US 3792302 A
A very high frequency electromagnetic energy device with a slow wave structure is disclosed having impedance matching means and lumped as well as distributed elements for energy propagation. An embodiment comprises a backward wave oscillator device incorporating elements used at microwave frequencies together with lumped capacitance and inductance elements to provide a device operative in a frequency range of, illustratively 100 to 200 MHz.
Description (OCR text may contain errors)
United States Patent 1191 Downing et al.
[ VHF SLOW WAVE STRUCTURE  Inventors: Arthur HrDowning, Woburn;
' Hans-Joachim Krahn, Burlington,
both of Mass.
 Assignee: Raytheon Company, Lexington,
[221 Filed: 'nec.22 ,'19172  Appl. No.1 317,586
 US. Cl 3l5/3.6, 315/39.3, 333/31,
333/32  Int. Cl. H0lj 25/34  Field of Search....'., 315/36; 333/31, 32, 34
 References Cited UNITED STATES PATENTS Martin et a1. 333/31 R 3,549,938
McDowell 333/31 1 Feb. 12, 1974 2,393,785 1/1940 Leeds 333/32 3,250,945 5/1966 Sample 315/39.73 3,400,298 9/1968 Krahn 333/31 R Primary Examiner-James W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr.
Attorney, Agent, or Firm-Har0ld A. Murphy; Joseph D. Pannone; Edgar O. Rost  ABSTRACT A very high frequency electromagnetic energy device with a slow wave structure is disclosed having impedance matching means and lumped as well as distributed elements for energy propagation. An embodimerit comprises a backward wave oscillator device incorporating elements used at microwave frequencies together with lumped capacitance and inductance elements to provide a device operative in a frequency range of, illustratively 100 to 200 MHz.
4 Claims, 6 Drawing Figures VHF SLOW WAVE STRUCTURE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to crossed field electron interaction devices and, particularly, a slow wave structure incorporating lumped and distributed elements.
2. Description of the Prior Art Crossed field electron interaction devices of the traveling wave type commonly include a periodic slow wave electromagnetic energy propagating structure such as an interdigital delay line. Such propagating structures are typically coextensive with and spaced from a continuous sole electrode to define therebetween an electron interaction path within an evacuated envelope. The sole electrode may be of either linear or circular configuration. An electron beam generation and direction means is disposed at one end of the interaction path and means for collection of the spent electrons is disposed adjacent to the other end. Suitable electrical biasing potentials are established between the sole electrode and slow wave structure to provide a transverse electric field. External magnets provide for the establishment of a magnetic field perpendicular to the electric field or parallel to the tube axis and such electron devices, therefore, are referred to as crossed field devices. The combined influence of the electric and magnetic fields provides for interaction of the electron beam in an energy exchanging relationship with the electromagnetic wave energy propagating along the slow wave structure. Adjustment of the electric and magnetic fields as well as the beam trajectory results in forward or backward wave interaction with the electron beam.
Traveling wave devices of the crossed field type operate in a nonreentrant manner. The term nonreentrant is defined for the purposes of the specification as referring to the requirement that the electron beam traverse the interaction path only once in an energy exchanging relationship and be a completely expended as possible after the first traverse. In addition, the term nonreentrant refers to the slow wave structure which provides for the propagation of electromagnetic wave energy in principally one direction with substantially no reflection of such energy by any discontinuities or mismatching of impedances. Such crossed field devices are utilized as amplifiers or oscillators in electromagnetic energy systems. In the amplifier configuration the input signal is coupled from an external source to one end of the slow wave structure and the amplified output signal is extracted from the other end. In the backward wave mode of operation the output coupling means is typically disposed adjacent to the electron beam generation means. In the forward wave mode of operation the opposite is true and the output coupling means is dis posed adjacent to the collector electrode. In an oscillator the electron beam current is adjacent until forward or backward wave oscillations occur with interaction of the beam and the electromagnetic wave propagating on the slow wave structure with the energy then coupled by a suitable coaxial transmission line to a utilization load.
In the microwave region of the electromagnetic energy spectrum traveling wave electron interaction devices are broadly utilized as amplifiers or oscillators incorporating delay lines having a plurality of periodically spaced conductive members resembling fingers in a comb. The interleaving of the elements provides for an interdigital delay line along which the energy is propagated. Such devices are highly efficient in the generation of microwave energy. For the purposes of the specification the term microwave" is defined as that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to l centimeter and frequencies in excess of 300 MHz. For the purposes of the present specification the term very high frequency (VHF) is defined as that portion ofthe spectrum having wavelengths in the order of 10 meters to 1 meter and frequencies in excess of 30,000 KHz. The generation and/or amplification of energy in the region of the spectrum having the longer wavelengths is becoming increasingly important, particularly, in long range electronic communication systems. To date, no adoption of microwave device techniques, particularly of the BWO type including an interdigital delay line, sole and collector electrodes has evolved for very high frequency applications simply because of the physical limitations which result in. costly and impractical devices by reason of size and weight. The efficiency and higher powers of microwave devices have not materialized in the very high frequency device art where vacuum devices of the klystron or radio tube art are being utilized.
A need arises, therefore, for a very high frequency device having a slow wave structure for energy exchanging relationship with an electron beam translated along an interaction path under the combined influence of perpendicular electric and; magnetic fields with a high power output capability, particularly, for long range system applications.
SUMMARY OF THE INVENTION In accordance with the present invention a VHF energy propagating slow wave structure is provided having distributed as well as lumped elements. Periodic conductive members are each terminated by an inductance element while capacitive coupling between adjacent members is provided to achieve the desired tuning characteristic. The combined components provide a structure having the characteristics of microwave frequency interdigital delay lines for supporting a fundamental forward or backward wave amplification and/or oscillation.
The terminal ends of the overall slow wave structure are provided with impedance matching means at the interface junction to handle the reactive component. The characteristic impedance of the new slow wave circuit is of the order of a few hundred ohms and is variable according to the frequency of operation. To suitably match this structure to the substantially lower characteristic impedances of coaxial transmission lines, typically, having a characteristic impedance of only 50 ohmsa transformation means of, illustratively, a quarter wavelength having a helical inner conductor is provided. The terminal ends of the slow wave structure are also provided with LC transformation sections to han dle the reactive component and reduce the impedance to approximately ohms. The end of the line opposite to the output is terminated by an attenuator having approximately 100 ohms resistance. In backward wave devices the output coupling means is disposed adjacent to the electron beam means. Efficient operation over relatively broad tuning ranges has been demonstrated.
BRIEF DESCRIPTION OF THE DRAWINGS The invention as well as details of an illustrative embodiment will be readily understood after consideration of the following description together with the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of the combined lumped distributed slow wave structure embodying the invention;
FIG. 2 is a schematic diagram of the combined slow wave structure and matching transformer sections embodying the invention;
FIG. 3 is an isometric view of a portion of the slow wave structure embodying the invention;
FIG. 4 is an isometric view of the slow wave structure adjacent to the output end with the components shown rotated 90 from plane of the view shown in FIG. 3;
FIG. 5 is a cross-sectional view, partly in elevation of a prior art microwave backward wave oscillator device; and
FIG. 6 is a partial-cross sectional view taken along the line 66 in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT Before proceeding with the description of an embodiment of the invention, illustrated in FIGS. 14 inclusive, a review of the prior art microwave device tech' nology, particularly, with regard to slow wave propagating structures will assist in understanding the invention. FIGS. 5 and 6 show a backward wave crossed field device 10 having within an evacuated envelope 12, a slow wave energy propagating structure 14 of the interdigital delay line type comprising a plurality of conductive finger members 16 supported in an alternating manner by the shoulder portions 18 and 20 of a cylindrical conductive base member 22. Upper and lower end plate members 24 and 26 form with the base member 22 the evacuated envelope 12 of the overall embodiment.
A sole electrode 28 is centrally disposed and coextensive with the slow wave structure 14 and defines therebetween an interaction path 30. Sole electrode 28 comprises a cylindrical member of electrically conductive material forming a web portion 32 having a recessed channel 34 to assist in confining the electron beam in the interaction path 30. The sole electrode is supported by a tubular member 36 positioned within end cover plate 24 and an inner tubular member 38 which is in turn secured to a tubular member 40 within the sole electrode web portion 32.
Electron beam generation means 42 are positioned within a notch 44 in sole electrode 28. The electron beam assembly includes a cathode 46 as well as a heater, control grid and accelerating electrodes, all supported by mounting plate 48. Electrical leads 50, 52, 54 and 56 extend within tubular member 38 and terminal glass seal 58 supports the leads as well as hermetically sealing the tube envelope. A dielectric sleeve member 60 joins conductivemember 36 to an outer conductive sleeve member 62 to provide forthe electrical isolation of the sole electrode with respect to the slow wave structure.
The electric fields which extend transversely between the slow wave structure 14 and the sole electrode 28 are provided by means of unidirectional voltages supplied, for example, by a source 64 connected between tubular member 36 and cathode lead 50. The slow wave structure 14 which is connected to the envelope base member 22 is grounded as at 66. Sole electrode 28 is negatively biased with respect to the cathode by source 68 coupled between cathode lead 50 and sleeve member 62 which is in turn eonductively coupled to the sole electrode supporting member 38. The accelerating electrode of the electron beam assembly 42 is positively biased relative to the cathode 46 by means of an appropriate voltage source 70 connected between leads 50 and 56. The control grid is connected through lead 54 to a terminal 72'to an appropriate beam optic control means. A source 74 coupled between cathode lead 50 and a heater lead 52 provides for the heater voltages.
The magnetic field which extends perpendicular to the electric fields and parallel to the axis of the tube is indicated by the circle and cross 76. The magnetic field producing means includes cylindrical pole piece members 78 and 80 which are positioned adjacent to the end plates 24 and 26. Permanent magnets or suitable electromagnetic means contact the pole piece members to supply the required magnetic field intensity. The combination of the crossed electric and magnetic fields in the interaction path 30 influence the energy exchanging relationship between the electron beam and the energy propagating along the slow wave structure 14.
The electron beam generation means is disposed adjacent to one end of the interaction path 30. An electron collector electrode structure including a precollector electrode 82 formed as an extension of the slow wave structure 14 together with a director 84 secured to plate 86 is disposed at the other end. The director 84 is connected to the sole electrode 28 by plate 86 and is biased at substantially the same potential as the sole. The opposing precollector structure 82 is biased at the slow wave structure 14 potential which is substantially the same as that of the base member 22. A postcollector 88 provides for the total collection of the spent electrons after traversing interaction path 30.
The illustrative device operates in the backward wave mode of oscillation and, therefore, the output energy coupling means 90 are disposed adjacent to the electron beam generation assembly 42. The coupling means 90 comprise a coaxial transmission line having an inner conductor 92 secured to a terminal slow wave structure member 16. An outer conductor 94 completes the coaxial transmission line which is coupled to a utilization load.
Referring now to FIG. 1 the combined distributed and lumped components of the invention are schematically illustrated. The problem of mechanical dimensions to achieve a device for operation at very high frequency wavelengths is overcome by the provision of a plurality of periodic spaced conductive members 96, preferably in the form of a straight bar which is similar to the main vertical portion of L-shaped members 16, illustrated in FIGS. 5 and 6. The required elongation of the conductive members of the microwave type interdigital delay line is accomplished by the provision of lumped elements connected to the member 96. A portion of each element 96 is serially connected by an inductance 98 to the base member forming the device envelope. Additionally, capacitive elements 100 are coupled to adjacent slow wave structure members 96 to provide the desired frequency tuning characteristics.
The combined slow wave structure components provide the same basic electrical properties interdigital delay lines capable of supporting fundamental backward wave or forward wave oscillations and/or amplification in microwave frequency devices. The characteristic impedance of the illustrative lumped and distributed components is of the order of a few hundred ohms and varies with desired frequency of operation.
The matching of the combined slow wave structure results in essentially considering the reactive component at the junction of the slow wave delay line ends. Referring now to FIG. 2 the matching-transformation means for the output coupling means comprising a coaxial transmission line 102 having a typical characteristic impedance of'approximately 46.4 ohms is shown. To transform the delay line 104 characteristic impedance of 200 ohms to a characteristic impedance of, illustratively, approximately 100 ohms a first transformer section comprises a combination of two reactances, one in series (inductance 106) and one in shunt (variable capacitance 108). Utilizing impedance/admittance charts, the reactance and susceptance transformations for a slow wave structure capable of being tuned between 130 and. l 80 MHz would, illustratively, require a value for capacitive element 108 of, approximately 3.0 picofarads. The value for the inductance 106 was calculated to have a value of approximately 150 microhenries.
The second transformation means at the output end of the slow wave structure comprises a transformer section 110 of approximately A wavelength and providing a helical inner conductor 112. The impedance of the second transformer section 110 was calculated to provide a characteristic impedance of 68.1 ohms with a helical inner conductor having approximately 1 1 turns per inch; an overall length of approximately 2.42 centimeters; an inside diameter of the outer conductor member 114 of approximately 0.400 inches; and a diameter of theinner conductor 112 of approximately 0.240 inches. A dielectric spacer sleeve member be tween the outer conductor 114 and helical inner conductor 112 desirably has a dielectric constant characteristic of 9.0 (alumina). The transformation then of the first section including theinductance 106 and variable capacitance 108 provides a characteristic impedance transformation down to 100 ohms. The combination of the coaxial transformer 110 having a value of approximately 68.1 ohms will suitably match the combined slow wave structure to the conventional coaxial output transmission line 102.
The other end of the structure 104 is suitably transformed by means of an inductance 116 and variable capacitance 1118 having substantially the same numerical values as the first components 106 and 108. The structure is terminated at the other end by attenuator 120 having a value of, illustratively, 100 ohms resistance to thereby provide for matching at both ends optimum interaction. In accordance with the invention it is also possible to provide for variable inductances, in lieu of the inductance elements 106 and 116 in the form of an adjustable ferrite material slug to achieve the matching desired.
Referring now to FIGS. 3 and 4 a working embodiment of the invention is illustrated. To assist in the understanding of the invention only the slow wave structure together with the matching means will be described. In a complete embodiment all the remaining components, shown and described in FIGS. 5 and 6, would be employed. Slow wave structure 122 shown in FIG. 3 comprises a plurality of periodically spaced conductive members 124 spaced from the envelope base member 126 and defining with the sole electrode (not shown) the interaction path. A spacer and support member 128 is joined at alternating opposite ends of the slow wave members 124. Each support spacer member comprises conductive sleeve members 130 and 132 metallurgically bonded at opposing ends of a dielectric sleeve member 134, of an insulating material such as a ceramic. Sleeve member 130 contacts the envelope base member 124 and is brazed thereto, while opposing sleeve member 132 contacts the slow wave member 124. In accordance with the invention slow wave members 124 are capacitively coupled by elements 136 comprising a conductive wire 138 extending through a hollow portion of insulator member 140 and conductively joined to sleeve member 132 as well as slow wave member 124. An intermediate wind ing of a conductive wire 142 provides the other member to provide the capacitance coupling desired for frequency tuning.
Inductance element 144 serially connects each slow wave member 124 to the envelope base member. A knurled insulator member 146 has a conductive sleeve member 148 secured at one end by the same metallurgical techniques as those utilized for bonding similar components to the spacer and support member 128. A conductive wire 150 is supported on the insulator member 146 and end 152 is conductively secured to the slow wave member 124 while the remaining end 154 is conductively secured to the envelope base member 126. The length of the interdigital line, therefore, is suitably elongated to accommodate the propagation of energy at wavelengths in the very high frequency region of electromagnetic energy spectrum. This completes the description of the slow wave structure 122 incorporating both distributed and the lumped elements in accordance with the invention.
The output coupling means comprise A wavelength coaxial transmission line 156 with a helical inner conductor 158 dimensioned to provide an impedance transformation value, of, approximately, 68.1 ohms. An inductance 160 of a value of 3.0 picofarads is provided for the first transformation means connected to the terminal end of slow wave delay line 122. A capacitance of approximately 150 microhenries is provided between the inner and outer conductors'162, 164 of the output line. For the sake of simplicity the electron beam generation means previously described in connection with FIGS. 5 and 6 have been deliberately omitted from the view shown in FIG. 4.
The collector electrode structure is partially illustrated in FIG. 4 with the precollector electrode 82 disposed opposite to the director 84.. The electrodes may be corrugated to aid in absorbing the spent electrons. Additionally, end shields 166 are disposed adjacent the ends of director 84 to assist in the focusing of the trajectory of the electron beam at the terminal end of the interaction path.
There is thus disclosed an efficient slow wave structure for very high frequency high power devices incorporating all the advantages ofmicrowave frequency devices. A plurality of lump and distributed components comprise the structure and together with matching transformers provide for tuning capabilities over a frequency band. Numerous variations, modifications and alterations will be evident to those skilled in the art and the foregoing detailed description of an illustrative embodiment is, therefore, intended to be interpreted broadly.
We claim: 1. A crossed field electron interaction device comprising:
means including a slow wave electromagnetic energy propagating structure and a substantially coextensive spaced sole electrode defining therebetween an interaction path;
means for generating and directing a beam of electrons along said path to interact in energy exchanging relationship with waves propagating along said structure;
means for collecting electrons disposed adjacent to one end of said structure;
means for establishing mutually perpendicular electric and magnetic fields in said path; and
means for coupling energy from said structure to an output utilization load;
said slow wave structure comprising a plurality of periodically spaced conductive members and a plurality of lumped electrical elements including an inductance connected in series with each conductive member and capacitive coupling means between adjacent conductive members to provide electrical characteristics for propagation of energy in the very high frequency range of the electromagnetic energy spectrum.
2. The device according to claim 1 wherein said inductance comprises a winding of a conductive wire supported on an electrical insulator member with one end of said wire joined to said conductive members and the other end joined to said envelope.
3. The device according to claim 1 wherein said capacitive coupling means comprise conductive wires spaced apart by an electrical insulator member.
4. A slow wave structure for propagating very high frequency electromagnetic energy comprising:
a plurality of periodically spaced conductive members;
electrical inductance means comprising a winding of conductive wire supported on an insulator serially connected between each conductive member and said envelope; and
electrical means for capacitively coupling adjacent members comprising conductive wires spaced apart by an electrical insulator;
said conductive members, inductance and capacitive means providing predetermined operating frequency electrical characteristics.