US3381166A - Electron tube assembly having an incorporated mode suppressor - Google Patents

Description

P. GERLACH 3,381,166 SEMBLY HAVING AN INCORPORATED ODE SUPPRESSOR April 30, 1968 ELECTRON TUBE AS M 2 Sheets-Sheet 1 Filed July 9, 1964 Hams ieL/wb 009 I WMM. fixlwfl ,ar

April 30, 1968 P. GERLACH 3,381, 66

ELECTRON TUBE ASSEMBLY HAVING AN INCORPORATED MODE SUPPRESSOR Filed July 9, 1964 2 Sheets-Sheet 2 United States Patent Oflice 3,381,166 Patented Apr. 30, 1968 3,381,166 ELECTRON TUBE ASSEMBLY HAVING AN INCORPORATED MODE SUPPRESSOR Pierre Gerlach, Thonon-les-Bains, France, assignor to Compagnie Francaise Thomson Houston-Hotchkiss Brandt, Paris, France, a French body corporate Filed July 9, 1964, Ser. No. 381,380 Claims priority, application France, Aug. 6, 1963, 943,858, Patent 1,373,741 Claims. (Cl. 31539) ABSTRACT OF THE DISCLOSURE A VHF-UHF tube having coaxial tube elements, and designed for operation in the TEM mode has a TE mode suppressor in the form of an annular Wire of ferromagnetic material (e.g., Kovar) located within the tube and between the control grid and another element (screen grid, anode), to suppress parasitic oscillations at the location where the TB parasitic oscillations are a maximum, so that optimum suppressing action is obtained.

Electron tubes designed for operation at very high frequencies (this expression including the UHF and SHF ranges) are frequently constructed with coaxial electrodes. These are connected externally of the tube with coaxial line conductors which, in effect, constitute extensions of the electrodes and are short-circuited at their outer ends through suitable means such as capacitive tuning plungers.

The coaxial electrodes of such a tube and the coaxial lines extending them provide resonant circuits capable of oscillating at the desired high frequencies in a desired mode, which typically is the TEM mode. However, other modes of resonance are possible, chiefly the TE mode, and should resonance in such an unwanted mode be set up in the input circuit of the tube, i.e., between control grid and cathode, and the output circuit, as between grid and anode, then a parasitic oscillation at the resonance frequency will be excited, with e.g., two diametrically opposed halves of the electrode system oscillating in pushpull relation. Such a parasitic oscillation will seriously impair the desired operation of the tube, when used as an amplifier or other device.

It has been usual heretofore to prevent such unwanted secondary oscillations from being sustained in tubes of the class specified, through the provision of mode suppressor or oscillation dam-per elements positioned in the coaxial line conductors connected with the tube electrodes externally of the tube. The resulting operation, however, has in many cases been far from satisfactory. The mode suppressor members had to be placed in empirical fashion, by trial and error, in order to obtain a stronger damping action for the undesirable TE mode than the useful TEM mode. Frequently the resulting mode suppressing action was only effective within a relatively narrow range of operating frequencies, and it Was therefore necessary to reposition the suppressor element, again empirically, every time the operating frequency of the tube circuit was substantially altered.

It is an object of this invention to provide an improved mode suppressor arrangement for coaxial-electrode tubes, which will operate with superior efliciency as compared to conventional such arrangements over a broad band of operating frequencies, will not require tedious positional adjustments on installation and/or in service, and will achieve a highly effective suppression of undesired modes, specifically the TB mode, under all circumstances.

The invention provides a coaxial-electrode electron tube assembly having an incorporated mode suppressor sealed in the envelope of the tube. The mode suppressor is in the general form of an annular element of a material having relatively high electric resistivity, disposed in generally coaxial relation with the tube electrodes and positioned radially between a pair of the coaxial conductor members provided in the tube (i.e., the electrodes themselves or the coaxial conductors serving to connect the electrodes with the external coaxial conductors), and axially spaced from the electronically active parts of the electrodes.

One of the reasons why such an arrangement is considerably more effective than the conventional arrangements wherein a mode suppressor element is positioned in the coaxial line conductors outside the tube is the fact that the strength of the circumferential electric field component in a coaxial line oscillating e.g., in quarter-Wave resonance, increases steadily from the shorted to the open end of the line. Hence maximum efiiciency in suppressing the undesired TE mode is achievable with the circumferential mode suppressor element positioned within the tube envelope according to the invention.

Exemplary embodiments of the invention will now be described for purposes of illustration but not of limitation with reference to the accompanying drawings wherein:

FIG. 1 is a fragmentary view of a coaxial triode in axial elevation with part of the envelope broken away to show the tube in section, the tube being provided with the improved incorporated mode suppressor means of the invention;

FIG. 2 is a transverse section of the tube on line AA, FIG. 1;

FIG. 3 is a view similar to FIG. 1 but illustrating a tetrode according to the invention; and

FIG. 4 is a section on line B-B of FIG. 3.

The triode illustrated in FIGS. 1 and 2 is of a generally conventional coaxial construction Widely used in high-power UHF tubes, and includes an inner cathode 1, a control grid 2 and an external anode 3 which con stitutes the envelope of the tube. The cathode 1 is of the directly heated type and includes an active portion comprising a mesh of thoriated tungsten wire having its upper end connected by conductive radial spacer means to an axially extending central conductor rod 8. The grid electrode 2 is in the form of a tube closed at its upper end and having a perforate wall part adjacent the cathode and spaced radially outward from it. The anode 3 is a massive copper tubular member sealed at its upper end and preferably provided externally with the usual cooling means such as fins, not shown, for dissipating the heat generated in operation by means of a stream of cooling fluid in which the tube is positioned.

Each of the three coaxial tubular electrodes 1, 2, 3 mentioned above is connected at its lower end to a related connector member 5, 6, 7 respectively, which projects from the lower part of the tube and serves for the connection of the related electrode with a respective conductor of a coaxial line, not shown. The coaxial connectors 5, 6 and 7 are maintained in assembly in their prescribed spaced relationship by means of insulating spacer rings 4, usually made of ceramic. The axial rod 8 provides a second connector for the direct-heated cathode 1 and projects through an insulator insert mounted in the centre of the cathode connector 5.

In accordance with the invention, an annular element 9 made from a material having'relatively high electric resistivity, e.g., Kovar alloy, is supported between the anode 3 and grid 2 coaxially therewith, and is herein positioned in a circumferential recess formed in the lower end of the inner anode surface. While the element 9 may be supported in any of various ways in the position shown, in the present example said element 9 is shown as being composed of two semi-circumferential parts as clearly apparent in FIG. 2, each part having its ends bent radially outward and secured as by solder to the inner wall surface of anode 3 at diametrically opposed positions 10 and 11. At points intermediate between the areas of attachment 10 and 11 the element 9 is spaced radially inward from said anode wall suface.

In order to understand the significance of the modesuppressor element 9 of the invention, the following explanations will be useful.

In the operation of a coaxial-electrode tube of the kind shown in FIGS. 1 and 2, the electrode connector members 5, 6 and 7 are connected to the upper ends of respective tubular conductors (not shown) forming coaxial lines and which normally are short-circuited at their lower ends as by means of capacitive tuning plugs or plungers. These coaxial lines thus constitute with the electrodes 1, 2, 3 connected to their upper ends, resonant cavities which normally resonate in the so-called transverse electromagnetic or TEM mode. In this mode of resonance, the electric field vectors in any cross section of the coaxial line are all directed in symmetrical relation radially outward or inward and are equal in intensity, while the magnetic lines of force form circumferential lines concentric with the line conductors. However, such resonant cavities also admit of another mode of resonance, the TB mode, in which the electric field vectors, while again directed in radial directions, are nonsymmetrical circumferentially, and vary in sense and intensity all around the circumference of the line cross section so as to constitute an even number of voltage nodes and voltage antinodes around said circumference. The associated magnetic force lines are circumferences concentrically surrounding the electric field vectors.

The existence of this TE mode of resonance makes it possible for parasitic oscillations to be set up at any frequency higher than the cut-off frequency f for that mode, which corresponds to a cut-off wavelength A substantially equal to the length of the mid-circumference of the coaxial line (the circumference equispaced between the inner and outer line conductor) divided by the number 12 of pairs of voltage loops present. In most cases, the cut-off frequency f even with 11:1, lies in a frequency range wherein the gain characteristic of the tube is sharply drooping, so that the setting-up of a parasitic oscillation can practically occur only at frequency values very close to the cut-off value f This condition is usually fulfilled in present-day tubes of the type specified, because the electrode diameters, and hence the length of the midcircumferences of the coaxial lines, are made to approximate rather closely the desired operating wavelengths. Hence the cut-off frequency f of the coaxial lines connected to such electrodes, in the TE mode, approximates the useful operating frequency.

The reason why the dimensioning of the resonant circuits associated with the tube increases the likelihood of parasitic TE mode oscillations being set up at frequencies close to the cut-off value f can be explained by the following considerations.

If it were possible in all cases to tune the coaxial lines for quarter-wave oscillation in the useful TEM mode, with the useful wavelength being comparable in length to that of the afore-mentioned midcircumference of the coaxial line, waveguide theory shows that the TE mode resonant frequency would be much higher than f and there would be no danger of parasitic oscillations being excited. However, beyond a certain useful frequency value it is not feasible to use quarter-wave tuning, since this would require the short-circuit tuning plungers being placed within the tube envelope. It then becomes neces' sary to apply fii-wavelength and even /4;wavetuning in order to retain acceptably large line lengths. As line length increases, the TE mode resonant frequency asymptotically approaches the cut-off value f and hence varies very little for the relatively large values of line length obtained with A and wave operation.

Furthermore, since the difference between electrode diameters is relatively small, the f values for the cathode and anode lines are nearly the same, and hence the natural frequencies of the cathode and anode circuits will be close enough to each other to permit self-excitation due to the feedback coupling inevitably present in the tube as a result of inter-electrode capacitance. Thus the unwanted parasitic TE mode oscillations will quite easily be excited in such a tube even through the length of the cathode and anode lines may be substantially different.

The circumferential mode suppressor element 9 provided according to the invention within the tube envelope will very effectively prevent such TE mode oscillations from arising. The element 9 provides a path of low electric conductivity in the circumferential direction which must be followed by the conductive current in the TE mode in each cross section of the coaxial line if the TE mode of oscillation is to be sustained, and thereby interrupts any incipient parasitic oscillation in that mode. At the same time, the circumferential element does not interfere with, or only opposes a negligibly low resistance to, the conductive flow of current in the desired TEM mode, since such current flow is effected in the axial direction.

The element 9, since it is positioned inside the tube, is more effective than would be a similar element positioned outside the tube in the external coaxial line, since the strength of the circumferential current component tends to increase continuously from the shorted (lower) to the open (upper) end of the line, so that the circumferential suppressor element, when positioned in the tube, exerts a more effective cut-off action.

According to an important feature of the invention the annular suppressor element 9 is spaced somewhat from r the conductive walls of the electrodes to either side of it as shown, since this causes a concentration of the electric fields in the region of said elements and thus greatly increases its effectiveness.

The material and the dimensions of the annular suppressor element 9 are selected to provide a total circumferential resistance thereof, for alternating currents at frequencies in the ranges used, e.g., higher than about 1000 megacycles, which will be at least 10 ohms and preferably more. According to an advantageous feature of the invention, the element is made from ferromagnetic material. The use of such material makes it possible to provide the desired over-all A-C resistance at high frequencies with reasonably large transverse dimensions of the element. If non-magnetic materials were used, it would be necessary to reduce the transverse dimensions of the element to such small sizes that the element would be mechanically weak and the resulting tube assembly would not stand up well to shock and vibrations in service. A satisfactory ferromagnetic material for use in the construction of the circumferential mode suppressor element of the invention is the iron-nickel-cobalt alloy known as Kovar. In one practical embodiment the ring 9 was 6.3 cm. in inner diameter and was made from 2 mm. gauge Kovar wire, and operated extremely successfully in suppressing the parasitic TE mode oscillations in the tube in which it was mounted.

In the embodiment of FIGS. 3 and 4, the invention is shown applied to a tetrode rather than a triode. The tube is constructed essentially similarly to the tube of the first embodiment and corresponding parts of it have been designated by the same reference numerals. The main difference lies in the provision of an additional electrode, screen grid 12, interposed between control grid 2 and anode 3, and constructed generally similarly to the control grid. The screen grid 12 has an associated annular connector member 13 connected with its lower end and projecting downwardly from the base of the tube for connection with an external coaxial conductor, the connector member 13 being held in position by means of ceramic spacer rings 4.

An annular mode suppressor element, here designated 14, made from ferromagnetic material, e.g., Kovar wire about 2 mm. thick, is supported between the outer surface of control grid 2 near the base thereof, and the inner surface of screen grid 12, which is shown as having a radial offset or shoulder around the element 14. The element 14 may be supported in spaced relation from the surfaces of the grids 2 and 12 through any suitable means, including the means shown for supporting the mode suppressor 9 in FIGS. 1 and 2. In the present embodiment, however, the mode suppressor element 14 is shown by way of example as being in the form of a continuous, onepart wire ring and is supported by way of a number of, e.g., three, radial pins such as 15, having their inner ends soldered to the ring 14 and its outer ends soldered to the inner wall surface of the electrode 12.

This embodiment operates in a manner exactly similar to that earlier described.

In an alternative, not shown, of the tetrode shown in FIGS. 3-4, the mode suppressor 14 may be mounted between the control grid 2 and screen grid 12. High-frequency electron tubes constructed according to the teachings herein disclosed with a built-in mode suppressor operate in a manner greatly superior to that of conventional tubes with external mode suppressors placed in the coaxial-line circuitry connected with the tube electrodes. A more effective and reliable dampening of the unwanted oscillation mode is obtained over a wider range of frequencies, eliminating tedious initial adjustments on installation and re-adjustments in service.

It will be apparent that various modifications may be introduced and departures made from the embodiments shown and described without exceeding the scope of the invention. Thus, the whole circumference of the mode suppressor may be in actual contact with the inner or outer cylindrical surface of an electrode of the tube. Or else, more than one mode-suppressor-rings of the kind here specified might be provided, such as one between the cathode and control grid, and the other between the screen grid and anode, of a tetrode.

I claim:

1. An electron tube assembly having a number of coaxial, annular conductive electrode elements and electrode-connector elements positioned within the tube and connectible exteriorly of the tube with respective coaxial conductors constituting respective resonant circuits therewith, and at least one annular member of a conductive, relatively high resistance material, said member being supported within the tube between a pair of said conductive elements coaxially therewith to provide a circumferential path of relatively high electrical resistance and damping out unwanted TE mode oscillation in said resonant circuits.

2. A tube assembly according to claim 1, wherein said annular member is constructed to provide an over-all electric resistance of at least ten ohms around its circumference for alternating currents at substantially the operating frequency range of said tube.

3. A tube assembly according to claim 1, wherein said annular member is made from a ferromagnetic material.

4. An electron tube assembly having a plurality of coaxial, annular conductive electrode elements therein; coaxial, annular conductive electrode connector elements having their one ends connected to ends of said electrode elements and having opposite ends connectable with respective coaxial conductors exteriorly of the tube to provide respective coaxial resonant circuits, and an annular member means made from ferromagnetic material and supported within the tube between circumferential wall surfaces of a pair of adjacent ones of said conductive elements coaxially therewith to provide a circumferential path of relatively high A-C resistance in at least one of said resonant circuits near the upper end thereof to damp out unwanted TE mode oscillation in said resonant circuits.

5. An electron tube assembly having a plurality of coaxial, annular conductive members therein, means for connecting outer ends of said members exteriorly of the tube with respective coaxial conductors, at least one annular shaped mode-suppressor element made from ferromagnetic material positioned within the tube between circumferential surfaces of a pair of adjacent ones of said conductive members, and means supporting said modesuppressor element in radially spaced relation from said surfaces.

6. The tube assembly defined in claim 5, wherein said annular mode-suppressor element comprises at least two part-circumferential wire sections having radially bent extremities connected with one of said circumferential surfaces, said part-circumferential sections substantially abutting at the bend point of the extremities to constitute a substantially complete circumference.

7. The tube assembly defined in claim 5, wherein said annular mode-suppressor element comprises a ring of ferromagnetic wire and radial pins are provided for connecting spaced points of said ring with one of said wall surfaces.

8. The tube assembly defined in claim 5, which comprises a cathode, a grid and an anode electrode members, and said annular mode-suppressor element is positioned between said grid and anode members.

9. The tube assembly defined in claim 5, which comprises a cathode, a control grid, a screen grid and an anode electrode members, and said annular mode-suppressor element is positioned between said screen grid and anode members.

10. The tube assembly defined in claim 4, wherein said annular member is supported between circumferential surfaces of said electrode connector elements at locations nearer the base than the top of the tube.

References Cited UNITED STATES PATENTS 2,502,456 4/1950 Hansen et al 333-98 X 2,806,951 9/1957 Willwacher et al. 33056 X 2,867,726 1/1959 Preist 33056 X 3,209,276 9/1965 Fricke et a1 33383 HERMAN KARL SAALBACH, Primary Examiner. ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner.

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