US 3775709 A
High-frequency tubes, such as klystrons or travelling wave tubes for example, producing high-power outputs within wide frequency bands by means of output devices comprising a dielectric window which ensures a seal between the evacuated enclosure of the tube and the load circuits which take the form of pressurized waveguides. This window is arranged in a waveguide section whose advantageously circular cross-section is larger than that of the output waveguide of the tube and that of the load waveguides, the connections between the waveguides being effected through junctions. The invention provides means which ensure to the thus constituted output device a resonance frequency equal to the centre frequency of the operating band of the tube, and means which effect impedance-matching throughout this band.
Description (OCR text may contain errors)
United States Patent [1 1 Firmain et al.
[ Nov. 27, 1973 IMPROVED OUTPUT WINDOW STRUCTURE FOR MICROWAVE TUBES  Inventors: Gerard Firmain; Guy Egloff, both of Paris 8eme, France OTHER PUBLICATIONS Lebacqz et al., High Power Windows At Microwave Frequencies, lEE Paper No. 2675R 12/1958, pp.
Chen, T. S., Broadbanding of Resonant-Type Microwave Output Windows, RCA Review, 6/1954, pp. 204-207.
Primary Examiner-Rudolph V. Rolinec I Assistant E.raminer-Wm. H. Punter Attorney-John W. Malley et al.
[5 7 ABSTRACT High-frequency tubes, such as klystrons or travelling wave tubes for example, producing high-power outputs within wide frequency bands by means of output devices comprising a dielectric window which ensures a seal between the evacuated enclosure of the tube and the load circuits which take the form of pressurized waveguides. This window is arranged in a waveguide section whose advantageously circular crosssection is larger than that of the output waveguide of the tube and that of the load waveguides, the connections between the waveguides being effected through junctions. The invention provides means which ensure to the thus constituted output device a resonance frequency equal to the centre frequency of the operating band of the tube, and means which effect impedancematching throughout this band.
6 Claims, 8 Drawing Figures PATENTH] HUVZ 7 [S75 SHEET 1 OF 4 PAIENIEU 177K). ('08 SHEET 3 OF 4 MICROWAVE TUBES The present invention relates to improvements in high-frequency, high-power, wide-band tubes such as klystrons for example or travelling wave tubes, for developing powers whose peak levels may reach several megawatts and even several tens of megawatts.
It relates more particularly to the output elements of such tubes, these elements conventionally being constituted by wave guides in which a dielectric window is provided for the vacuum tight sealing of the enclosure forming the body of the tube.
The design of windows which do not interfere with the transmission of the power developed by the tube, throughout the whole of thefrequency band, is a particularly delicate matter, especially because the dimensions of these windows have to be quite large.
On the one hand, the area presented by the window to the HF wave being transmitted must be sufficiently large-to avoid any risk of a shutter effect due to the electric field prevailing in the window.
On the other hand, the thickness of this window should be adequate to allow it to withstand the pressure difference existing between its two faces. However, this pressure difference is very substantial since on one side there is the enclosure which is under vacuum and on the other there is the waveguide supplying the load circuits, which waveguide is usually pressurised to a level in the order of some few kilograms per square centimeter to prevent any unwanted ionisation there.
Generally, the tube output is through one, or sometimes two rectangular output waveguides whose crosssection is too small for the output windows to be arranged in them. Since the power delivered by the tube must not be dispersed amongst parasitic modes, but
.must be confined to the sole fundamental mode, generally the TE in the case of an output which is effected through one rectangular section of the waveguide as described, there can be no question of enlaying the section of this waveguide sufficiently to be able to install the window there if this were done, the cut-off frequency would be too low and parasitic modes would appear.
The solution generally employed consists, therefore, in inserting the window in a waveguide portion of larger cross-section than that of the output waveguide of the tube, a more or less progressive junction being provided to establish the connection between these two waveguides of different section.
In the usual way, the window is made circular and is inserted in a section of cylindrical waveguide, the junction thus being one of conical form. This kind of window has two advantages over a rectangular window. On
the one hand, it presents a larger area for the transmit-.
ted electromagnetic wave, whilst having roughly the same size. On the other hand, the parasitic modes in a circular waveguide are further away from one another than in a rectangular section waveguide so that the risk of the appearance of parasitic modes is not so high.
A major problem constituted by output devices of this kind, whether the window be circular or not, is that of matching them within the wide frequency band of operation of the tube, this matching being particularly difficult owing to the junction. Furthermore, this difficulty is generally aggravated by the presence of a second junction which establishes the connection between the output of the waveguide section containing the window, and a rectangular waveguide known as the load waveguide, the load circuits conventionally comprising rectangular section waveguides of smaller cross-section than the waveguide which contains the window.
A variety of solutions have already been proposed in an attempt to resolve this problem, that is to say in order to give the assembly of window, waveguide section containing same and junctions, a standing wave ratio of close to unity within a frequency band which as closely as possible corresponds with the operating frequency band of the-tube.
One of these solutions consists in utilisinga relatively long circular waveguide section so that the junctions are sufficiently far away from the window to produce a pure real admittance in the plane of its faces. This solution has several drawbacks. It is bulky there is the risk of stationary waves appearing in the circular wave-- guide section, which then behaves as a cavity which is the seat of a free-running oscillation in addition, the impedance-matching has to be carried out in the circular waveguide section and setting up is thus rendered a delicate operation.
A second solution consists in utilising more progressive junctions with a very flat slope. This solution has the same drawbacks as before.
A third known solution consists in incorporating a matching obstacle within the thickness of the dielectric window itself. Although this solution enables good matching to be achieved throughout a wide frequency band, the setting up and manufacture of this kind of window are extremely delicate operations.
One object of the present invention is to produce high-frequency, high-power, wide-band tubes, by the introduction of an improved output device which enables excellent transmission of "the output power from the tube throughout its operating frequency range to be achieved, whilst escaping the drawbacks of the devices thus far known and in particular providing a system which is simple to manufacture and set up.
In an improved tube in accordance with the invention, the matching of the output device which comprises at least one dielectric window arranged in a portion of waveguide whose section is larger than that of the rectangular output waveguide section of the tube, and a junction linking these two waveguides, is achieved by metallic obstacles arranged in front of the junction and in the vicinity thereof, within the output waveguide itself. A first set of obstacles corrects the effects of the junction whilst a second advantageously matches the impedance of the output device assembly to that of the rectangular output waveguide.
In the case of a second junction located after the window and connecting the waveguide containing it, with the load waveguide, another set of metallic obstacles is arranged after said second junction in the load waveguide, symmetrically vis-a-vis those arranged in the output waveguide.
Generally, although this is by no means limitative, and for the reasons already indicated the output window is circular and the waveguide-section containing it is cylindrical.
Other objects and features of the invention will become apparent from the ensuing description, given by way of non-limitative example and illustrated by the attached figures in which FIG. 1 is a schematic perspective view of a conventional output device for a high-power high-frequency tube FIG. 2 illustrates different resonance curves for a window, matched or unmatched FIG. 3 illustrates a schematic view of an embodiment of matching means in accordance with the invention FIG. 4 is a schematic view of an output device for a high-frequency tube in accordance with the invention FIG. 5 is a schematic view of an example of klystron tube comprising an output device according to the invention FIGS. 6, 7 and 8 are schematic views of other embodiments of matching means in accordance with the invention.
FIG. 1 schematically illustrates a conventional output device for a high-frequency, high-power wide-band tube, utilising a circular dielectric window.
The tube has not been illustrated here it is symbo lised by an arrow together with the word tube" and by its rectangular output waveguide 1. The interior of this waveguide, which forms an integral part of the tube, is under vacuum. The load circuit has not been illustrated here it is symbolised by an arrow together with the word Load. A second rectangular waveguide 2, forming part of the output device of the tube, enables the tube to be connected to the load circuit in the manner already described this load waveguide 2 is advantageously pressurized and carries a gas pressure in the order of some few kilograms per square centimeter for example. The waveguides 1 and 2 are linked by a circular waveguide portion 3 containing a circular vacuumtight window 4 made of a strong dielectric material, ceramic for example, and having a thickness 2 and a radius a, the connections between said waveguide portion 3 and the rectangular waveguides l and 2, being effected through progressive junctions t of approximatly conical form, generally referred to as conical junctions or tapered junctions.
As already described, the dimensions of the window 4 must be sufficiently large. Moreover, they are chosen in such a way that the window transmits the TE. mode corresponding, in the circular waveguide 3, with the fundamental TE mode which is transmitted by the output waveguide 1 so that no parasitic modes appear.
FIG. 2 illustrates the resonance curves of a circular window under different conditions, and enables the object of the invention to be more readily understood. In this diagram, on the abscissa there is plotted the frequency F and on the ordinance the standing wave ratio 5. The operating frequency band AF of the tube is defined between F, and F The continuous curve 5 is the resonance curve of a circular window such as that 4 inserted in a circular junction-less waveguide, the dimensions of the window being such that it resonates in the TE mode with a standing wave ratio of unity at the centre frequency F of the band AF, the impedance-matching being assumed to be effected by conventional means.
The dots and dashes curve 6 represents the resonance curve of the same window arranged in a circular section waveguide disposed between two conical junctions connected to rectangular waveguide sections in the manner indicated in FIG. 1.
It is very clear that the presence of junctions produces a substantial shift in the matched frequency of the system. The conventional matching means, which are adequate to produce suitable matching in the absence of any junctions, in this case no longer allow the system to be matched within the operating frequency band AF of the tube, which band is radically displaced in relation to the resonance frequency F of the system.
In accordance with the invention, the output device of a tube comprises, within the rectangular waveguide connected to the waveguide portion which contains the dielectric window by a junction 1, metallic obstacles which transform the curve 6 of FIG. 2 into a curve 7 centeredon the center frequency F of the band AF. The resonance frequency of the window inserted in the circular waveguide portion and of the junctions, thus having been made equal to the centre frequency F of the band AF, matching this band is quite readily achievable using conventional means brief mention of which will be made at a later point.
FIG. 3 schematically illustrates an embodiment of such metal obstacles, constituted in this case by two metal bars 8 and 9 arranged in the rectangular waveguide G near to the junction 1 which connects said waveguide G with the circular waveguide portion 3 containing the window (not shown here). These bars are arranged in the same sectional plane of the waveguide G, in such a way to be parallel to the electric component of the electromagnetic wave propagating through said waveguide and to be symmetrically disposed in relation to the waveguide axis, each bar being connected to one of the two wider faces of the waveguide they act as an inductive obstacle.
In the case of the tube output device containing only one junction between the output waveguide 1 and the circular waveguide portion 3, said two bars enable the resonance frequency of the window and of the associated junction to be made equal to the centre frequency F of the band AF of the tube. More generally, the device, as FIG. 4 shows, comprises a second junction between the circular waveguide portion 3 and the load waveguide 2. In this case, the load waveguide 2 itself contains metal obstacles in the form of bars 10 and 11 for example, identical to those in the output wave guide 1 and disposed symmetrically vis-a-vis the window 4. I
Thus, thanks to the presence of these metal obstacles, the design and setting up of which are extremely simple to put into effect in particular because they are located in rectangular waveguide sections, the disturbances caused by the junctions are suppressed and the resonance frequency device is made equal to the centre frequency F of the operating band of the tube.
As far as the matching of the device throughout the whole of said band AF, is concerned, this is something which is then simple to carry out and in particular has the advantage of being independent of the correction effected by the bars 8 and 9.
9. Two bars and 15 are symmetrically arranged in the load waveguide 2.
An output device of this kind conveniently enables the output power of the tube to be transmitted throughout its whole operating frequency range, whilst at the same time being quite simple to manufacture and set up. Moreover, since the matching has been achieved without increasing the length of the system of circular waveguide portion and junctions, the risk of the appearance of free-running oscillation in this system is very much reduced.
FIG. 5 schematically shows an example of highfrequency tube comprising an output device according to the invention, the tube thus improved being here a klystron tube.
Such a klystron comprises within a tight exhausted enclosure 80, the classical elements of a klystron not shown here for clarity of the figure. The klystron here represented as an example comprises two cavities, an input one 81 provided with an input coupling device 83 and an output one 82 provided with an output device 84 according to the invention. The emissive cathode is disposed within the lower part of the enclosure 80 and may be heated by means of connections 85, while the collector electrode is disposed in the upper part 86 of said enclosure.
The output cavity 82 and the output device 84 are shown in a partly sectional view for showing the matching means of the invention. The output device 84 is equivalent to that shown on FIG. 4 and the same references correspond to the same elements. Of course two only of the four metallic rods constituting the matching means in each output 1 and load 2 waveguides are shown here owing to the sectional view. The dielectric window 4 is represented with dashed lines for the sake of clarity.
As earlier said, an output device such as 84 may also be used for other high-frequency high-power tubes, traveling wave tubes for example.
Such output devices may also be used for several output tubes, klystrons for example which may have two output devices such as 84 connected to the output cavity. I
FIGS. 6, 7 and 8 schematically illustrate other practical embodiments of metal obstacles which, in accordance with the invention, enable the effects of a junction I between a rectangular waveguide G and a circular waveguide containing a dielectric window (not shown), to be compensated.
In the example of FIG. 6, the two metal bars of FIG. 3 are replaced by two flat metal fins 58 and 59 arranged in the same plane of section of the rectangular waveguide G, substantially at the same distance from the junction 1 as the bars 8 and 9 of FIG. 3 these two fins are connected to the smaller faces of the rectangular waveguide, along a line parallel to the electric component of the electromagnetic wave propagating there, and act as an inductive window.
In the example in FIG. 7, the inductive window of FIG. 6 is replaced by a capacitive window located in the rectangular waveguide G, at a greater distance from the junction, in order to produce the same effect as such inductive window of FIG. 6. This capacitive window of FIG. 7 is constituted by two flat metal fins 68 and 69 fixed to the larger faces of the waveguide G in the same plane of section thereof and perpendicularly to the electric component of the electromagnetic wave.
In these two embodiments, FIGS. 6 and 7, the impedance-matching within the operating frequency band of the tube, is effected by a technique which is well known per se, through the agency of a second set of metal obstacles, not shown in these figures, but equivalent to the bars 12-and 13 of FIG. 4 for example. This second set of obstacles can itself be constituted rather than by bars such as those 12 and-13, by flat fins constituting either an inductive'window or a capacitive window and suitably arranged in the relevant rectangular waveguide.
Another possible embodiment of the invention is that of FIG. 8 where the two sets of obstacles, one of which restores the resonance frequency F, FIG. 2) to the centre of the band AF, whilst the other effects impedance-matching within this band, are combined, the system being constituted by a resonant window 70 arranged in the relevant rectangular waveguide G. This window can be considered as consisting of the conbination of capacitive obstacles (such as those 68 and 69 of FIG. 7) which bring the frequency F to F (FIG. 2), and inductive obstacles (of the kind represented by 58 and 59), which effect impedance-matching within the band AF. In order for the planes containing these two sets of obstacles to coincide and for the obstacles to combined to constitute the resonant window 70, it is merely necessary to select the length of the circular waveguide portion 3 appropriately. This arrangement, however, has the drawback of being rather more delicate to set up and of lengthening the waveguide portion 3, with the attendant risk of the development of parasitic modes.
As already indicated, --the description isconcerned more particularly, albeit not exclusively, with a circular window tube output device. The invention is applicable likewise to any device in which the output window, al-
though not circular (but rectangular), has a larger section than that of the tube output waveguide, thus necessitating a waveguide junction.
In a particular embodiment, an improved tube in accordance with'the invention is a klystron operating at a frequency range extending from 2900 to 3200 Mc/s and delivering peak powers of as much as 25 megawatts. For such a tube, the output window is made for example of glucine. It is circular and thick its diameter being in the order of 68 mm and its thickness 20 mm. The parasitic modes close to the operating band AF are in the T5 mode at frequency 2860 Mc/s and the TM mode at frequency 3220 Mc/s they are thus outside said band AF. I
This window is arranged in a circular waveguide portion linked by two conical junctions to the output and load waveguides which latter two contain the two sets of metal obstacles in accordance with the invention, as illustrated in FIG. 4 for example. Thanks to this arrangement, the output system, throughout the band AP, has a standing wave ratio very close to unity and in any case of less than 1.1. Moreover, theshort length of the system constituted by the circular wave-guide system and its two junctions, its length is less than mm, avoid the appearance of any free-running parasitic resorrance.
' What I claim, is
l. A high frequency tube comprising, an exhausted enclosure, at least one rectangular-output waveguide tightly connected to said enclosure, a waveguide portion of larger section than said output waveguide, a progressive junction connecting said output waveguide and said waveguide portion, a dielectric output window arranged within said waveguide portion for maintaining the vacuum tightness of said tube enclosure, metallic obstacles, known as the first obstacles, arranged within said output waveguide for ensuring to the system comprising said waveguide portion, said output window and said progressive junction, a resonance frequency which is substantially equal to the central frequency of the operating frequency band of said tube, and further metallic obstacles, known as the second obstacles, arranged within said output waveguide for matching the system comprising said waveguide portion, said output window, said progressive junction and said first obstacles, throughout the operating frequency band of said tube.
2. A high frequency tube according to claim 1 comprising, a rectangular load waveguide of smaller section than said waveguide portion, a further progressive junction connecting said load waveguide to said waveguide portion opposite to said output waveguide, further metallic obstacles, known as the third obstacles, said third obstacles being identical with said first obstacles and being disposed within said load waveguide in an arrangement which is symmetrical of that of said first obstacles within said output waveguide with regards to said waveguide portion, and further metallic obstacles, known as the fourth obstacles, said fourth obstacles being identical with said second obstacles, and being disposed within said load waveguide in an arrangement which is symmetrical of that of said second obstacles within said output waveguide with regards to said waveguide portion. a
3. A high-frequency tube according to claim 1, wherein said first obstacles comprise two cylindrical metal bars located in a plane of section of said output waveguide and linking its wider faces to which they are perpendicular, said bars furthermore being symmetrically spaced from the axis of said waveguide and located in the neighborhood of the smaller faces thereof, and wherein said second obstacles comprise two further cylindrical metal bars located in a further plane of section of said output waveguide, said further plane being a little furtheraway from said output window than said plane of section of said first obstacles, said further metal bars linking said output waveguide wider faces to which they are perpendicular, said further metal bars furthermore being symmetrically spaced from the axis of said waveguide and located closer to the smaller faces thereof than are the bars constituting said first obstacles.
4. A high frequency tube according to claim 1, wherein said first and second obstacles are each constituted by two flat metal fins attached to the smaller faces of said output waveguide in the same plane of section thereof, the plane of said first obstacles being closer to said'output window than the plane of said second obstacles and said two planes being in the vicinity of said progressive junction connecting said output waveguide and said waveguide portion.
5. A high-frequency tube according to claim I,
wherein said first and second obstacles are each constituted by two flat metal fins attached to the larger faces of said output waveguide in the same plane of section thereof, the plane of said first obstacles being closer to said output window than the plane of said second obstacles, and said two planes being slightly spaced away from said progressive junction connecting said output waveguide and said waveguide portion.
6. A high-frequency tube according to claim 1, wherein the assembly of said first and second obstacles is constituted by a resonant window arranged within the output waveguide in the neighborhood of the junction connecting said output waveguide and said waveguide portion.