US 2129711 A
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Sept. 13, 1938. G. C. SOUTHWORTH GUIDED TRANSMISSION OF ULTRA HIGH FREQUENCY WAVES '7 Sheets--Sheei'l l Filed March 16, 1933 -E-m-ez-m-m- :1 .mug
fr?" 4" lll/EzNgoR G. C. 50mi/Malik ATTORNEY Sept 13, 1938 G'. c. souTHwoRTH l2,129,711-
GUIDED TRANSMISSION OF ULTRA HIGH FREQUENTIV WAVES Filed arch 16, 1933 l 7 Sheets-Sheet 2 A'YTORNEY Sept.V 13, 1938. G. c. soUTHwoRTH GUIDED TRANSMISSION OF' ULTRA HIGH FREQUENCY WAVES Filed March 16, 1933 7 Sheets-Sheet 5 'oull'ce of Casel ATTORNEY Sept. 13, 1938. G. C. soUTHwoRTH 2,129,711
GUIDED TRANSMISSION 0F ULTRA HIGH FREQUENCY WAVES Filed March 16, 1933 7 Sheets-Sheet 4 Fama@ wles 0f ,mgl'utable length,
Shielded Conductors fljfower and JLgnaZ fflou Shielded sclator /-aJ/0/087' .Plate l 66 'Glass -co/per Sends )6 ATTORNEY Sept. 13, 1938. G. c. soUTHwoRTH 2,129,711
GUIDED TRANSMISSION 0F ULTRA HIGH FREQUENCY -WAVES Filed March 16, 1933 '7 Sheets-Shes*I 5 87 L Vig/mane I?. 25 ha@ INVENTOR BY W ATTORNEY Sept. 13, 1938.
G. c. souTHwoRTH 2,129,711
GUIDED TRANSMISSION OF ULTRA HIGH FREQUENCY WAVE'S Filed llarci 16, 1933 '7 Sheets-Sheet 6 (I Il INVENTOR G. C. Sauthwvl/v 6 ATTORNEY Sept. 13, 1938. G. c. souTHwoRTH 2,129,711
GUIDED TRANSMISSION OF ULTRA HIGH FREQUENCY WAVES F-ild arch 16, 1955 7 She'ebS-Sheet 7 INVENTOR 6'. C. Soudan/waa@ Piuma sept. 1s, 193s `UNITED STATES PATENT OFFICE GUIDED TRANSMISSION F HIGH FREQUENCY WAVES Application March 16, -1933, Serial No. 681,154
nomma An object of my invention is to provide a new and improved system for the transmission of electrical effects from one place to another place at a distance therefrom by means of electromagnetic waves associated with adielectric guide extending between the two places. Another object of my invention is to provide for signaling along such a guide by means of such waves. Another object is to provide for the generation of high frequency 0 electric conduction currents in a suitable medium and the application of their energy to generate corresponding "displacement current waves for transmission along a guide of dielectric material.
An object complementary to the foregoing is to 5 provide for the translation of the energy of received displacement currents in a dielectric guide into conduction currents in associated receiving apparatus. Still another object is to provide suitable apparatus and a proper method so that elec- 0 tric waves may be transmitted along a dielectric guide without excessive dissipation of their energy in the guide or in the medium adjacent thereto. In some examples of my invention the guide employed may be partly dielectric and partly conductive.
All these objects and various other objects and advantages of my invention will .become apparent on consideration of a limited number of examples of practice in accordance with the invention which I have chosen for presentation in this speciiication. It will be understood that the following disclosure relates principally to these particular examples of the practice of the invention and certain scientific principles involved in such practice, and that the scope of theinvention will be indicated in the appended claims.
Referring to the drawings, Figure 1 is a simpli fied diagram of a system by which my invention may be practiced; Fig. 2 is a perspective diagram showing a coupling at one end of the dielectric guide that may be employed in practicing my invention; Fig. 3 is a corresponding Sectional eleva tion; Fig. 4 is a set of curve diagrams giving electric and magnetic intensities for various frequencies in and about the dielectric guide; Fig. 5 is a curve diagram for wave slowness as a function of the wave length in relation to the radius of the dielectric guide; Fig. 6 is a set of diagrams showing wave shapes in an all-dielectric guide for diiierent wave lengths; Fig. 7 is a perspective diagram showing apparatus for the generation of higher order waves of shorter wave length; Fig. 8 is a sectional diagram of apparatus for demonstrating the laws of guided dielectric waves by means of standing waves; Fig. 9 is a detail section von the line 9 of Fig. 8; Fig. 10 is a diagrammatic sectional view of certain testing apparatus for the standing waves in connection with Fig. 8; Fig. 11 is a curve diagram that extends the showing of Fig. 5 to different diameters 5 of wave guides and to the case of composite wave guides each made up of a dielectric body surrounded by. a metallic sheath; Fig. 12 is a set of diagrams for such a composite wave guide corresponding otherwise with Fig. 6; Fig. 13 is a dial0 gram of sending end signaling apparatus for a dielectric guide; Fig. 14 is a corresponding dia` gram for the receiving end; Fig. l5 is a plan view of a generator and coupling for sending waves in a dielectric guide; Fig. 16 is a corresponding side 15 elevation; Fig. 17 is a circuit diagram for the apparatus of Figs. and 16; Fig. 18 is a section showing sending end apparatus alternative to that of Figs. 15 and 16; Fig. i9 shows still another alternative form of such apparatus; Fig. 2O 20 is a diagram showing receiving end apparatus employing a detector; Fig. 21 is a diagrammatic end elevation showing how one or more detector units may be connected in the system of Fig. Fig. 22 is a modification as compared with Fig. 20 25 and shows an adjustable inductive coupling; Fig. 23 shows receiving end apparatus with adjustable means for supplying the carrier current at the receiving end; Fig. 24 is a diagram illustrating the reception of signals in two successive stages; Fig. 30 25 is a curve diagram for characteristic impedance of an all-dielectric wave guide; Fig. 26 is a diagrammatic end elevation of a modified coupling electrode; Fig. 27 is a cross-section of the dielectric guide showing the lines of force for a 35 certain case of polarization; Fig. 28 is a perspective diagram showing how multiplexing may be accomplished by the superposition of diametrical ly polarized waves; Fig. 29 is a diagram for one form of a highgpass iilter; Fig. 29a is a corre- 40 sponding section, modified to comprise an ab- .sorbing member for unwanted energy; Fig. 30 is a diagrammatic view showing a high pass dielectric guide lter interposed in a concentric conductor system; Figs. 31 and 32 are diagrams 45 for band pass iilters; Fig. 33 is a diagram for a band pass iilter for polarized waves; Fig. 34 is a diagram illustrating the phenomenon of radiationI from a part of a dielectric guide having its diameter reduced for that purpose; Fig. 35 illus- 50 trates the same phenomenon attained by shielding the non-radiating part of the guide and leaving the radiating part unshielded; Fig. 36 is a diagrammatic broadside elevation of an antenna array based on the principle discussed in connection with Figs. 34am 35; and Fig. 3v is a diagram of a device for modulating or detecting at the receiving end by means of a section of dielectric guide material having a non-linear characteristic.
As a result of theoretical studies and experiments I have determined that under certain conditions electric waves may be transmitted a considerable distance along a dielectric guide, and that such waves may be utilized for signaling along the guide. A system for signaling in this way is illustrated diagrammatically in Fig. 1. Various elements of apparatus are represented by boxes, but certain of these may be broken apart into distinct elements or they may be consolidated with others as I shall show, for example, in Fig. 17, where the generator G and modulator M may be merged together in a unitary device. In Fig. 1 high frequency electric currents are generated in the generator G and passed along the conductor pair 3| to the modulator M. These currents from the generator G are of a single frequency, say 1750 megacycles per second. S is a source of electric currents of composite frequency, say ranging in cycles per second from 1,000,000 to 3,000,000. Thus, the currents from S comprise a frequency band having a band width of 2,000,000 cycles per second. If a greater width is needed it can readily be attained. These currents of from 1,000,000 to 3,000,000 cycles are signaling currents and by their wide band range they may carry a large amount of intelligence. For example, they may comprise a large number of voice telephone channels. It is known that a band width of about 3,000 cycles is suicierit for intelligible telephone transmission. With a guard band of 1,000 cycles at each end of the 3,000 cycle band, this means an appropriation of a 5,000 cycle band for each voice channel. Hence with the range of 2,000,000 cycles there may be 400 voice channels. That is, the system shown in Fig. 1 may represent a multiplex one-way transmission comprising 400 voice channels. Or, the symbol S may represent a source of a considerably greater number of telegraph channels. Or, it may represent both telephone and telegraph channels. ,Again, it may represent a band of currents obtained by means of a television sender. It is Well known that for good television transmission a wide frequency band is necessary, and the range suggested by way of example for the source S in Fig. 1 would be suitable for good television.v transmission. Thus in general, the symbol S in Fig. 1 represents a source of currents extending over a wide frequency range and capable of carrying a large amount of intelligence expressed in signaling form.
The currents from the source S, ranging in frequency from 1,000,000 to 3,000,000 cycles, are applied through the conductors 30 to the modulator M to modulate the current of the frequency that I have assigned at 1,750 megacycles coming from the generator G. This frequency of 1,750 megacycles corresponds to a Wave length in free space of about 17.1 cm. The output from the modulator M in the conductor pair 32 may comprise currents of the carrier frequency 1,750 megacycles and the upper and lower side bands, the upper side band ranging from 1,751 to 1,753 megacycles, and the lower side band ranging from 1,749 down to 1,747 megacycles. 'Ihese high frequency currents going over the conductor pair 32 from the modulator M, will be delivered on the input side of the coupling device T. The structure and method of operation of this device T will be explained later in this specification. For the present it will suffice to state that it receives high frequency conductionv currents on its input side through the conductors 32 and generates corresponding displacement currents in the proximate end of the dielectric guide D. Various specific designs that may be chosen for the guide D fall under two general classes that will be discussed later: (1) without lassociated metal elements; and (2) with such metal elements. For the present disclosure in connection with Fig. 1 it will be convenient to consider the guide D as of the first type. A guide of the second type will be considered in connection with Figs. 13 and 14.
As I have stated, the frequencies involved in the currents delivered over conductors 32 to the coupling device T lie in the range from 1,7 47 to 1,753 megacycles, which is a comparatively narrow frequency range, lying within about 17/100 of 1 per cent above or below the carrier frequency. The wave length in free space being about 17.1
cm., the diameter of the dielectric guide D is of comparable magnitude, and it is assumed in this case to have a radius of 6.6 cm. These data and their relation and significance will be discussed more deflnitely'farther along in this specification.
In Fig. 1 a repeater R is shown interposed between the sending end on the left and the receiving end on the right. If the attenuation from end to end is not too great, such a repeater will be unnecessary. According as the attenuation is greater, one repeater or more than one may be interposed at proper intervals.
Waves of displacement current will travel along the dielectric guide D from left to right, and will be received in the coupling device T whose function is the inverse of that of T. .That is, at T' the energy of the displacement current waves arriving in the dielectric guide D will be translated into conduction currents in the conductor pair 32. These conduction currents passing in the conductors 32 will go to a detector M', and its detected output current in the output conductor pair 30 will comprise the various frequencies mentioned heretofore lying within the range from 1 to 3 megacycles. The conductors 30 lead to a device S which comprises suitable means for performing the inverse vfunction compared to S at the transmitting station. For example, if the intelligence transmitted should consist of a large number of telephone conversations on respective Voice channels, the apparatus S will be employed for separating these various channels into respective conductor pairs at normal voice frequency. Or, the symbol S' may represent apparatus for receiving television currents and translating them into the corresponding appropriate variations of luminosity to generate a moving picture at the receiving end corresponding to the changing scene at the transmitting end. If one or more repeaters R are employed, each may consist of a coupling device Ti like T', a frequency-step-down unitv FSD, an amplifier A, a frequency-step-up unit FSU, and a coupling device Ti like T. If preferred, the step-up and step-down units may be omitted, and the amplifier may operate at high frequency. l
For a discussion of the utilization of a wide frequency range for conveying a large amount f signaling intelligence, I make reference to the specification of the United States patent to Espenschied and Affel, No. 1,835,031, granted December 8, 1931.
Having indicated in connection with Fig. 1, in a general Way, how a complete signal transmitting and receiving system may utilize a dielectric guide, I will now explain more in detail some `principles involved in the generation and transmission of high frequency electromagnetic waves in a dielectric guide.
Referring to Fig. 2, this is a simplified diagrammatic representation of apparatus'by means of which the theory of these waves may be explained in a particular case. D is a circular cylinder of dielectric material extending an indefinite distance to the right, and cut square across at the left. Its radius is b; this was taken at the value of 6.6 cm. in Fig. 1. Attached centrally on the circular end face of this cylinder D is a metal disc electrode 34, and also attached concentrically around the edge of this circular end face is the annular plate electrode 35. The two conductors 32 of Fig. 1 are connected respectively to these two electrodes 3l and 35. Thus, in Fig. 2 the dotted line rectangle T corresponds to the box 'I' in Fig. 1.
Assume that the current in the conductors 32 is of a singleA frequency varying harmonically, and that this frequency is within or about the frequency of the range suggested for the conductors 32 in Fig. 1. 'Ihen there will be displacement currents" in the proximate end of the dielectric guide D corresponding to the conduction currents in the conductors 32. Whereas Fig. 2 shows the end of the dielectric guide D in perspective, it has been redrawn in Fig. 3 to represent an axial or longitudinal section. The displacement currents in the dielectric guide' D will extend along such paths as indicated by the dotted lines B0. These lines may be looked upon not only as displacement current paths, but they are lines of force that wax and wane with the alternating electromotive force applied on the conductors 32.
Under the circumstances assumed in connection with Figs. 2 and 3, we proceed to ascertain the nature of the displacement currents and the associated electric and magnetic forces in and about the dielectric guide D. If, at any point in or near this guide D, we represent the electric force as a vector by the letter E, and the magnetic force at that same point as a vector by the letter H, then, according to the laws of electromagnetism, we must have the following fundamental equations:
vxH=lK+ 4ms] (i) vxE= l- ,im (2) v.H= o (s) V.E= (4) In these equations (l) to (4), Vx is the differential operator sometimes called curl, and V. is the differential operator sometimes represented by div for divergence; c is the velocity of light in free space; and a, K and e are, respectively, the permeability, dielectric constant and specific conductivity of the medium under consideration. E and H represent respectively et a of component perpendicular to these. Also, the vector H is tangential, that is, at any point it is perpendicular to the plane of the corresponding radial and axial components of E.
By suitable steps, solutions may be deduced from the foregoing Equations 1) to (4) that are physically appropriate to the conditions of the present problem. These solutions, yput in convenient lform for our purpose, are as follows:
For the region outside of the dielectric guide D:
For the region inside the dielectric guide D:
These Equations (5) to (10) are expressed in cylindrical coordinates, the coordinate z being measured along the axis of the cylindrical guide and the coordinate r being measured radially. Ez and Er are the corresponding components of the electric force E at the point whose coordinates are z, r, and at the time t. Il is the magnetic force at the same point and same time. For any given value of these coordinates a and T, each of El, Ez and H is a single valued function of iime t.
Each of the Equations (5) to (l0) has the cornmon factor ef (b1-MM5, and it will be recognized at once that in the exponent of e, the term -iact/b represents the variation with time, and the term iz/b represents the propagation along the wire, involving the attenuation whatever that may be. That is, is in general a complex number and its imaginary component represents attenuation. The absolute value of this imaginary component of will be relatively small, and for our present purpose it will not be advantageous to consider it further, and we shall assume that is a real number. This simply means that we are ignoring attenuation along the line but We are getting the relative wave forms with substantial accuracy, and when We interpret our equations'as in Fig. 4, which will be discussed presently, it will be seen that we deal Wih relative intensities and our conclusions are equally applicable no matter what the attenuation.
Therefore, for the present purpose, we define and ,S as follows:
a=21rb/ and --21rb/L (11) where i( is the wave length as it would be in free space at the given frequency of the electromotive force that is applied across the electrodes 34 and 35 in Fgs. 2 and 3, and L is the wave length as it is for propagation at that frequency along the guide D. For some purposes it is convenient to notice, as may be seen from Equation (1l), that a or is the circumference of the guide D measured in wave lengths i or L respectively.
Other constants and dependent variables of the foregoing Equations to (10) are dened as follows:
x=1/s,=a22 and y=4sfa22 (12) where Si is the wave slowness (reciprocally proportional to velocity) for a medium such as exists outside and around the guide D, and Sz is the wave slowness for a medium such as Within said guide. More specifically, in the present case, S1=1 and S2=n, where n is the so-called index of refraction of the material of the guide D; n2 is equal approximately to the dielectric constant. This index of refraction n is to be taken at its value for the range of frequencies involved.
Jo(y) is the Bessel function of y of zero order, and Jo'(y) is its first derivative with respect to y.
Ho(:c) is the Hankel function of :c of zero order, and Hu'(:c) is its first derivative with respect to (These functions Ju, Ju', Ho and Ho' for the particular values of the respective arguments involved may be ascertained from suitable tables as, for example, Jahnke and Emdes Funktionentafeln, published in 1909 by B. G. Teubner in Berlin.)
Incidentally, it may be noticed that the coordinates z and t enter Equations (5) to (10) only in the factor eWFM/b, which is common to al1 these equations and from which it is apparent that the electrical and magnetic effects in and about the guide D constitute a wave transmission.
From Equations (5) to (10) and having reference to the data' on which they are based, information may be deduced to enable us to plot the lines of electric force in and near the guide D. To get this information involves somewhat tedious computations which may be facilitated somewhat by the employment of graphical methods. The value of H must approach one and the same limiting Value as 1' approaches b, whether this approach is from within or without the guide D, that is, whether r approaches b with increasing or decreasing values; this consideration yields the equation:
Where y is real and :c is imaginary. For any particular value of n, we may make a graph of this Equation (13) and from it we may read off corresponding values of :r/i and y. We assume representative values of the frequency (expressed in the wave length A, on which a: and y are dependent) and from these and from our graph we get the corresponding values of :c/z' and y. From these values, may be calculated by use of Equation (12) above.
Further employing these values in Equations (5) to (l0) we get the corresponding values for the components of E and for H. Having thecomponents of E, we may combine them to get the magnitude of E. For a concrete example, assume the value of the index of refraction n to be taken as 9 (this is the approximate value for pure water) and assume various representative frequencies expressed in the ratio Mb; thus the curves of Fig. 4 are obtained. In these curves the intensities of the radial electric force E (continuous lines) and magnetic force H (dotted lines) are plotted as ordinates against radial distances r (in terms of radius b as unit) as abscissas. These intensity ordinates are purely relative (in decibels above an arbitrary level); hence they serve for any point along the length of the guide D, that is, for any value of z, no matter what the attenuation may be from the origin to that point. It will be understood that at any given point whose coordinates are z and r, the actual intensities vary sinusoidally at a frequency corresponding to the wave length )c In each of the Equations (5) to (10) this variation is expressed in the common factor efHwO/b, as has been stated heretofore. The ordinates in the diagrams of Fig. 4 may be regarded as expressing the relative eective or root-meansquare values. i
According to Fig. 4, it will be'seen that at the comparatively low frequency for which 7l=23.23b, the forces are not much less outside the guide'D than within it, and are of considerable magnitude at points as far distant from the axis of the guide as three times its radius. This state of affairs may be expressed by saying that the energy of the waves resides largely in the space outside the guide D. With increasing frequency (that is, with decreasing values of IMb), We pass along the series of diagrams of Fig. 4 and flnd the forces are relatively less and less intense without the guide until at the comparatively high frequencies corresponding to A=l0.24b and 7\=7.87b, the energy of the waves resides almost wholly within the guide D.
Utilizing such graphs of /z' against y as mentioned heretofore, we may obtain values of the ratio /a, which I call the slowness factor and designate by K. This quantity corresponds to the velocity of. propagation of the actual waves as they exist, having their energy partly within and partly without the guide D. Fig. 5 gives values of K as ordinates for various frequencies expressed in terms of A, the wave lengths as they would be in free space, as abscissas; a reason for introducing this Fig. 5 is that its immediate data are susceptible of experimental verification. as will be pointed out presently. It Will be noticed that the parameters that distinguish the various diagrams of Fig. 4 are particular values along the scale of abscissas of Fig. 5. The frequencies increase as we go along the scale of abscissas from right to left in Fig. 5. At the lower frequencies on the extreme right the velocity is the same as in free space, and indeed we have seen that at these frequencies there is no substantial wave propagation within the guide. But at a critical frequency value corresponding to Equation (16) to be considered presently, we begin to have waves in the guide and these are propagated at velocities less than in free space, as indicated by the upward bend of the curve of Fig. 5. Then at yet higher frequencies the velocity of propagation in the guide approaches as a limit the value which it would have in an unbounded medium of the same material as the guide, this value corresponding to the index of refraction n, which we have given the particular value 9 in the present instance.
The diagrams of Fig. 6 are compiled from data that have been explained heretofore, including Fig. 4 on which especially Case II of Fig. 6 is based. 'I'hese diagrams are not rigorously exact; they are intended to show the main trends of the lines of force at different wave lengths. A few points of these lines of force have been computed, but to a considerable extent they have been completed by interpolation and extrapolation. We assume a fixed instant of time t, and the same value of n as before, namely n=9. Having ascertained the values of E for Fig. 4 by combining the component values, we have those components already available, and from them we get the slope of E at each of various points corresponding to various coordinate pairs z and r. Waves ofv different length may be superposed in the dielectric guide D, but for clearness as well as for other reasons I show the different wave lengths in different parts of Fig. 6, which I distingulsh as Cases I, II, III and IV. A threedimensional picture of the iields depicted in Fig. 6 may be obtained by imagining the figures illustrating each of the four cases to be rotated about the axis of the guide so that the electric lines describe surfaces of revolution.
Case I.-Here we assume the frequency at such a low value, that is, the wave length is so great, that propagation first becomes possible. This being a critical or limiting condition of affairs, special mathematical procedure is necessary which it will not be necessary here to set forth, except very briefly as follows: In connection with the derivation of Equations (5) to (1-0) from Equations (l) to (.4), it may readily be shown that may take any value between the limits given by naza and that no wave power will be propagated through the guide D until the frequency is suiliciently high to satisfy the relation This relation obtains when J (y) :0, of which last mentioned equation, y=2.40 is a root. Subject to the condition expressed in Equation (l5), :1::0 and 1:5 and therefore )\=L, which means that the effect in and about the guide D'is propagated with the velocity of light in free space. We ascertain that outside the guide the lines of electric force are everywhere radial, inside and near the axis they are parallel with the axis, and inside and near the boundary they are radial. These general characteristics of the field are depicted in the diagram of Fig. 6 labeled Case I. The longitudinal electric intensity in both Case I and Case II is maximum at the axis of the guide and it varies with respect to time and also with respect to distanceA along the guide in the same manner as the applied electromotive force, sinusoidally, therefore, in the case of. a sine wave source. Accordingly, there is a longitudinal flow of displacement current through the dielectric medium. The accompanying magnetic field may be represented by closed loops concentric with the guide and disposed in planes transverse to the axis.
Case II.-Here ywe assume the frequency at such a value that Mik-13.5. This determines a, a: and y, and accordingly We get the values of E as a function of z and r; the corresponding llines of force are plotted in the part of Fig. 6
designated Case II. It should be noticed that the value of A/b=13.5 which was chosen for this case, lies between the values for the fifth and sixth diagrams of Fig. 4.
Cases III and I V.-With increasing frequencies, waves of higher order appear as may be deduced by proceeding further in consideration of the same data that were relied on for Cases I and II. Thus, y=2.40 is not the only root of the equation J (y) there are other roots, y=5.52, y=8.65, etc. It is in connection with such higher values for y that we learn of the possible existence of these higher orders of waves, but for the pur-` pose of this specification it will not be necessary to do more than illustrate them qualitatively, as
tudinal electric intensity and displacement current flow are maximum at the axis of the guide and also at a cylindrical surface lying between the axis and the periphery. The accompanying magnetic field may be represented by closed loops ccncentric with the guide; and with respect to the radial direction it varies in accordance with a Bessels function from a zero value at the axis.
When desirable to transmit over a dielectric guide by means of higher order waves such as illustrated in Cases III and IV of Fig. 6, their generation may be facilitated by employing concentric annular electrodes at the sending end, as illustrated by way of example in Fig. 7. In this case five such electrodes are shown, numbered consecutively from the center. Those numbered 2 and 4 are conductively connected and energized in opposite phase with the intermediate electrode 3, so that there will be two sets of lines of force 2-3 and 3 4, as shown in Fig. 7, and these will be snapped off and propagated along the dielectric guide D in closed loops constituting electric displacement current waves, as shown in Case III of Fig. 6.
These higher order waves may be utilized for multiplexing. Thus, in Fig. 7 lines of force are also shown extending between the central electrode l and the outer electrode 5. These lines of force are due to superposed electromotive forces of appropriate lower frequency generated in the circuit associated with these electrodes. In this way we may have superposed in one and the same dielectric guide D a system of waves corresponding to Case II of Fig. 6 and a system of waves corresponding to Case III of the same figure. Similarly, higher orders of waves may be generated and transmitted, as in Case IV of Fig. 6, and various Superpositions of waves of lower order may be practiced in that connection.
We have already seen that at the critical Value of y=2.40, 1:5, and from equations (1l) it is further apparent that a==21rb/A. Substituting these values in the second of equations (12) we get the result that where Ae is the critical Wave length below which propagation is not possible and above which it becomes possible. The beginning of this possibility is shown in Case I of Fig. 6. For illustrative purposes we have assumed that 11,:9, and introducing this value in Equation (16) We get Ac=23.33b. It will be seen that the iirst diagram of Fig. 4 has a value for )i only slightly less than this critical value Ac, and it is this Value which is employed in Case I of Fig. 6.
From. Fig. 4 we see that in the case of wave lengths not much less than M, the wave power resides partly in the guide but to a great extent in the surrounding space, and from Fig. 5 we see that in this case propagation is at velocities little less than of light ln free space. As the frequency is increased, more and more of the power comes to reside in the guide as illustrated in Case II oi Fig. 6, and turning to Fig. 5, the velocity of propagation decreases toward a limiting value which is the characteristic velocity in an unbounded rnedium of the same material as the guide. With increasing frequencies, higher order waves may appear at certain shorter critical wave lengths, as indicated in Cases III and IV of Fig. 6.
In connection with Fig. 1, the wave frequency was suggested to be taken at 1,750 megacycles, and an alternating electromotive force of this frequency was generated and applied at the sending such asbakelite.
end of the system'of Fig. l. This frequency gave us a wave length in free space of 17.1 cm. stituting this value of i in Equation (16) and solving for b, we get where be is the critical radius which is at the least value possible for propagation corresponding to Case I of Fig. 6.
In connection with Fig. 1 it was suggested that the radius of the guide D might properly have a value of 6.6 cm. The basis of this assignment was as follows. In Equation (16) for the critical wave length at which propagation is impossible on one side and possible on the other side, we have the constant factor 2.61. This corresponds closely with the first of the diagrams of Fig. 4. Guided by the sequence of these diagrams and other considerations, we arbitrarily alter -this constant to a value about 40 per cent less, that is, we make it 1.5, and thereby we get the equation for a practical wave length,
This we regard as a practical relation for the propagation of Waves with substantial eiIect along the dielectric guide. It should not be inferred that the wave length could not be shorter or even longer.
In connection with the theoretical study that began herein with Equations (l) to (4), we have assumed a value, n=9, for illustrativepurposes and also because it enters largely into experimental verication of this theoretical work, as will be described presently. For variety of illustration, we assume the alternative value, n=2, for the guide D of Fig. 1. Already, in connection with Fig. 1, we have assigned 1,750 megacycles as a practicable frequency and have noticed that this corresponds to a wave length-7i of 17.1 cm. in free space. Substituting these values -of n and x in Equation (18) and solving for b, we get b=6.6 cm., which is the radius given heretofore for the guide D in Fig. 1.
In addition to a mathematical study like the foregoing, I have made experimental studies of these displacement currents in the dielectric. Of course, the vectors represented by E and H in connection with Figs. 2 and 3, are elusive for experimental study, traveling as they do with velocities comparable with the velocity of light, or varying at any one place with frequencies of the order of a thousand million per second. But, it is a well known device in the study of wave transmission, to set up standing waves, and I will indicate how this can be vdone for these guided dielectric waves. Referring to Figs. 8 and 9, `the tall vessel 36 of circular crosssection has a wall of suitable dielectric material The diameter is made large enough so that the interior is conveniently accessible. This vessel 36 may be filled with water D, or other suitable liquid dielectric, to any desired height. The column ofv water within the vessel,36 is the dielectric corresponding to the guide D in Figs. 2 and 3. It is made tall enough so that it can have a system of stationary electric waves set up therein, but there is no need to make it taller than necessary for this purpose. The bakelite wall is thin so that its presence can be neglected in respect to the electric wave effects.
The bottom wall of the vessel 38 is a thin horizontal plate of dielectric material, and at' tached to the bottom wall and coaxially s .r-
Bub-- ranged with the cylindrical vessel 36 are the central disc electrode 34 and the surrounding annular electrode 35. A pair of Lecher wires 31 hang vertically with their upper ends conductively connected respectively to the electrodes 34 and 35. At 38 is an adjustable conductive bridge that may be clamped at any desired height. A source of high frequency oscillatory currents is represented at S in Fig. 9 and these currents ow in a circuit comprising the conductor 39 bent around three sides of a rectangle. This source S" is positioned (see Fig. 9) to bring the intermediate side of the rectangular frame 3S close to and parallel with the bridge 38. Thus. the oscillatory currents in the circuit 39 induce electromotive forces of the same frequency in the bridge 38, and the corresponding induced currents flow in the portion of the Lecher Wire conductors 31 between the bridge 38 and the electrodes 34 and 35. The complete circuit of these induced currents comprises the path of the displacement currents in the delectric material D within the vessel 36, this path being represented by the dotted lines of force 40 in Fig. 8.
y A reason for interposing the Lecher wire System between the source S and the dielectric guide D in Fig. 8 is that it serves as a convenient intermediate coupling that is capable of a wide range of adjustment, and it permits of impressing on the dielectric a definite and measurable electric force. By adjustment of the bridge 38 one mayascertain the wave length and corresponding frequency of the source S". A microammeter 4l to which is connected a small rectier is held with its terminals near the two Lecher wire conductors 31 so as to pick up a small portion of the wave power in this circuit. Starting with the bridge 38 close to the electrodes 34 and 35, it is then adjusted away therefrom till the microammeter reading is a maximum. Then we know that the distance from the bridge 38 to the electrodes 34 and 35 is a fraction of a wave length, just how much depending onv the conguration and properties of the associated elements at the upper. ends of the Lecher wires 31.
By adjusting the bridge 38 further downward, the intensity reading on the meter 4l diminishes to a certain value and then increases to another maximum as with the bridge at 38'. Then we know that the distance between the two positions of the bridge, vthat is, the distance from 38 to 38', is a half-wave length. Knowing that the waves associated with the Lecher Wires travel with the velocity of light in space, the frequency can easily be computed from the wave length.
The standing waves in the water column of Fig. 8 were demonstrated in three ways. 'I'he irst of these will now be described. Varying the 'height of the water column Within the vessel 36, and relying on the meter 4I as shown in Fig. 8. 'we find that at certain water levels the intensity indicated in the meter is a maximum and at others a minimum. If we vary the height of the water column from one maximum to the next maximum, we know that we have increased its height one-half wave length. Thus we can demonstrate the presenceln the dielectric guide D of the electric waves corresponding to those discovered by a theoretical study such as was presented above in connection with Figs. 2 and 3 and Equations (1) to, (4).
According to a second procedure for demonstrating standing waves in the water column D,
amar/11y the container was nearly lled with water and the Lecher wires were adjusted for resonance. A horizontal circular disk of sheet copper which contacted the walls of the column D was then lowered into the water. Its eifect at various heights in limiting the top of the wave range was the same as changing the water level. Here the meter 4| is used in the same way as in the iirst method. Inasmuch as the energy of the waves under some conditions lies partly outside but adjacent to the dielectric guide, I provided an adjustable annular plate around the container for the water column D, and adjusted itto lie in registry in the same horizontal plane with the copper disk just mentioned.
In a third method of demonstrating the standing waves, a composite disk plunger was employed in somewhat the same way as the simple disk in the second method, but in this case the disk was made up of annular copper electrodes 43 with water-proof crystal detectors 44 connected between consecutive electrodes. This is shown in Fig. 10. Connecting wires were brought out to an external microammeter 45 which indicated the maximum eifects, and these could be readily correlated with the vertical adjustments of the disk. By means of the switches 4S the electric forces could be tested at various distances from the axis of the guide D. Y
Of these three methods the second was found to be most practicable, and was most generally used.
By adjusting the frequency of the source S" of Fig. 9 to various values, we can get experimental results corresponding to the results that were deduced by mathematical reasoning in connection with Figs. 2 and 3. For example, at low frequencies, that is, below the critical frequency given by Equation (16), the presence of waves in the dielectric D cannot be discovered, but at frequencies only a little higher, the presence of electric force becomes measurably apparent, both within the vessel 36 and around it, showing that the lines of force within the dielectric D extend out into the adjacent space. Exploring for longitudinally directed forces outside the vessel 36 but adjacent thereto, we iind that these forces wax and wane as we go along the length of the vessel. Also at any location in this region, if the direction of test for the exploring meter is turned from a longitudinal direction to tangential, a difference of effect is generally apparent, and this is different at different places. By such exploratory tests we are able to make substantial checks on the theory that has been developed in connection with Equations (5) to (10) By tests at adjustments for various wave lengths the data were observed that have been plotted as points near the curve of Fig. 5. Their proximate agreement with the theoretical curve of that figure conflrms it and the associated theory as truly representing the actual phenomena in a dielectric guide.
It will be understood that the apparatus shown in Figs. 8 and 9 is presented for the purpose of illustrating some theories that are involved in or linked with the invention. This is accomplished by an experimental study of conditions and processes in a dielectric guide, such as can be made by means of standing waves.
The dielectric guide of my invention may comprise parts of different properties, as for example, it may consist of a core and one or more concentric cylindrical shells of different dielectric constants. Again it may consist oi a dielectric member associated with a conductive member or members. extending longitudinally therewith. Examples that are amenable to theoretical and experimental study are:
1. To surround the cylindrical dielectric guide with a conductive cylindrical shell.
2. To provide a conductive axial core along the cylindrical dielectric guide, and
3. To provide both such shell and core.
On the theoretical side, a study may be made of each of these cases similar to the study that was made for Fig. 2, starting with the same general Equations (1), (2), (3) and (4) from which the start was made for the dielectric guide without any associated conductive member. I'he rst of' the foregoing three cases may be treated the same as the case of the all-dielectric guide to the point where the boundary conditions are set up. In such case of a composite guide we have in place of Equation (13), the condition that Ez=0 when r=b. This yields equations similar to (8), (9) and (10) for the region inside the guide. It is obvious that there willbe no appreciable field outside the guide. It will not be necessary to write out these equations as the principles involved have been illustrated for the earlier case.
'Ihe case of a dielectric guide surrounded by a conducting sheath differs from that of an alldielectric guide in that waves are possible when the dielectric has any index of refraction whereas in the case of the all-dielectric guide the index of refraction must in general be greater than unity. A special case of this guide having the more general range of index will therefore be that of a hollow conductor either evacuated or filled with air or other gas.
It may be established that for a guide of dielectric material within a metallic cylindrical shield K= n1/1-)\/}\ (approximately) (19) where K is the slowness factor as defined in connection with Fig. 5; n is the index of refraction of the dielectric material ofthe guide; i is the wave length in free space corresponding to an assigned frequency; and kc is the critical wave length in free space such that at frequencies for which \c there is propagation, but at frequencies for which )\c there is none. Fig. 11 should be compared with Fig. 5. In Fig. 5 the abscissas are in terms of the radius of the guide, whatever that may be. In Fig. 11 the abscissas are in centimeters, and four different guides have been represented, corresponding to Fig. 8. Two 0f these are all-dielectric, respectively 10 and 6 inches in diameter. Points were ascertained experimentally and plotted as shown for the 10- inch all-dielectric guide. Also, by means of the foregoing Equation (19) the theoretical continuous line curves were plotted in Fig. 11 for composite guides, respectively l0 and 6 inches in diameter. In all these guides the dielectric was water. In the all-dielectric guides the Water was within thin cylindrical bakelite shells and in the composite guides the water was Within cylindrical copper shells. Points were ascertained experimentally for both composite guides and these have been plotted in Fig. 11 and the dotted line curves sketched to correspond therewith.
The optimum diameter of a shielded dielectric guide may be regarded as depending on the relative dissipation losses suffered respectively in the dielectric core and the metallic shield. At the lowest frequencies at which dielectric Waves are possible in such a guide, the wave power may be supposed to reside near the exterior, so that the shield may be expected to contribute considerable loss. At extremely high frequencies the field resides nearer to the axis of the core, and the shield may play a relatively unimportant part and the dielectric losses may be controlling and the conductive losses in the metal shield may be relatively insignificant. It iis furthermore possible, for various circumstances of frequency and radius,
to alter this optimum condition by varying the` index of refraction of the dielectric. In fact, when other sufficiently low loss materials are not available, .it may be desirable to use air as a medium. In the latter case, the losses will be almost entirely those contributed by the resistance of the external sheath. According to this plan the medium will be air Within the cylindrical metal shell shown in each of Figs. 12, 13 and 14, that is at 56 in Figs. 13 and 14.
Fig. 12 for the metal sheathed dielectric guide corresponds to Fig'. 6 for the all dielectric guide. When the lines of force extend to the boundary of the metal sheathed dielectric as in Fig. 12 they end in the metal sheath instead of extending out into surrounding space as for the all dielectric guide of Fig. 6.
The metal sheathed guide has the advantage that it aiords immunity from outside inductive influences. The sheath may be extremely thin. A plurality of such sheathed guides may be grouped in a cable without appreciable crosstalk effects. Reference is made to my application, Serial No. 701,711, filed December 9, 1933, which is directed generally to systems and methods for the transmission of waves through a dielectric guide comprising a metallic sheath.
In connection with Figs. 13 to 24, I will point out how my improved dielectric guide may be utilized to transmit a large amount of signal intelligence requiring a Wide frequency range, as for example, a theatre scene transmitted by television. As is well understood, a good television transmission of a scene having considerable detail requires a very wide frequency range. Whatever the range necessary to transmit,. forexample, a single face, a far wider range is necessary to transmit with as good definition a. plurality of faces and other details on a theatre stage. For such a purpose my system is well adapted. The advantage of a wide frequency range may be desirable for a short transmission distance as well as for a long distance; it might be advantageous to employ my invention to transmit a theatre scene to an annex in the same or a neighboring city. Using the specific data assigned heretofore in connection with Fig. 1, we assume a carrier current frequency of 1,750 megacycles and its modulation by currents having a frequency band width of 2,000,000 cycles. Probably this is a suicient band width to give satisfactory television detail for a theatre stage.
Referring to Fig. 13, this shows the proposed television transmitting system inl-diagrammatic outline. The box 50 represents the apparatus by which the theatre scene is scanned and thevarying light impulses from various successive parts of the eld of view are converted photo-electrically into currents having components ranging from zero to 2,000,000 cycles, which are then stepped up in frequency to a range from 1,000,000 to 3,000,000 cycles and delivered to the output conductors 5l. The oscillator 52 generates a. current of 1,750 megacycles and delivers it over the conductors 53 to the input of the modulator 54. There the varying photoelectric currents from the conductorsy 5| are applied to modulate the carrier current of 1,750 megacycles, and the output modulation current comprising the bands from 1,747 to 1,749 megacycles, and 1,751 to 1,753 megacycles and the carrier of 1,750 megacycles are filtered in the band pass filter 54; and the band from 1,751 to 1,753 megacycles is applied to the coupling unit 55. This is connected to the proximate end of the composite wave guide, which consists of the cylindrical dielectric core 56 surrounded by the copper sheath 51. Alternatively, the coupling 55 and the guide 5'6-51 may be made to serve as the high pass filter by properly proportioning the diameter of the guide in relation to the wave length as will be explained in connection with Figs. 29 and 30. In the latter case it may be desirable to separate the two side bands somewhat more widely than is assumed above. This may be done by a double or triple modulation whereby the nal carrier is modulated by say, 11,000,000 to 13,000,000 cycles.
The guide 56-51 of Fig. 13 may be suitably armored or otherwise -protected and laid in a conduit. It constitutes a long cylindrical copper container with the dielectric therein, and extends from the transmitting station associated with the source 50 to the receiving station shown in Fig. 14. At the receiving end the guide 56-51 goes into a coupling unit 58, Where the energy of the dielectric waves in the medium 56 is translated into conductive currents in the conductor pair 59. It will be remembered that these currents comprise a band of from 1,751 to 1,753 megacycles, a modulation side band of frequencies extending over a range of 2,000,000 cycles. At 6l there is a local oscillation generator that generates a current of the carrier frequency that is delivered over the conductor pair 62 to the demodulator 63. 'Ihis demodulator 63 also receives the incoming currents on the conductor pair 59, and the demodulation output product of frequency range from 1,000,000 to 3,000,000 cycles is filtered through and goes to the amplifier 64 and thence to the apparatus 60 where the original band of to 2,000,000 cycles is reproduced. Here these currents are applied to control a beam of light so that it shall vary in correspondence with their variations, and this light beam is distributed over a screen in synchronism with the scanning system at the sending end so that on this screen the whole theatre scene is continuously reproduced.
In Figs. 15 and 16 I have shown apparatus to perform the same functions as the apparatus indicated symbolically by the reference numerals 52, 54 and 55 in Fig. 13. A three-electrode vacuum tube comprises the glass envelope 10 which is highly evacuated and contains the usual three-electrode arrangement except that the design is carefully adapted for the comparatively high frequencies that are here involved. The plate circuit electrodes 1| are connected with the oscillatory output circuit which comprises the two parallel metal bars 12 connected at their distal ends by the blocking condenser` 13. This blocking condenser consists of interleaved and insulated metallic plates, and it functions to Complete the primary circuit for alternating currents of the conductors 12. The axis of the blocking condenser 13, that is, the line joining its terminals, is parallel with the intermediate part mary currents in the circuit 1i-12-13 induce currents in the secondary circuit 14-15-18 and as explained before, lines of force extend in the dielectric material 56 between the electrodes 15 and 1liy and are snapped of! and propagated as electric waves through the dielectric core 55.
As described thus far, the system of Fig. 15 'corresponds to the oscillator 52 of Fig. 13 as if it were feeding directly into the coupling unit 55 of that same figure. In Fig. 17 the circuit connections of Fig. 15 are shown in schematic form and we see how theA modulating currents are applied on the conductors 5i to modulate the oscillatory currents set up in the circuit 1l-12-13.
An alternative generating system at the sending end is shown in Fig. 18. Here a so-called electronic oscillator tube is employed in which the frequency does not depend n the electrical design of an external circuit but rather on certain dimensions and adjustments of and related to the electron paths in the tube itself. Accordingly the intermediate coupling circuit may be omitted and the central electrode 15 on the end of the dielectric guide is connected directly with the grid of the vacuum tube, the annular electrode 16 being connected with the plate of the tube. Thus these electrodes 15 and 16 become integral parts of the oscillator which is enclosed in an extension of the same sheath that surrounds the dielectric guide. This arrangement is such as to avoid as far as possible any electrical discontinuity between the generator tube and the guide. y
Yet another generator is shown in Fig. 19 which shows a cylindrical metallic shell that is at the same time a part of the tube envelope and is the plate electrode of the tube. V'I'his makes metallic contact with the sheath 51 of the composite dielectric guide 56-51; The grid 65 is of squirrel-cage type and is connected 4to the central electrode 15 on the proximate end of the guide. The modulating potential is applied through a separate grid lead 66. The external space to the right of the exciting electrodes 15 and 16 and within the sheath 51' is filled with a dielectric having the same properties as the wave guide so that there will be a minimum electrical discontinuity between the oscillator and the wave guide. The tube and oscillator in combination are attached to the wave guide either by a bayonet connection or by a threaded union. It is important that there shall be no wave discontinuity at the junction surface of the dielectric material.
At the receiving end the processes are essentially the reverse of those at the transmitting end and therefore they may be performed by the same general type of circuit elements, and it will not be necessary to describe these in much detail. A simple form of receiver is shown in Fig. 20 where 61 is a rectifier or detector. The choke coils 68 permit the passage to the amplifier 69 of only the currents of comparatively low frefrequency, that is, ranging from 1 to 3 mega.-
'cycles. and exclude the comparatively high frequency currents around 1,750 megacycles. Of course, the detector 61 should be chosen and designed so that its resistance will match the characteristic impedance of the dielectric guide and prevent the reflection of currents into the guide. This desideratum may be secured by putting the detector unit radially across the electrodes 15 and 16 as shown in Fig. 21, and one or several such units may be employed. In each unit of this detector of Fig. 21 a suitable crystal may be employed, such as zincite, perikon, carborundum, galeria or silicon.
When it is not practicable to match impedances in the manner indicated in connection with Fig. 21, an adjustable coupling may be employed as indicated in Fig. 22.
If the carrier current has been suppressed in transmission so that it must be supplied at the receiving end, this may be done as in Fig. 23. Here the oscillation generator of carrier frequency feeds into a short dielectric guide 80 across which is an adjustable iris 8|, which provides essentially a metallic diaphragm having an axial aperture of adjustable diameter by which the proper level of local carrier energy may be admitted to the detector.
If the received power is very low, detection may take place in two or more stages with suitable amplication at the lower frequencies, as indicated in Fig. 24.
Of course, there should be a suitable matching of impedances wherever the wave energy is translated from one form or medium to another, as for example, referring to Fig. 2, where the energy of the electric conduction currents in the circuit 32 is translated into the energy of the dielectric displacement currents in the guide D. A suitable impedance match means that energy is transmitted without reflection at the junction, and this may involve a suitable degree of looseness of coupling. If the ring electrodes 3| and 35 are wide with a. narrow gap between them, this gives a loosev coupling and the reflection is largely of the metallic type. On the other hand, if the rings are narrow and the gap is wide, the coupling is not loose, and the reflection is substantially of dielectric type. What is wanted is such an optimum width of the gap 34-35 that there will be no reection.
To match impedances in a dielectric guide or with a dielectric guide, and for other purposes, it will be advantageous to know the impedance characteristic of the dielectric guide. I'he mathematical development given earlier in this speciiication puts us in position to compute what may be called the impedance characteristic of a dielectric guide. At any point within or without the guide of Figs. 2 and 3, we can get the Poynting vector, which is the vector product of E and H, both of which factors we know from Equations to (10). We integrate the Poynting vector over the cross-sectional area and divide by the square of the eifective current (which, of course, is a displacement current). Integrating thus and dividing for the internal cross-section of the guide gives a result that may be called the internal characteristic impedance, and similarly for the external cross-sectional area we get the external characteristic impedance. To produce a plot for these impedances we assume the particular value 1::9, and accordingly in Fig. 25, we have shown a semi-logarithmic diagram of these two impedances, internal and external. The impedance values expressed in ohms are plotted as ordinates against wave lengths in free space as abscissas.
To facilitate matching impedances at the sending end, and to get the proper degree of loose coupling at that place, we may modify the electrodes somewhat from the annular shape as shown at 34 and 35 in Figs. 1 and 2. An alternative construction is shown in Fig. 26. The circular plate shown here has the same diameter as the guide D. Its marginal part 35' corresponds to the electrode 35 and its central part 34' to the electrode 34 and betwen the electrodes. comparing with Fig. 2, it will be seen that in Fig. 26 the two conductors 33' of the electric conduction circuit are conductively connected by the radial members 83 lying between the sector-shaped slots 82. By suitable designof the number and size of the slots, an optimum coupling between the conductive circuit and the dielectric guide can be attained.
In any case the dielectric guide, whether provided with a metallic sheath or not, will be suitably protected for overhead suspension or for carrying it in an underground conduit. A leady sheathing may be employed in the case of a metal sheathed guide and the complete guide may be drawn into an ordinary conduit or suspended by rings from a messenger wire. If an alldielectric guide is employed it may be enveloped in an impervious dielectric covering such as duct cloth or paper impregnated with bakelite.
It is well known that so-called resistance noise is one of the factors that sets a limit to the attenuation that is permissible on signaling conductors. Static and other interference also are limiting factors. With my improved all-dielectric guide, so far as electrons are involved in the transmission they are bound electrons, not free as in metals, and for that reason resistance noise will be comparatively low. L This advantage will be present to a considerable degree with the metal sheathed dielectric guide, and moreover in that case, static and other interference Will be obviated entirely or to a very great extent.
As alternative dielectric materials that may be employed in addition to those mentioned heretofore, there may be mentioned a mixture of paraiiin and finely subdivided mica, or rubber withv which powdered zinc oxide has been mixed. The latter dielectric material has the advantage of giving a semi-flexible guide and has a low power factor and a favorable dielectric constant lying between the values 6 and 1. The rubber and zinc oxide may be in the proportion of about 35 and 65 per cent respectively. As already pointed out, the dielectric may be either air or vacuum when a metal sheath is used. Such an arrangement would necessarily involve somewhat larger diameters than would otherwise be needed.
vIn all cases, low conductivity of the dielectric is an important desideratum. In order that the guide be as small as practicable it is desirable also that the material have a high dielectric constant. While Water has a high dielectric constant, about 80, it does not have a very low conductivity and hence, on the whole, a medium with lower conductivity than water though with lower dielectric constant, may be preferable. I will mention a few additional dielectrics among which choice may be made of a suitable medium for my improved electric Wave guide; these are terpineol, camphor, borneol, halowax, superlowax, rubber, victron and resoglaz.
In electromagnetic wave transmission the lines of electric force and the lines of magnetic force are mutually orthogonal a'nd give ywhat may be called a complete picture of the wave. Such a Wave picture suggests the possibility of interchanging the lines of electric and magnetic force. 'I'hus we might have a dielectric guide in which the lines shown in Figs. 6 and 12 are lines of magnetic force and the lines of electric force are coaxial circles. Of course, this would involve an appropriate change of vcoupling at the ends of the guide to effect the proper translation the sector-shaped slots 82 correspond to the gap of energy between the guide and the conduction circuits at its ends.
In still another vcase the electric and magnetic lines in the dielectric medium may be approximate ellipses or circles or perhaps straight lines that are mutually orthogonal. Instead of annularly disposed electrodes such as 34 and 35 in Fig. 2, one may employ two diametrically opposite electrodes as shown in Fig. 28 or Fig. 33 and -in this case the electric waves are polarlzed'in the plane of the axis of the guide and the diameter joining the electrodes. That is, instead of the lines of electric force all being radial they-lie somewhat as shown by the transverse continuous lines in Fig. 27, which is a cross-section of the dielectric guide. The corresponding magnetic lines are dotted. In this type of wave the electric and magnetic fields have both transverse and longitudinal components. 'I'he lines of electric force, in an all-dielectric guide, form closed loops lying in surfaces parallel with the axis of the guide, one such surface being a diametral l plane. In a metal-sheathed guide the loops may still be identifled although they are closed through the metal sheath. The magnetic field, in both types of guide, may be represented by closed loops lying in surfaces parallel with the axis of the guide and substantially at right angles to the surfaces in which the lines of electric forcelle. By a suitable design of the coupling between the conductive circuit and the guide, the pattern of Fig. 27 can be varied to some extent as may be desired.
By such polarization as in Fig. 27, multiplexing may be accomplished. Thus in Fig. 28 we have a normal system i that gives waves with radial lines of force as described heretofore. In addition we have the pair of diametricallyopposite electrodes of system 2 that give lines of force like those shown in Fig. 27. These are superposed on the radial lines of system l. The waves of each system may be modulated independently for multiplexing. Additional waves polarized along a different diameter may be superposed as shown in Fig. 28 for system 3. By employing two sets of electrodes as shown for system 2 and system 3, with their axes at a right angles and energizing them in quarter-phase relation, the waves may be given a rotary polarization and propagated as an advancing rotary field. WhatI have called system i may be multiplexed as explained in connection with Fig. 7. Another way in which-multiplexing may be practiced is to make the guide with an enclosing metal sheath of radius b and with a smaller coaxial sheath of radius b separating the dielectric into two parts, a central axial part and an annular part. There may be independent wave systems in each part, and obviously the number of innerv coaxial shells may be increased so as to have more than two parts.
While a cylindrical cross-section for the guide.
is obviously preferable for many purposes and is easier for theoretical study and for practical test, other cross-sections may be preferred in some cases. A square or hexagonal cross-section may be easier for assembling the guides in cables, land an elliptical or rectangular cross-section may be preferable when the waves are polarized diametrically in distinction from radially.
It has already'been pointed out in connection with Equation (16) that for a given dameter of a dielectric guide there is a limit to the length of Wave that can be transmitted therein. Therefore, an abrupt reduction of diameter in a dielectric guide performs a lter function, as may be seen in connection with Fig. 29. That is, suppose waves are' being propagated from left to right in the part of the guide of large cross-section and that these waves are composite, comprising various wave lengths up to the limit capable of transmission in a guide of this larger diameter. At the point where the diameter is reduced the waves of longer length will be stopped. The energy of these longer waves may be reflected in the guide, or radiated therefrom, or absorbed in an appropriate resistance, such as the carbon annulus 98 in Fig. 29a. But the shorter waves will be transmitted past the junction along the part of the guide of less diameter. Thus we have a high pass filter effect at the junction point.
In Fig. 30 I have shown how my dielectric guide may be interposed in a. concentric conductor system to operate as a high pass filter. The concentric conductor system comprises the conductor pair consisting of the cylindrical conductor shell 85 and the axial conductor 86, with no material between them except air and the necessary spacers. The lines of electric force extend radially between conductors 85 and 86. Between the points 81 and 88 the outer concentric conductor 85 is enlarged and lled by a dielectric guide 84 with the annular electrodes 96 and 81 at its ends connected respectively with the conductors 86 and 85. 'I'hus the conduction currents coming from the left to the point 81 are converted into dielectric displacement currents in the guide 8l and their energy is reconverted into conduction currents to go on from the point 88 on the right. But since the wave lengths are limited to a certain ratio to the diameter of the 1 guide 84, according to Equation (16)', only the wave lengths shorter than this limit-get through from the left to the right.
Fig. 31 shows a band pass filter interposed in a concentric conductor system. As in Fig. 30, a dielectric guide section is interposed in a concentric conductor system between the points 81 and 88, and likewise as in Fig. 30, the combination shown in Fig. 31 stops low frequency components. The annular enlargement is bounded by a shell |02 of semi-conductive material, for example, graphite mixed with clay. 'I'he longitudinal dimension g is adjusted to proper value so that short-length waves, that is,'waves of frequency above a certain limit, are diverted into this annular enlargement |0| and their energy is absorbed in the resistance material |02. Hence the waves that get through from left to right are those lying .between lower and upper limiting frequencies, and the device functions as a band pass filter.
Fig. 32 shows a band pass lter that differs from Fig. 31 principally in that -instead of the annular enlargement |0| of Fig. 31, this Fig.- 32
vhas a plurality of lateral branch dielectric guide stubs of various suitable diameters, each terminated by a non-reflecting energy-absorbing element. These stubs, so terminated, absorb the shorter waves, permiting only longer waves to go through from left to right. But waves longer than a certain limit will be stopped as explained explained heretofore in connection with Fig. 32. The incoming currents in the conductor pair |03 on the left may have various frequencycomponents. The low frequencies will be stopped at 81 and the high frequencies will be absorbed in the lateral stubs and only the intermediate band of frequencies will get through on the right. The shell surrounding the dielectric material |05 may be of dielectric material or it may be conductive. Aside from its filter function, this figure shows in simple form a coupling between a conductor ,pair and a dielectric guide adapted for diametrically polarized waves in the guide.
In connection with the subject-matter of Figs. 29 to 33, reference is made to my U. S. Patent No. 2,106,768, issued February 1, 1938, entitled Filter system for high frequency electric waves.
Consider a dielectric guide of two parts of different diameter as in Fig. 34. Let it carry waves progressing from left to right that are of a single wave length such that their lines of force are in the form of closed or nearly closed loops in the part of greater diameter (like Case II of Fig. 6), but the lines extend out into adjacent space for the part of less diameter (like Case I of Fig. 6). With propagation from the part of greater diameter to that of less diameter, that is, from left to right,'as in Fig. 34, there will be very little radiation from the guide on the left but considerable radiation from the guide on the right.- Thus the part of the guide of reduced diameter may be regarded as a radiating antenna and the part of greater diameter as the feeder thereto.
In Fig. 35 the dielectric member is of uniform diameter, and in its-non-radiating part it is surrounded by a metallic shell. Here the transmission of energy is as in Case I of Fig. l2. At the transition fromthe non-radiating part to the radiating part the shell is ended; and in the radiating part the lines of force extend out into the surrounding space as shown in Fig. 35.
In Fig. 36 a directive antenna array is shown based on the principle explained in connection with Figs. 34 and 35. Each of the guides 80, 9|, 92, 93 and 94 is fed from below upward with energy of proper wave length and in proper phase relation. From each section of reduced diameter there will be radiation, and in. accordance with the known principles of directive antenna arrays, all these foci of radiation lying in a two-dimensional array will give a narrow radio beam. Instead of enlarging the diameter in the non-radiating parts of the antennas, the radiation may be inhibited by surrounding these parts with metallic shells, as explained for Fig. 35.
Note is made here of my pending application, Serial No. 743,753, iled September 12, 1934, in which is further developed and claimed the subject-matter of Figs. 34 to 36.
For demodulation at the receiving end of the guide we vmay use finely divided carborundum and clay, mixed and baked, for a section of the guide at that end. The current-voltage relation in this substance is non-linear, and this makes it a good modulator for high frequencies. This section of the guide should have its conductivity and dielectric constant chosen at the proper values to make the impedance match the impedance in the adjacent normal part of the dielectric guide. Referring to Fig. 37, the electric waves in the dielectric guide coming from the left-meet the disc 11 of distorting or detecting material which, by virtue of its non-linear characteristic gives rise to electromotive forces having frequencies that comprise not only the carrier and side band frequencies, but also twice the carrier frequency plus or minus the side band frequencies. By proper choice of the transmitting frequency or of a locally introduced carrier whose source is not shown in Fig. 37, the desired side bands may be obtained as currents in the coaxial conductor system that extends to the right. In this way we come down, say, cfrom a frequency range in the neighborhood of 1,750
megacycles to a range from 1 to 3 megacycles in the concentric conductor system on the right. 'I'hese currents are then dealt with in the usual way in wave conductor systems to get the signal indications therefrom. Instead of the carborundum-clay mixture mentioned above, one may use finely divided zinc oxide and clay, mixed and baked, or fused silicon or copper oxide or crystals such as galeria or iron pyrites may be used. The moulded materials may be so compounded as to give a wide range of conductivities and effective dielectric constants thereby making it possible for the detecting material used at the terminal to be selected to match the characteristic irn-v pedance of the guide.
From the beginning of the practice of electrical signaling, the tendency has been toward the use of Wider bands of frequencies. The simple, slow dots and dashes of early telegraphy required but relatively narrow bands of frequencies, but the y speed of signaling has increased until now the band of frequencies necessary for transmission taxes the existing facilities, as for instance, in the case of ocean telegraph cables. In telephony there has been a gradual trend toward the use of wider frequency bands in order to attain increased intelligibility and naturalness. In television there has been a corresponding extension of the required frequency band width, and it is recognized by those familiar with the problem that, ultimately, television will require a frequency range yet Wider and of the order of one or several million cycles per second. Such a wide frequency range is needed also to enable a plurality of narrow communication bands to be grouped and transmitted together.
My improved system of electromagnetic transmission by means of guided dielectric waves opens the way to transmit on higher frequencies than have been employed commonly heretofore. It is well understood that conductive currents develop the skin effect at high frequencies and for this reason and for other reasons, it has not been a common practice to transmit conductive currents effectively at very high frequencies over long distances. Ordinarily such currents have been subject to high attenuation, and for this reason and other reasons their long distance transmission has not been practiced to any great extent.
It is Well known that a certain amount of signal intelligence requires a certain frequency band Width, whether at high frequency or at low frequency. For example, to transmit a telephone conversation with a certain rather good quality may require a band Width of 5,000 cycles. This means a width from 5,000 to 10,000 cycles, or from 95,000 to 100,000 cycles, or from 995,000 to 1,000,000 cycles. The width in frequency range is thesame wherever it occurs along the frequency scale. The percentage width is very different in these different cases, being 50 per cent in the case rst mentioned, 5 per cent in the next case, and one-half of 1 per cent in the next case.
There are certain transmission diiculties which are more dependent on the percentage width of the band than on its absolute width. In the case assumed in connection with Fig. 1 of the drawings, the absolute width of the frequency band that is transmitted is 2,000,000 cycles. Compared with the frequency ranges commonly considered, this seems like a very wide frequency range, and it is wide in absolute measure. lIt is 400 times the range that is necessary for a good telephone conversation, and it is about double the whole range for all the various stations and channels of public radio broadcasting in the United States, which is from 550 kilocycles to 1,500 kilocycles. This broadcasting range from 550 kilocycles to 1,500
. kilocycles is a range of about 63 per cent, assuming a 100 per cent base at the upper limit of 1,500 kilocycles. But the 2,000,000 cycle range assumed in connection with Fig. 1 is less than 11i/100 of 1 per cent compared with the upper limiting frequency of 1,753 megacycles. This indicates the enormous gain in absolute frequency range that may be attained by going to very high frequencies. That is, with very high frequencies one may have an enormously wide frequency range, and a corresponding amount of intelligence may be carried, although that frequency range is small from the percentage standpoint.
In the transmission of intelligence by means of electromagnetic waves, there have been two general methods employed heretofore: (1) by conductive waves along metallic guides, and (2) by radiated displacement waves in free space. The frequencies commonly employed along conductive guides have seldom been greater than 30,000 cycles or somethingl of that order, and, of course, with that limit, the available frequency range for carrying intelligence on one conductive circuit is of no greater magnitude than this 30,000 cycle range. In many cases the frequency range on conductor guides is far less than this, as for example, in the case of long ocean cables where the' telegraph speed is only a few hundred words a minute and telephone communication has not yet been practiced. Land wire lines are capable of transmitting wider frequency ranges. f 'I'he use of loading coils and the use of repeaters are more practicable with land lines than with ocean cables and they are effective in a limited degree tov widen the frequency range that can be transmitted over long distances.
For radiated waves in free space, Wave lengths have been employed commercially from as long as several kilometers in length, down to as short as of the order of about 4 meters. The longer space Waves follow the earth in its curvature, and tnus signals maybe transmitted by such waves over non-optical paths, but at the shorter wave lengths mentioned, that is, of the order of 10 meters down to 4 meters or less, the waves will follow a non-optical path imperfectly, or not at all; that is, such short waves will not always bend perfectly around the earths surface and will go from transmitter to receiver only along a straight or nearly straight line outside the earths surface. This puts a severe limit on the distance to which they can be transmitted.
As distinguished from systems that employ guided conductive waves and systems that employ radiated displacement current waves, my system involves in some cases guided displacement current waves and in others a combination of guided displacement currents and conductive currents, It is true that antenna arrays have been employed that direct the waves of displacement current in free space, but that involves an initial directing of the wave, and does not involve any guiding of them as occurs in connection with conductive waves along metallic conductors.
By my invention it becomes possible to employ displacement current waves guided along dielectric guides, and thus to go to high frequencies with their corresponding favorable band widths.
The propagation of waves along dielectric guides appears to be sui generis in the realm of electromagnetic wave transmission and readily distinguishable from other forms of guided wave propagation. Comparison of dielectric guide systems with other systems for guiding electromagnetic waves may serve to emphasize some of the distinguishing features. In some cases the nature of the structure along which the waves are guided is itself the salient feature of differentiation; in other cases the guiding structures may be identical and the nature, characteristics and behavior of the guided electromagnetic waves must be looked to. In most cases, both the guiding structure and the waves will be seen to be essentially different.
Thus, dielectric guide systems may be contrasted with ordinary conduction current systems utilizing as the guiding structure a pair of metallic wires, shielded or unshielded, or a pair of coaxial conductors. Considering first the nature of the guiding structure, it will be apparent that no structure falls in the category of conduction current systems that does not provide two or more conducting members suitable for the go-and-return flow of conduction current. There can be no confusion therefore between conduction current systems on the one hand and such typical dielectric guides, on the other hand, as consist wholly of dielectric material or of dielectric material with a single metallic core or of a single metallic pipe containing only a dielectric medium. A dielectric guide, however, may comprise a plurality of metallic conductors, and where it comprises, for example, both a metallic sheath and a metallic core, as disclosed in this speciiication, the structure is essentially the same as that of a coaxial pair, and if the one system is to be distinguished from the other the nature, characteristics, etc., of the waves guided along the structure must be examined.
In the ordinary coaxial conductor transmission system, as in all conduction current systems, the go-and-retum flow of conduction currents is an essential and significant feature. In a dielectric guide system such current flow may be absent or there may be conduction current in only one conductor of the guide, e. g., in a metallic sheath. In the conduction current system again there is no component of either the electhric or magnetic field that lies in the direction of wave propagation, excepting, of course, to take account of the trailing of the wave front in the vicinity of imperfect conductors, this trailing representing a fiow of energy into the conductors equal to the energy loss occurring therein. In all of the dielectrically guided waves herein disclosed, on the contrary, either the electric field or the magnetic field has a substantial component in the direction of wave propagation, entirely independent of energy loss in metallic elements, this longitudinal component, in all of them too, being evident in a longitudinal flow of magnetic current or of displacement current through fairly well defined regions within the dielectric medium.
In other respects, too, the dielectrically guided waves herein disclosed differ radically from the `waves associated with ordinary conductor sys.
tems. The velocity at which they are propagated along the guide, and therefore the wavelength within the guide, depends in a marked degree on a transverse dimension of the guide. T'he diameter is the significant dimension in the case of a simple cylindrical guide. 'I'his characteristic is in strong contrast with ordinary conductor systems, where the velocity and wave-length are substantially independent of the transverse dimensions.
Another striking characteristic of the waves herein described is the existence of a cut-oil frequency separating a high frequency range of easy transmission from a lower frequency range of zero or negligible transmission. 'I'he frequency at which this cut-off occurs depends on a number of factors, such as the field pattern of the wave, the dielectric constant of the medium, and a transverse dimension of the guiding structure. Regardless of the factors involved, however, it is true that for any particular guide and type of dielectrically guided wave herein disclosed this anomolous behavior of the attenuation-frequency characteristic may be observed.
Electromagnetic waves of optical frequencies have been propagated within quartz rods and within polished tubes, but this involves a manner of transmission entirely distinct from that of a dielectric guide system. There are myriads of independent waves, each arising from an atom within an incandescent source, and all are in random frequency and phase relation. Each wave progresses along optical paths and is confined, not guided, within the dielectric medium by repeated internal reflection from the dielectric or metallic surfaces. Such waves have none of the important characteristics, hereinbefore described, that are generally attributable to dielectrically guided waves.
From the foregoing comparisons it is evident that the novel waves described in this application are essentially different from any waves heretofore known and used, as different in fact as radio waves and ordinary conduction currents diifer from each other. It may well be, however, that the speciiic forms of waves herein disclosed are only representative of a broader class of waves, the limits or boundaries of which are yet to be accurately fixed, and it is impossible at this time to predict what feature or features of the several herein discussed might be found to be the common link between them all. 'I'he term "dielectrically guided" as used in the appended claims, therefore, is intended to embrace the various novel waves so denominated in this specification and such other waves as may fairly be found equivalent thereto.
By dielectric guide is to be understood any wave-guiding structure capable of sustaining dielectrically guided waves. All such guides appear to be characterized in that they comprise a dielectric medium having an enclosing boundary defining a discontinuity in electrical properties.
What is claimed is:
l. The method of transmitting electromagnetic effects which comprises applying electromagnetic waves to a dielectric guide and propagating these waves along said guide with the energy flow largely in the one direction of propagation and deriving the power available within the guide at some other point along the guide, the propagation being characterized by a critical existence relation between the frequency of said waves, a transverse dimension of said guide and