US2190511A - Ultra short wave system - Google Patents

Description

Feb. 13, 1940. J M A ULTRA SHORT WAVE SYSTEM Filed Marchl. 1938 3in, E 7/0-l0V Inventor: John MCage, by 51 H ttorney.

Patented Feb. 13, 1940 PATENT OFFICE.

ULTRA SHORT WAVE SYSTEM John M. Cage, Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York Application March 1, 1938, Serial No. 193,316

'1 Claims.

The present invention relates to improvements in ultra short wave systems and more especially in amplifiers, detectors, oscillators and converters for use on wave lengths on the order of 1 meter to five centimeters or less. While not limited thereto, the invention is particularly applicable to velocity modulation-systems such as are described and claimed in Wm. C. Hahn application S. N. 153,602 filed July 14, 1937, and assigned to the same assignee as the present application.

Inasmuch as an adequate explanation of the invention necessarily involves the use of various terms of more or less technical character, I have, in the following paragraphs, set forth the meanings which I desire to attach to certain such terms.

By conduction current I intend to designate a stream of moving charges, such, for example, as an electron beam current passing through an evacuated or gas-filled conduction space.

By conduction current modulation I mean to designate the controlled production of irregularities in a conduction current stream. Thus, a conduction current modulated electron beam is a beam in which at any given time, systematic irregularities in electron velocity or electron density exist from point to point along the beam.

By charge density modulation I mean the controlled production of irregularities in the distribution of charges within a conduction current stream. Thus a charge density modulated electron beam is a beam in which at any given time the electron density varies from point to point along the beam in accordance with some controlled pattern of variation.

By velocity modulation I mean the controlled production of irregularities in charge velocities within a conduction current stream. Thus a velocity modulated electron beam is a beam in which at any given time the electrons at various points along the axis of the beam are moving with different velocities according to some controlled pattern of variation.

Quantitatively, any type of modulation may be measured as the ratio of the magnitude of the maximum departure of the modulated quantity from its average value to the magnitude of such average value. Thus a charge density modulated electron beam in which the electron density along the beam axis varies from zero to twice the average density may be said to possess 100% charge density modulation.

In conventional electronic vacuum devices the control member is ordinarily so constructed and arranged as to afiect directly the electron emission of the cathode, thus producing a type of charge density modulation" as hereinbefore defined. It may be shown that the conduction current variations so produced by the grid have the effect of inducing a similarly varying current in the grid circuit. Under ordinary conditions and at low frequencies this induced current, which is caused by instantaneous difierences in the electron charges approaching and receding from the grid, is relatively small and is approximately out of phase with the grid voltage so that it produces no appreciable power loss. However, if the operating wave length is decreased so that the electron transit time becomes appreciable with respect to the reciprocal frequency (l/f) of the control grid potential variations, the induced current not only increases but becomes more nearly in phase with the grid voltage. These two effects combine to produce the result that the apparent shunt resistance of the grid circuit varies inversely as the second power of the frequency of the operating voltage. It is for this reason that at very high frequencies (1. e., very short wave lengths) the conventional type of grid attains such a low shunt impedance and involves such a large power loss as to be practically unusable.

In the application S. N. 153,602 previously referred to, novel discharge devices and methods of operation are described such that the shunt impedance of the control circuit may be maintained at a very high value even when control potentials of very short wave lengths are involved. In one view of the invention disclosed in the said application this is accomplished by providing control electrode structures which are so constructed and operated as to produce primarily velocity modulation of the discharge current without the occurrence of appreciable charge density variations in the vicinity of the control electrode. The velocity modulation thus produced is subsequently converted into charge density modulation under conditions which have no adverse reaction on the control electrode circuit. It is found that proper application of the basic principles of the Hahn and Metcalf invention makes possible the construction of amplifiers, detectors, oscillators, and converters which are eminently suited to operation at very short wave lengths. Insofar as the principles of the invention are relevant to an understanding of the improvement described herein they will be further explained in the following.

In dealing with ultra short wave devices of the type referred to, considerable difficulty is experienced in transmitting power at short wave lengths from an antenna system to a control electrode structure, or conversely, from an output electrode to an antenna or other energy-converting device. It is a primary object of the present invention to provide means whereby such transmission may be greatly facilitated. In accordance with one aspect of the invention this is accomplished by the provision of a resonant transmission line hav ing its respective conductors physically and electrically continuous with electrode elements of a discharge device from which, or to which energy is to be transmitted. (In referring to electrical continuity between two sets of connected elements I wish to specify the condition in which transition from one set to the other will involve no significant change in characteristic or surge impedance, the latter terms being used in the sense in which they occur in conventional transmission line theory.)

The features of novelty which I desire to protect herein will be pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the drawing in which Fig. 1 illustrates in partial section a short wavetube suitably embodying the invention; Fig. la is a plan view of a fragmentary portion of Fig. 1; Figs. 2, 3 and 4- are fragmentary sectional views representing structural modifications of the embodiment of Fig. 1; Fig. 5 illustrates the invention as applied to a super-regenerative detector, and Fig. 6 shows the invention as applied to an amplifier.

In the drawing above referred to I have indicated certain voltage ranges as belngsuitable for the operation of the various electrodes. It should be understood, however, that the values given are exemplary only, and that they may be varied within wide limits even to the extent of changing their order of magnitude.

Referring particularly to Fig. 1, I have shown a discharge device which comprises a sealed glass envelope l terminating at one end in an external press II and at the other end in an internal stem and press I2.

Within the envelope there is provided means for developing an electron beam of substantially constant average intensity and velocity. Such means may include any known type of "electron gun, and that illustrated constitutes only one example of the many possible constructions. In the arrangement shown, the electron source comprises a filamentary heater l and an enclosing electron emissive cylinder l6. Surrounding the cathode as a whole there is provided a focusing electrode in the form of a conducting tubular member l8. This latter element is in turn surrounded by an insulating bushing l9 and by a second conducting tubular member 20.

In the use of the device the filamentary heater I 5 is energized by means of a suitable energy source, such as a battery 22, connecting with the heater through suitable lead-in connections 23. The focusing electrode 18 is maintained at cathode potential or at a potential which is slightly negative or positive with respect to the cathode so as to concentrate the emitted electrons into a beam of generally cylindricaloutline. Such a beam may be given the desired velocity by impressing an appropriate potential between the cathode and a transversely extending tubular conductor 25. This latter conductor is supported on the member 20 and is supplied with potential by means of a lead-in connection 28. The magnitude of the potential to be applied will vary within wide limits depending on the conditions of operation. For a particular case it may be on the order of from 200 to 400 volts and may be provided by means of a battery 21 connected between the cathode and the lead-in conductor 28.

The tubular conductor 25 is provided with aligned openings 29 adapted to permit the passage of electrons therethrough. It serves two functions, but will first be described in its capacity as an electrode element. As such it defines a modulating space having entrance and exit boundaries which are at a fixed potential with respect to one another. With the connections illustrated, their potential may also be fixed with respect to ground.

concentrically arranged within the modulating space there is provided another conductor 30 having an opening 3| therein which is aligned with the openings29. The upper end of the conductor 30, which also functions as an electrode element, is electrically separate from the conductor 25 and may be made to vary in potential with respect thereto.

Let it be assumed that the potential of the inner electrode element (i. e., the upper end of the conductor 30) is caused alternately to rise above and fall below that of the outer element (1. e., the upper end of the conductor 25). If this is accomplished at such a rate that the electron transit time through the opening 3| corresponds to a half cycle or an odd number of half cycles of the potential variations, then the velocity of any given electron will be similarly affected as such electron approaches and recedes from the inner electrode. That is, an electron which is accelerated as it approaches the electrode 30 will, if its transit time through the electrode is proper, be again accelerated as it recedes from it after leaving the opening 3|. (Since the electron transit time is an aspect of the average beam velocity it can obviously be adjusted by changing the accelerating potentials to which the beam is subjected.) Similarly, under the conditions predicated, an electron which approaches the electrode at such a moment as to be retarded by its field will also be retarded on leaving the electrode, this being due to the reversal of potential which occurs during passage of the electron through the electro-statically shielded region within the electrode interior.

With the specified electron transit time relationship fulfilled, an electron beam which traverses the modulating space will be velocity modulated in accordance with the amplitude variation of the potential applied to the inner electrode element. That is to say, controlled variations in electron velocity will be produced from point to point along the beam.

If the approach and recession spaces between the conductors 25 and 30 are short. the velocity modulation produced in the manner described will be ineffective to create appreciable "charge density variations within the confines of the modulating space. In other words, little sorting or regrouping of accelerated and retarded electrons will occur in the vicinity of the electrode structures. Since the velocity variations themselves are of small relative magnitude the resulting current induced in the control circuit (i. e., in the circuit elements associated with the electrode elements 25 and 30) as a result of disparities in the rate at which charges are approaching and receding from such electrode element will be substantially negligible. Consequently,

the power loss in the control circuit will be extremely small.

No consideration has so far been given to the means by which the desired potential variations of the inner electrode are produced, nor to the manner in which the relatively minute velocity variations created in the modulating space are converted into charge density variations of useful magnitude. This point will be discussed in the following.

Referring again to Fig. 1 it will be seen that to the right of the modulating space there is provided an additional electrode 35 positioned in the path of the electron beam. A hollow cylinder 34, electrically connected to the conductor 25 serves to shield the beam and to direct it toward this electrode.

By means of a battery 36 the electrode 35 may be maintained at a low potential, saya few volts negative or positive with respect to the cathode It, so that it is effective to reverse all or a part of the approaching electron beam. In

-either case, as is explained in the aforesaid application S. N. 153,602, the reversed component will be charge density modulated to an extent which is dependent on but is of a higher order of magnitude than the velocity modulation imparted to the beam in the modulating space. I

its efiect in approaching and recedingfrom the inner electrode element 30 is to induce oscillations in the circuit to which the element is coupled. If the various parts of the system are properly related these oscillations may be of such frequency and phase as to render the system selfsustaining. This means of obtaining high frequency oscillations is described and claimed broadly in Geo. F. Metcalf application Serial No. 201,953, filed April 14, 1938, and is not intended to be claimed herein except as modified by the use of my improved electrode arrangement as described in the following.

For the practical operation of the device, some means must be provided for utilizing the oscillatory energy developed. In the arrangement illustrated such means comprises a radiating antenna 38 joined to the conductor 30 at a point near its lower end. This antenna may appropriately be of a length corresponding to an odd number of quarter wave lengths of the frequency at which the system is to operate and should be insulated from direct contact with the outer conductor 25.

For effectively transmitting energy between the electrode elements and the antenna, there is provided a concentric transmission line comprising the two axially symmetrical conductors 25 and 30. In order that this line may serve as a transforming element for matching the relatively low impedance of the antenna to the very high impedance of the electrode system, it is preferably an odd number of quarter wave lengths long and is closed at its lower end by a conducting body 39 which electrically connects the inner and outer conductors. The oscillating system thus formed is adapted to sustain a standing wave having a voltage node at the closed end of the line and a voltage anti-node at the open end.

As a corollary of the impedance matching function of such a line it maybe noted that it serves to transform high voltage low current energy at the electrode end of the line into low voltage high current energy at the antenna end. With the arrangement described a standing wave may be maintained on the antenna, the voltage anti-node being at the outer extremity.

In order that the energy conversion may be as efficient as possible it is deslrablethat the efiective length of the transmission line be as close as possible to a quarter wave length or to an odd number of quarter wave lengths long. However, this can be done and the system still be maintained in resonance only if the surge impedance of the electrode system coupled to the transmission line is equal to the surge impedance of the line itself. Any departure from this condition will require a compensatory shortening or lengthening of the line to maintain resonance and will decrease the overall effectiveness of the system.

In accordance with my present invention fulfillment of the theoretically ideal condition is attained at least to a very close approximation by making the electrode elements physically and electrically continuous with the line conductors to which they are connected. That is, as shown in Fig. l, the electrodes are respectively constituted of the upper ends of the conductors 25 and 30. (Assuming that the overall length of the conductor 30 corresponds precisely to a quarter wave length, the opening 3| should be as near its upper extremity as possible.) With this arrangement the electrode surge impedance is necessarily substantially identical with that of the remainder of the transmission line.

The precise form of transmission line illustrated in Fig. 1 is in no way essential for the purposes of my invention and in Figs. 2.and 3 I have illustrated alternative forms which may be .used. In both of these figures elements corresponding to parts already described are similarly numbered. Referring to Fig. 2 I have shown an arrangement which diifers from that already discussed only in that the central conductor, numbered 30', is of hollow configuration. It will be understood that while this change may affect the constants of the line to a slight extent, it will not alter its mode of operation in any important way.

In Fig. 3 the transmission line shown is symmetrical with respect to a plane passing through the axis of the electron beam. It comprises a solid inner conductor 40 arranged symmetrically within a cylindrical outer conductor 4 I, both conductors being of a length corresponding to a half wave length (or odd number of half wave lengths) of the frequency at which the illustrated device is intended to operate. Each end of the line is terminated by a short circuiting conducting body 42 at a point a quarter wave length distant from the axis of the electron beam, that is to say, from the common center line of the openings 44 through which the beam is projected. Near one of the short circuited ends of the line there is provided a quarter wave antenna 43, corresponding in essential respects to the similar element 38 described in connection with Figxl.

In this case, the concentric transmission line may be made to resonate in such a way as to maintain a standing wave having voltage nodes at the extremities of the line and a voltage antinode at its center. Due to the fact that the line is closed at each end by the bodies 42, it is free of the end eflects which necessarily exist to a certain extent in connection with the arrangements of Figs. 1 and 2. Otherwise, its operation is similar.

The manner of coupling the antenna to the transmission line may also be varied from that described in connection with Figs. 1, 2, and 3. For example, in Fig. 4, I have shown an arrangement in which the antenna comprises simply an extension of the inner conductor of the transmission line.

In this case, as in the constructions previously referred to, both the transmission line and the antenna are enclosed within the confines of the envelope ID. The transmission line comprises an inner conductor 45 and an outer conductor 46, both extending on opposite sides of the axis of the electron beam. A short-circuiting connection, between the two conductors is made at 41, a quarter-wave length from the beam axis. By this means the transmission line is caused to sustain a standing wave having voltage nodes at 4! and at a point X spaced one-half wave length from 41.

The exposed portion 45', which constitutes the antenna, is capacitively coupled to the transmission line by means of a conducting part 48 extending from the outer conductor 46 to a region closely adjacent to the inner conductor.

This part is preferably spaced from the nodal point X such a distance as to match the impedance of the antenna to that of the electrode system. A convenient length for the antenna is a quarter wave length from the point of capacitive coupling.

In Fig. I have illustrated the invention as applied to a discharge device used as a detector. I

In this case the discharge envelope comprises an elongated cylindrical metal portion 50 closed at one end by an abutting header 5|. Lead-in connections 52 brought through the header tobayonet terminals 53 permit energization of the enclosed electrodes in the manner previously described in connection with Fig. 1. The electrodes include a cathode 55, a focusing element 56 and concentrically arranged conductors 51 beam to a desired extent.

and 58 arranged in the path of the electron beam. The outer one of these conductors is directly connected to the envelope 50 and in common with the envelope, is maintained at a high potential sufficient to accelerate the electron This may be accomplished, for example, by means of a suitable potential source such as a battery 60. The conductors are provided with aligned openings 62 and 63 and conjointly serve to velocity modulate the electron beam in accordance with the principles previously set forth. As in the arrangements previously described, the conductors 51 and 58 constitute a quarter wave concentric transmission line which is short-circuited at one end. A receiving antenna 65 is coupled to the inner conductor 58 through a suitably tuned circuit including an inductance 66 and a capacitance 61. A leadin conductor 88, insulated from the conductor 51 serves to make the connection and should be tapped in at such a point as to match the impedance of the antenna system to that of the electrode system provided at the upper end of the transmission line.

The antenna 65 is adapted to receive incoming signals and to impress such signals on the transand 11.

mission line to effect velocity modulation of the electron beam passing through the openings 62 and 63. In the desired operation of the device this modulation is a function of the received signal. An additional electrode positioned in the path of the electron beam is so biased by means of a battery H as to reverse at least a portion of the beam, thus converting its velocity modulation into charge density modulation in accordance with the principles previously set forth herein.

By analogy with the description given in connection with Fig. 1 it will be seen that with the elements so far described the device of Fig. 5 should operate as an oscillator. That is to say, the reverse component of the electron beam on returning to the velocity modulation space enclosed by the conductor 5! should produce sustained oscillations of the electrode system and its associated transmission line. This operation may be prevented, however, and satisfactory detector action obtained by supplying a quench voltage to the system somewhat in the manner of the schemes practiced in connection with super-regenerative detectors of other types. Such a quench voltage may be provided for example, by means of a tuned circuit including a condenser 12 and an inductance 13 connected in series with the electrode 10 and coupled to a voltage source I4 having a frequency only a fraction of that of the received frequency. With a circuit such as that described the device, of Fig. 5 may be made to operate effectively as a rectifier (detector), the rectified voltage appearing across the resistor 19, that is between the terminals 16 A condenser 18 connected as shown will serve to filter out the quench frequency.

In Fig. 6 I have shown a device, usable as an amplifier, in which my invention is utilized in connection with both input and output electrode systems. In this case, an electron gun including a cathode 80 and a focusing cylinder 8| serves as an electron beam source. Concentric conductors 83 and 84 constitute a combined velocity modulating electrode system and quarter-wave transmission line. Control voltage is supplied to the system from a receiving antenna 85 which is shown as being connected to the inner conductor 83 near its lower end.

During passage through the modulating space provided by the hollow conductor 84 the electron beam is velocity modulated in accordance with the principles explained in connection with Fig. 5. In order that this velocity modulation may be converted into charge density modulation it is permitted to pass through a conversion space provided by a series of axially aligned hollow conducting cylinders 81. due spreading of the beam as a result of the dispersive forces acting within it, certain of the cylinders are charged to a relatively high potential and alternate ones are charged to a relatively lower potential so as to exert a focusing effect on the beam. During passage through the conversion space the retarded electrons tend to be overtaken by the accelerated ones so that a bunching of electrons occurs. Such bunching is equivalent to charge density modulation as hereinbefore defined.

After passage through the conversion space the charge density modulated beam is projected into a second electrode system generally similiar to that constituted by the conductors 83 and 84. This may comprise, for example, a quarter wave concentric transmission line made up-of axially In order to avoid unsymmetrical conductors 89 and 90. Passage of the charge density modulated beam through these electrode elements will serve to induce current variations in the resonant system formed by the conductors 89 and 90. As is more fully explained in the aforesaid application S. N. 153,602, this eflrect will be most pronounced if the axial length of the hole 92 in the conductor 89 corresponds at least approximately to the spacing between adjacent charge density maxima and minima in the charge density modulated beam or to some odd multiple of such spacing; If this be true, the approach of a charge maximum toward the conductor 89 will coincide with the recession of a charge minimum and the resultant effect as viewed in the external circuit will be at itsgreatest possible value.

The energy transmitted along the conductors 89 and 90 may be utilized in a suitable load circuit by connecting the same to terminals 94 and 95. These terminals should be coupled to the transmission line at such points that the impedance of the load is substantially matched to that of the electrode system. After passage through the various electrode elements referred to, the beam, robbed of a portion of its energy, may be caused to impinge on a collecting anode 91. This is preferably biased to a sufficiently high positive voltage to insure collection of the entire beam.

The system described in the foregoing pararaphs is especially adapted for use as an amplifler wherein a relatively weak signal received at the antenna 85 is caused to produce a relatively more powerful response at the terminals 94, 95.

While I have described my invention in connection with a particular type of discharge device, it is applicable to any system in which closely adjacent electrode elements are caused to vary in potential with respect to one another at high frequency. I therefore, aim to cover in the appended claims, any combination of transmission line conductors and electrodes wherein the latter are electrically continuous with the former and wherein the principles of my invention are otherwise incorporated.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, means for producing an electron beam and a pair of electrically separate concentric conductors arranged transversely to and in the path of the electron beam, said conductors being provided with aligned openings to permit passage of the beam therethrough and being electrically connected to one another at a point spaced from the axis of the beam by a distance corresponding approximately to an odd number of quarter wave lengths of an electrical quantity desired to be transmitted along the conductors. p

2. In high frequency apparatus, the combination which includes means for producing a beam of electrons, a. par of parallel conductors of such dimensions and spacing as to constitute a transmission line which is resonant at a desired frequency of operation of the apparatus, said conductors being arranged transversely to and in the path of the electron beam and being provided with aligned openings which permit the beam to pass completely through both conductors, thereby to facilitate efiective mutual electrical reaction between the transmission line and the beam.

3. In a short wave system, the combination which includes means for producing an electron beam, an electrode system including apair of juxtaposed, electrically separate electrode elements positioned in proximity to the beam so that each of the elements is traversed by the beam, a transmission line having inner and outer conductors which respectively comprise electrical and physical continuations of said electrode elements, said transmission line being adapted to resonate at a particular frequency, and means for exciting the transmission line at the said particular frequency, thereby to effect a corresponding modulation of the beam.

4. In combination, a short wave tube including means for producing a stream of electrons, a pair of axially symmetrical conductors arranged transversely to and in the path of the electron stream, said conductors being provided with aligned openings to permit passage of the electrons therethrough, means electrically connecting the conductors to one another at a point spaced from the electron stream by a distance corresponding approximately to an odd number of quarter wave lengths of an electrical quantity desired to be transmitted along the conductors, and an antenna system connected to one of the conductors adjacent to their said point of connection.

5. In combination, means for producing an electron beam, 'a concentric transmission line resonant to a particular frequency, means for projecting the electron beam transversely through the inner and outer conductors of the transmission line at .a point near a voltage anti-node thereof, and means coupled to the transmission line to effect excitation of the line at the said particular frequency and thereby to cause modulation of the electron beam.

6. In combination, means for producing a beam of electrons, a pair of concentric conductors positioned in the beam path and having aligned openings to permit passage of the beam transversely through both conductors, the said conductors being of such length as to constitute a resonant transmission line adapted'to oscillate at a particular frequency and having transverse dimensions which are so correlated to the velocity of r the beam as to assure effective modulation of the beam when the line is excited at the said particular frequency, and means for exciting the line at such frequency.

7. In combination, means for producing an electron beam, means for modulating the beam at a particular frequency, a pair of concentric conductors arranged in the path of the modulated beam and having aligned openings therein to permit passage of the beam transversely through both conductors, the transverse dimensions of the conductors being correlated to the beam velocity so as to assure effective mutual reaction with the beam at the said particular frequency of modulation, and means coupled to the conductors for receiving energy therefrom.

JOHN M. CAGE.

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