure for tuning the antenna and counterpoise system to a specified frequency is as follows: First, the loading coil 16 is adjusted to a tap which is known to be appropriate for resonating with capacitance 28 at the frequency to be radiated. Second, the counterpoise tuning coil 26 is adjusted to a position at which coil 26 will be approximately resonant with capacitance 34 at the desired frequency. Third, the transmitter is operated so as to excite the antenna at the desired frequency, and the variometer which is part of loading network 16 is tuned for maximum output as indicated by a meter on the transmitter or in the far field. This procedure may be repeated if desired for finer tuning.

For one system which was tested and found to result in great improvement over the prior art, the adjustable loading coil 16 in series with the main vertical element has an inductance range of 0.17 to 1.6 millihenries. The variable inductance 26 in series with the insulated counterpoise has a range of adjustment of 0.04 to 0.2 millihenries. Capacitance from the main vertical element 12 and the top hat 18 to the counterpoise 24 for the antenna tested was approximately 320 picofarads. The approximate capacitance from the counterpoise structure 24 to ground 32 was 2440 picofarads. The static drain 36 was a 100 ohm resistor. This range of parameters permits tuning the antenna over the frequency range of 260 to 530 kilohertz. The antenna's effective input resistance ranges from approximately 7 ohms at 530 kilohertz to 10 ohms at 260 kilohertz.

It is necessary to treat the antenna as a distributed parameter system even through it is electrically small because the losses which it is sought to minimize arise in consequence of the distribution of the parameters. The capacitance from the top hat to the counterpoise, from the counterpoise to the earth, and from the top hat to the earth are distributed throughout a circular area centered on the antenna. Also, the resistivity of the earth or other support surface under the antenna causes power loss, due to currents of small magnitude, to be distributed throughout the earth under the counterpoise and somewhat beyond it. The operation of the invention can nevertheless be described by reference to a lumped parameter equivalent circuit diagram shown in FIG. 3 wherein the distributed parameters are shown as three groups of partial capacitances similar to those of an artificial transmission line. In FIG. 3, the top hat is shown as the upper line 18 of the transmission line. The counterpoise structure is shown as the lowest line 24 of that equivalent transmission line. The earth 32 is shown between lines 18 and 24 although mechanically the counterpoise is not located there unless it is buried underground, which it may be in some cases. The line 32 representing the earth has series resistors to symbolize that it is a lossy path. The lines 18 and 24 are shown as lossless because they are good conductors; no resistances are included in their equivalent circuit because they would be negligibly small for purposes of this discussion.

A transmitter 14 connects to the input terminals of this equivalent transmission line, which is a tapered line because of the variation in values of the parameters with varying radius from the center of the structure. Current driven into the transmission line by the transmitter passes through the loading coil 16 and out to various partial capacitances 28a, 28b, 28c in the equivalent circuit. These capacitors represent the distributed capacitance between top hat 18 and counterpoise 24, and have a total value equal to that of capacitance 28.

Distributed capacitance between the counterpoise 24 and earth 32 is designated in FIG. 3 as 34a, 34b and 34c. Capacitance from top hat 18 directly to the earth 32 is represented by capacitors 30a, 30b and 30c. A main current flows from the transmitter 14 through loading coil 16, then through capacitors 28a, 28b and 28c to the counterpoise 24 and then through variable inductor 26 and back to the transmitter. The transmission lines 18 for top hat and 24 for counterpoise are not equally balanced electrically to earth 32; instead, they have a relationship based on a ratio of the capacitance 30 to the capacitance 34. This relationship to earth is established by the unavoidabe presence of the capacitance 30 and the capacitance 34. Inductor 26 permits the counterpoise 24 to assume a voltage corresponding to this asymmetrical relationship to earth of the lines 18 and 24. It is believed that no voltage of appropriate value could develop on line 24, the counterpoise, if inductor 26 were replaced by a short circuit to ground.

In the antenna of this invention, the ratio of the inductive reactance of loading coil 16 to the capacitive reactance of capacitance 30 is approximately equal to the ratio of the inductive reactance of counterpoise tuning coil 26 to the capacitive reactance of capacitance 34. Thus, the inclusion of tuning coil 26 permits current from the counterpoise to earth to be controlled in the area under the counterpoise.

Failure to control the potential of line 24 would cause circulating currents to flow through the partial capacitances 30a, 30b, 30c and 34a, 34b and 34c into the earth, whose resistivity is symbolized by the resistances 38a, 38b, 38c, 38d, 38e, 38f. In the absence of inductor 26, circulating currents would exist in the earth.

The potential of counterpoise 24 is controlled with respect to earth in the present invention. The counterpoise 24 is not symmetrically disposed to earth as compared with top hat 18; that is, the earth's potential is asymmetrically related to those of the top hat 18 and the counterpoise 24 but the counterpoise potential is maintained so as to minimize the flow of circulating currents in the earth. It is a flow of these earth currents that would otherwise cause heating loss in the earth and which would reduce the efficiency of the antenna. FIG. 4 is included to show how in the absence of an inductor such as 26 a small phase imbalance 42 in two phasor currents 44 and 46 induced in the earth by capacitances 30 and 34, respectively, causes a significant imbalance current 48 to flow in the lossy earth.

The antenna system of this invention can be used equally well for receiving radio waves as for transmitting them. When used as a receiving antenna, the signal-to-noise ratio is improved by comparison with antennas of the prior art because the usable signal-to-noise ratio depends upon the power losses in the receiving antenna, and smaller losses correspond to greater usable signal-to-noise ratio.

Although this invention has been described by reference to one particular embodiment thereof, it is clear that various other antennas may be constructed that utilize the inventive concept described herein and it is intended that such variations be included in the scope of this patent. One such variation is to control the alternating potential of the counterpoise by connecting a source of alternating voltage to the counterpoise instead of using the inductor 26. The source of alternating voltage could then be synchronized with the transmitter with an appropriate phase relationship thereto, by conventional techniques, in order to minimize earth losses.

Both electrically large and electrically small antennas are included.

While the invention relates to antennas of all electrical size, it is particularly useful for those whose mechanical dimensions are small in terms of a wavelength of frequency of excitation, this class being known as electrically small antennas.

Claims

What is claimed is:

1. In an antenna system for use upon an electrically lossy supporting material and employing a superstructure having a substantially non-uniform distribution of capacitance per unit area to the material, apparatus comprising first and second antenna terminals for connection to a source of AC excitation, a central support device extending upwardly away from the lossy material, an elevated conductive superstructure connected with said central support device and sloping radially outward and downward therefrom toward the lossy material to define an annular opening between the superstructure and the lossy material, different portions of said superstructure at different radial distances from said support device having substantially different capacitances per unit area of superstructure to respective portions of the lossy material, a first inductance connected in a series circuit extending from said first antenna terminal to a point on said superstructure approximately at said central support device, a conductive counterpoise structure located closer to the lossy material than is said superstructure and sloping radially outward and downward from said central support device toward the lossy material and insulated from the lossy material, different portions of said counterpoise structure at different radial distances from said central support device having substantially different capacitances per unit area of counterpoise structure to said respective portions of the lossy material, a second inductance connected between said second antenna terminal and the approximate center of said counterpoise structure, and a conductive path connected between said second terminal and the lossy material, and wherein, for the lossy material that is generally under said superstructure, said substantially different capacitance per unit area of counterpoise structure is proportioned to said substantially different capacitance per unit area of superstructure in a ratio that is approximately the same ratio for each and every said portion of the lossy material.

2. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, apparatus as defined in claim 1 and wherein said same ratio for each and every portion of the lossy material is approximately equal to a ratio of said first inductance to said second inductance.

3. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, apparatus as defined in claim 1 and further comprising a plurality of insulating lines each having one end fixedly connected with the lossy support material, and wherein said elevated main conductive superstructure comprises a plurality of conductors each extending from said central support device to the other end of one of said insulating lines respectively, and wherein said conductive path comprises a resistance.

4. In an antenna system for use upon an electrically lossy supporting material and employing a superstructure having a substantially non-uniform distribtion of capacitance per unit area to the lossy material, apparatus comprising mechanical spacing means connected with the lossy material for establishing a distance from said lossy material, a main conductive superstructure connected with said mechanical spacing means and spaced away from said lossy material thereby for producing electromagnetic waves, different portions of said superstructure being capacitively associated respectively with different portions of the lossy material to form distributed capacitances, said different portions of said superstructure having substantially different capacitances per unit area of superstructure to said respective portions of the lossy material, conductive counterpoise means disposed intermediate said main conductive superstructure and said lossy material for forming distributed capacitances between said superstructure and said said counterpoise means, different portions of said counterpoise means also being associated respectively with said different portions of the lossy material to form distributed capacitances, the capacitance per unit area between said different portions of said counterpoise means and said respective different portions of the lossy material being of substantially different capacitance values, each of said substantially different capacitances per unit area of counterpoise means being proportional to said substantially different capacitance per unit area of superstructure for the same portion of said lossy material in a common ratio that is substantially equal for each of said different portions of the lossy material, means for energizing said main superstructure with a first AC signal at a point on said superstructure to produce first displacement currents flowing from each portion of said superstructure to the respective portion of the lossy material, and means for energizing said counterpoise means with second AC signal at a point on said counterpoise means to produce second displacement currents flowing from said counterpoise means to each of said portions of said lossy material, and wherein for each portion of said lossy material said first displacement current is equal in magnitude and of opposite phase to said second displacement current.

5. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, apparatus as defined in claim 4 and wherein said means for engaging said main superstructure with a first AC signal comprises an AC source and first phase shift means connected intermediate said AC source and said point on the superstructure for shifting the phase of said first AC signal, and wherein said means for energizing said counterpoise means comprises said AC source and second phase shift means connected intermediate said AC source and said point on the counterpoise means for shifting the phase of said second AC signal.

6. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, apparatus as defined in claim 5 and wherein said first phase shift means comprises a first inductance and said second phase shift means comprises a second inductance.

7. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, apparatus as defined in claim 6 and wherein said first and second inductances are connected with different terminals of said AC source having signals phased 180.degree. apart, and wherein an inductance ratio of said first inductance to said second inductance is approximately equal to said common ratio.

8. In an antenna system for use upon an electrically lossy supporting material and employing a superstructure having a substantially non-uniform distribution of capacitance per unit area to the lossy material, a method for reducing electrical losses from currents induced in the lossy material, comprising the steps of providing a counterpoise between the superstructure and the lossy material having a substantially non-uniform distribution of capacitance to the lossy material that, for each portion of the lossy material which capacitively communicates with the superstructure, is proportioned by a constant ratio to the non-uniform capacitance between the superstructure and the lossy material, exciting the superstructure at a first point with a first AC signal, and exciting the counterpoise at a second point with a second AC signal, said AC signals being established relative to a potential of said lossy material, and adjusting the relative phases and amplitudes of said first and second AC signals to maximum antenna efficiency.

9. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, a method as defined in claim 8 and wherein exciting the superstructure at a first point comprises exciting said first point from one terminal of an AC source through a first series inductance, and wherein exciting the counterpoise at a second point comprises exciting said second point from another terminal of the AC source through a second series inductance, and wherein adjusting the relative phases and amplitudes comprises establishing relative values of said first and second inductances in approximate proportion to said constant ratio.

10. In an antenna system employing a superstructure having a substantially non-uniform distribution of capacitance per unit area, a method as defined in claim 9 and further comprising a step of providing a resistive current path from said other terminal of the AC source to said lossy material.