US2806976A - Impedance matching device - Google Patents

Description

Sept. 17, 1957 K. G. HERNQvlsT IMPEDANCE MATCHING DEVICE Filed Nov. 26, 1952 v INIENTOR. KHRLE. HERNQYIST Patented Sept. 17, 1957 tice 2,806,976 MPEDANCE MATCmG DEVICE Karl G. Hernqvist, Princeton, N. E. Application November 26, 1952, Serial No. 322,722 12 Claims. (Cl. S15-5.15)

This invention is a device for obtaining a good match between the relatively high impedance of a density-modulated electron beam and the relatively low impedance of free space or a transmission line.

@ne feature of the device according to the invention is the arrangement of a density-modulated electron beam to pass through a region having low dielectric constant due to the existence in said region of an electronic medium, which has a dielectric constant increasing towards free space or towards a transmission line.

The device according to the invention is an electronic impedance transformer for microwaves, having the advantage compared to hereto-known matching devices for microwaves, such as resonators and delay lines, that the transmitting medium has very small losses and that the geometry limitations, which are very severe in the millimeter-wave region, can be overcome. Thus this device improves the possibilities of utilizing the velocity modulation principle for the generation of millimeter waves.

Two embodiments of the device according to the iuvention will be described here in connection with Figures l and 2 on the accompanying drawing. Fig. l is a partially schematic longitudinal cross-section View of a transformer for matching the impedance of a densitymodulated electron beam to the impedance of free space. Fig. 2 is a partially schematic longitudinal cross-section of a transformer for matching of the impedance of a density-modulated electron beam to the impedance of a coaxial line.

In the device according to Fig. l, an electronic medium in the form of a conical diverging electron ilow or stream l is obtained in a cathode ray or electron beam tube having a concave cathode 2, an accelerating grid 3 concentric to the cathode, and a cylindrical anode 4 having a through passage or chamber in the form of a double-cone with the apices together. The right-hand part of the double-cone denes a region which is substantially closed at the sides and at the apex end and open at the base end of the cone. The base end of the cone is closed by a wall or window W of dielectric material permeable to electromagnetic waves. The heater of the cathode 2 is heated by power source S. A high voltage from a voltage source 6 is applied to the anode 4 and the accelerating grid 3 with respect to the cathode 2. The conical region bounded by the grid 3 and the anode 4 is a :field-free space in which the space charge of the converging electron stream is neutralized, whereby a stream of great density is produced at the apex of the cone.

in the anode 4 a device is built in for obtaining a density-modulated electron beam, which device consists of a concave cathode 7, an accelerating grid 8 and a cavity resonator 9. The heater of the cathode 7 is heated by the power source 16. To the cathode 7 is applied a 1row potential with respect to the anode 4 and the accelerating grid 8 from the voltage source 11. Electromagnetic energy is supplied to the cavity resonator 9 by means of the coaxial line i2. The electron beam from cathode 7 is velocity-modulated by the cavity resonator and, in traversing a passage in the anode 4 which serves as a drift tube, becomes density-modulated at a point 13 which is situated at the apex of the conical diverging electron stream 1. The modulated electron beam is here shown directed transverse to the axis of the conical diverging electron stream 1, but this is not necessary and the angle between the modulated electron beam and said axis may have other values. However, it is necessary that the modulated electron beam cross the conical diverging electron stream 1 at or near the cone apex. The conical diverging electron stream 1 is an electronic medium, have ing a dielectric constant which increases continuously from a low value at the cone apex, where the plasma frequency of the electron stream is equal or nearly equal to the frequency of the electromagnetic energy, to the value 1 of free space at the window W. This impe-dance transformer makes a matching between the impedance of the density-modulated electron beam (about 100,000 ohms) and the impedance of free space (about 370 ohms) possible.

In the device shown in Fig. 2, the impedance of a density-modulated electron beam is matched to the impedance of a coaxial line having an inner conductor 14 and an outer conductor l5. At one end of the coaxial line is arranged an evacuated tapered annular chamber having an outer wall 16 and a centrally-located cylindrical cathode 17 forming an extension of the inner con ductor 14. The tapered annular chamber is separated from the coaxial line i4, l5 by a wall or window W of dielectric material permeable to electromagnetic waves. The heater of the cathode 17 is heated by a power source 18. The outer wall 16 around the cathode 17 consists in part of a cylindrical envelope portion having much less diameter than the outer conductor i5 and in part of a tapered envelope portion connected to the cylindrical portion and to the conductor 1S and having a diameter increasing towards the outer conductor 1S. A voltage from a voltage source 19 gives the outer wall i6 a higher potential than the cathode 17. A solenoid 26, enclosing the chamber, provides a magnetic field, which is coaxial to the cathode 17, and which decreases in strength towards the coaxial line 14, 15. Electrons emitted from the cathode 17 are accelerated towards the outer wall' 16 and due to the magnetic eld an electronic medium in the form or an electron stream or cloud rotating around the cathode 17 is obtained according to the magnetron principle. The annular chamber is surrounded at its smaller diameter portion by a device for obtaining a density-modulated electron beam, which device consists of a concave annular cathode 2l, a concave annular accelerating grid 22 concentric to the cathode, and an annular cavity resonator 2S. The heater of the cathode 2l is heated by a power source 24. From a voltage source 2S a low potential is applied to the cathode with respect to the accelerating grid 22, Electromagnetic energy is supplied to the cavity resonator 23 through a coaxial line 26. The electron beam from cathode 2i is velocity modulated by the resonator 23 and forms a circular disc with radial electron paths directed inward toward the cathode 17 and through the electron cloud. Because the cathode i7 has a negative potential with respect to the resonator 23, the electrons are reected baci: and densitymodulated as in a reflex klystron. The voltages of the sources 25 and 19 are so chosen that the point of maximum density-modulation is located inside the electron cloud. Due to the interaction between the density-modulated electron beam and the rotating electron cloud the latter is brought into plasma oscillations, whereby an electromagnetic wave is initiated through the window W and along the coaxial line 14, 15. By varying the magnetic field the dielectric constant of the electron cloud can be adjusted to a low value in the region of the density-modulated electron beam, to make the plasma frequency of the electron cloud at the density modulated-electron beam equal or nearly equal to the frequency of the electromagnetic wave. Thus the dielectric constant of the electron cloud has a low value at the density-modulated electron beam and increases continuously in the direction towards the coaxial line 14, 15, due in part to the increasing distance between the outer wall 16 and the cathode 17 in the direction towards the coaxial line 14, and due inpart to the magnetic iield decreasing in the same direction. This impedance transformer makes a matching between the impedance of the density-modulated electron beam and the impedance of the coaxial line possible.

The invention is of course not limited to the embodiments illustrated, but may be modied in several ways withoutvdeparting from the spirit and scope of the inventic-n. The resonator 9 in Figl, for example, may be omitted, and the density modulation may be produced by the velocity modulation of the electron beam 'from cathode '7 by the electronic medium itself, in the manner described by G. vWehner, Journal of Applied Physics, 2l (1950) pp. 52-3.

What is claimed is:

1. A device for matching the relatively high impedance of a densitynnodulated electron stream to a space having relatively low impedance comprising means defining the boundaries of a region having relatively small transverse dimensions in a first portion and relatively large transverse dimensions in a'second portion, and means for causing said region to have a dielectric constant which increases from a low value in said rst portion to substantially unity in said second portion, said `last-named `means including means for producing an electronic medium in said region having a relatively high density in said first portion and a relatively low density in said second portion, said second portion of said region being terminated by means permeable to electromagnetic waves, means for projecting a density-modulated electron beam transversely through said rst portion of said region for generating oscillations in said medium and thereby initiating electromagnetic waves toward and through said wave permeable means.

2. A device according to claim l, wherein said region comprises a chamber which diverges continuously from said first portion toward said second portion, and said electronic medium producing means is adapted to produce a medium which substantially lls said divergent chamber.

3. A device according to claim l, wherein said means for producing said electronic medium comprises an electron gun for producing a conical diverging electron stream, which diverges from an apex in said tirst portion of said region toward said second portion.

4. A device according to claim 1, wherein said region comprises a divergent chamber dened by a section of tapered coaxial transmission line having inner and outer conductors, and said means for producing said electronic medium comprises a cathode and an anode formed as extensions of said conductors and means for providing a magnetic field coaxial to said cathode and anode, whereby during operation of said device the electrons emitted by said cathode are formed into a rotating electron cloud in said region the density of which decreases toward said coaxial line.

5. A device according to claim 4, wherein said density modulated beam is projected by said beam projecting means into said region symmetrically with respect to its axis of rotation.

6. An electron tube comprising means defining the boundaries of a region which is closed to electromagnetic waves at the side and open at one end and has a small aperture at the other end, means for producing within said region an electronic medium the density of which decreases from a relatively high value near said aperture to a relatively low value near said open end, whereby said region has a dielectric constant which increases fromV a relatively low value near said aperture to substantially unity near said open end, and means for exciting said electronic medium near said aperture at the plasma frequency of said medium at that point, for generating oscillations of said frequency in said medium and thereby initiating electromagnetic waves toward and through said open end. Y

7. An electron tube according to claim 6, wherein said exciting means comprises means Vfor projecting a densitymodulated Velectron beam transversely through said electronic medium near said aperture.

8. An electron tube accordingv to claim 7, wherein said stream projecting means includes a cathode, a velocity modulating cavity resonator and a drift tube.

9. An electron tube according to claim 6, wherein said iirst-named means defines a cone-shaped region and said electronic medi-um is a'conical diverging electron stream which substantially fills said region.

l0. An electron tube according to claim 9, wherein said exciting means comprises means for projecting a density-modulated electron beam -through the apex of said cone-shaped region. A

11. An electron tube according to claim 6, wherein said iirst-named means comprises a vsection of coaxial transmission line closed at oneend and open at the other end and including inner and outer conductors spaced apart a substantially greater distance at said open end than at said aperture, and said Velectronic medium producing means comprises a-cathode and an anode forming portions of said conductors, and means for establishing aV magnetic iield coaxial to said conductors.

12. An electron .tube according to claim ll, Vwherein said last named meansisadapted to establish a tapering magnetic eld decreasing in strength from said aperture toward said open end.

References Cited in the le of this patent UNITED STATES PATENTS 1941 2,241,976 Blewett et al. May 13, 2,338,237 Fremlin Ian. 4, 1944 2,413,251 Smith ,Dec. 24, 1946 2,416,302 Goodall Feb. 25, 1947 2,439,387 Hansen et al. Apr. 13, 1948 2,445,771 Fremlin etal. July 27, 1948 2,475,646 Spencer July 12, 1949 2,477,317 Spencer July 26, 1949 2,567,674 Linder Sept. 11, 151 2,591,696 Hansen Apr.8, 1952 FOREIGN PATENTS 665,505 Great YBritain Ian. 23, 1952

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