US2886742A - Broadband output coupler - Google Patents

Description

y 1959 J. F. HULL BROADBAND OUTPUT COUPLER Filed 001:. 23, 1957 United States Patent BROADBAND OUTPUT COUPLER Joseph Frederick Hull, Redwood City, Calif., assignor to Litton Industries of California, Beverly Hills, Calif.

Application October 23, 1957, Serial No. 691,855

4 Claims. (Cl. 315-39) This invention relates to a broadband output coupler for ultra-high frequency electron discharge devices, and more particularly to a broadband output coupler which functions to provide a substantially continuous transmission line from the ramp output of 'an ultra-high frequency electron discharge device to an associated ridged output waveguide.

For many years one of the limitations in the design of high power microwave electron discharge devices has been the inability of these devices to provide operation over a broad frequency range. In the prior art most effort has been expended to provide broadband tuners which are capable of tuning over the requisite bandwidth. However, even when satisfactory broadband tuners have been constructed there has remained the very real problem of how to provide broadband transmission of high power from the R.F. generator to an external load, since prior art output couplers have proven to be relatively narrow band devices.

In the prior art, attempts have been made to eliminate output coupler limitations by the use of coupling transitions such as quarter wave transformers, variable impedance ramps and similar devices which serve as impedance matching devices for transforming the high impedance of a conventional waveguide to the characteristically low impedance of high frequency resonator structures. It has been found that the use of these coupling transitions alone are not adequate, because of the limitation imposed by the structural characteristics of output terminations. As a further attempt toward a solution, R.F. chokes have been used, in combination with tuners and output coupling transitions, to provide a more effective electrical connection between the output termination and the adjoining waveguides. Although these and other methods have been employed with minor success, they have not successfully provided a truly broadband output coupler which would permit the extraction of power over a sufiiciently broad frequency range.

The present invention obviates the foregoing and other disadvantages of the prior art and fills the above-described void in the prior art. In accordance with the basic concept of the present invention there is herein disclosed a broadband output coupler which is operative over a two to one frequency range and thereby functions to provide :a continuous broadband transmission line from an ultrahigh frequency electron discharge device to an associated output load. More particularly, in accordance with the present invention the extraction of energy from microwave tubes is accomplished through the use of a pair of "exponentially tapered output ramps which function to deliver power from the microwave generator to a ridged output waveguide, the transmission path between the ramps and the ridged guide being made substantially continuous and non-reactive through the utilization of a pair of high frequency Waveguide chokes disposed on opposite sides of an output window which separates the output ramps and the ridged waveguide.

It is, therefore, an object of theinvention to provide ice" an improved broadband output coupler for high frequency electron discharge devices which is operable over a two to one frequency range.

Another object of the invention is to provide an -.improved broadband output coupler which employs a pair of output ramps and aridged output wave guide linked .to provide a substantially continuous transmission line.

A further object of the invention is to provide an output coupler which employs a pair of waveguide chokes disposed on opposite sides of the output window to provide a substantially non-reactive broadband interconnection between the output ramps of a high frequency electron discharge device and an associated ridged output waveguide.

The novel features which are believed, to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood fromthe following description considered in connection with the accompanying drawing in which one embodiment ofthe invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is intended for the purpose of illustration and description only, and is not intended as a definition of thelimits of the invention.

Figure 1 is an elevation view, partly in section, of the output coupler of the invention illustrating the manner in which the various elements thereof cooperate to provide broadband operation;

Figure 2 is a cross-sectional view of the output coupler of the invention, taken along line 2-2 of Figure 1; and

Figure 3 is another cross-sectional View of the output coupler of the invention taken along line 33 ofFigure. 1, illustrating the ridged output waveguide employed therein.

With reference now to the drawing, there is shown in Fig. 1 a broadband output coupler, constructed in accordance with the teachings of the invention, for transmitting microwave energy generated within an ultra-high frequency electron discharge device 10, only a portionof which is shown, to an associated load. As shown in Fig. 1, the output coupler of the invention comprises four basic elements, namely, an output horn 12 including a pair of ramps 14 and 16, avacuum-tight seal including an output window 13 disposed adjacent to the end of the ramps, a ridged waveguide 20 positioned adjacent the opposite side of the window, and a coupling structure, generally designated 22, for intercoupling horn 12 and window 1% with waveguide 20, the coupling structure also being operative to form a waveguide choke joint on both sides of the output window.

Before describing the detailed structure of the foregoing elements, it should be pointed out that electron discharge device it? maybe a magnetron, klystron, traveling wave tube, backward wave oscillator or any like device for generating or emplifying microwaves. Accordingly, it should be expressly understood that the basic concepts herein taught are not restricted to use with any particular form ofmicrowave vacuum tube.

Returning now to the specific elements of the output coupler of the invention, horn 12 is preferably conical in shape and is afiixed to the tube body 10, as by brazing or the like. The ramps 14 and 16 are fixedly mounted within horn l2 and function as a relatively broadband impedance transformer to provide a transmission line' for the transmission of ultra-high frequency energy from the high frequency structure within tube 10' to the output window 18. In Fig. 2 there is shown a cross-sectional configuration of the ramps illustrating one typical manner in which the output structure is illuminated with energy, this particular technique involving the use of an output iris 23 which intercouples the internal resonator structure of the tube with the output ramps14 and 16.

As shown in Fig. 1, window 18 is located adjacent the divergent ends of the ramps, the window being made of low loss glass, ceramic or the like, and being preferably circular .in shape. As will be appreciated from the description set forth below, the diameter and thickness of this window are not critical when used in accordance with the present invention, thus permitting a selection of the optimum configuration for various tube types.

Referring now to Fig. 3 there is shown a cross-sectional view of the output coupler illustrating the ridged waveguide 20 employed therein. As shown in this view the ridged waveguide 20 includes a pair of ridged members '24 and 26 which are fixedly positioned diamentrically opposite each other in a cylindrical body member 28.

Returning once more to Fig. 1, it will be noted that the spacing between the ridges on members 24 and 26 is substantially the same as the spacing between the divergent ends of output ramps 14 and 16 within the output horn. In addition it should be noted further that the upper end of the cylindrical body 28 has a smaller diameter than the central region thereof, thereby providing an external flange which forms part of a wave guide choke joint as disclosed in more detail below. The ridged waveguide assembly is completed by an output flange 29 which is brazed or otherwise afiixed to the lower end of cylinder 28, as viewed in Fig. 1.

Consider now the details of coupling structure 22. As will be more clearly understood from the description hereinafter set forth, the coupling structure performs three functions, namely, it provides a vacuum tight seal for window 18, it rigidly positions the ridged waveguide 20 relative to output ramps 14 and 16, and in addition, it provides in conjunction with the flanged portion of cylinder 28 a pair of waveguide chokes on opposite sides of the output window to thereby provide an effectively continuous electrical connection between the ramps 14 and 16 and the corresponding ridges in the output waveguide.

As shown in Fig. 1, output horn 12 is hermetically enclosed by the cup-like annular member 30, formed of Kovar or a like material, to which the output window is sealed, and a second annular member 32 having an outwardly flared upper end for interconnecting member 30 and output horn 12, the external diameter of the unflared portion of member 32 being smaller than the internal diameter of member 30. Consequently the dimensioning of these members provides a choke joint between output ramps 14 and 16 and output window 18, the internal volume of the choke comprising the annular passageway designated 33 which is formed between members 30 and 32, and the region between the output window and the ends of ramps 14 and 16. A more detailed analysis of the choke dimensions is set forth hereinbelow.

Continuing with the description of Fig. 1, coupling structure 22 is completed by an annular spring contact member 34 whose upper end is slotted longitudinally to provide a plurality of contact fingers, and a protective shroud 36, both of which are affixed at their lower ends to the central region of cylinder 28 which forms part of the ridged output waveguide as described previously. By dimensioning the upper end and central region of cylinder 28 to correspond with the outer diameters of annular members 32 and 30, respectively, it will be seen that spring contact fingers of member 34 will slidably engage the external periphery of annular member 30 thereby forming with the upper end of cylinder 28 a second or external waveguide choke for reflecting an apparent electrical continuation of the ridges in the ridged waveguide up to the output window.

It should be noted here that the desired physical spacing between the waveguide ridges and the output window may be assured by controlling the length of the protective shroud 36 so that it bottoms against tube body when the desired physical spacing is achieved between the output window and the ridged waveguide.

Before discussing the electrical operation of the output coupler of the invention and the advantages provided thereby, consideration will be given first to the mechanical problems which the coupler overcomes. Firstly, it is apparent that the use of the two relatively thin annular members 30 and 32 for intercoupling the output window and the output horn 12 provide a rigid structure which nevertheless is sufliciently flexible to prevent damage to the output window and its glass-to-metal seal by differential expansion during the exhausting and bake-out of the tube. Secondly it will be recognized that the several annular members employed in coupling structure 22 may be fabricated by punching or stamping, and thus reduce manufacturing costs while simultaneously eliminating sharp corners in the choke structure which could otherwise create arcing at higher power levels. Finally, the use of contact spring fingers on member 34 for engaging the periphery of member 30 permits rapid assembly or disassembly of the component parts of the output coupler. It will also be recognized by those skilled in the art that the contact fingers engage member 30 at a point of substantially zero current in the external waveguide choke, thus substantially eliminating electrical breakdowns and losses which might otherwise occur.

Considering now the electrical operation of the output coupler of the invention, it will be recalled that the two quarter wave length chokes formed in the output coupler are operative to form a substantially continuous transmission line between the ramps 14 and 16, and the ridges in the output waveguide. Although it is generally known to the art that quarter wave chokes may be employed to reflect an apparent short circuit, it is important to note that as utilized in the present invention the chokes do not reflect a short circuit directly intercoupling the ends of ramps 14 and 16 and the ridges in the output waveguide; instead, as disclosed below, the choke joints are used to further broaden the frequency range of the output coupler by eliminating at least one source of impedance matching problems normally associated with output couplers.

More specifically, although the output window in a microwave electron discharge device is often considered to be lossless and electrically transparent, it does in fact reflect a portion of the incident energy, and appears as a shunt capacitance in the output transmission line. While this elfect may be tolerated in vacuum tubes operating at a fixed frequency, it can seriously limit the bandwidth of a tunable broadband device.

In accordance with the present invention, the quarter wavelength chokes utilized in the output coupler are dimensioned to reflect a short circuit to the right and left of the ends of ramps 14 and 16, respectively, so that the region between the ramps and the ridged output waveguide appears at the center frequency of the operating band as a notch in the transmission line formed by the ramps in the output horn and the ridges in the output waveguide. Stated differently, the distance between the reflected short circuits taken along the electric field vector is larger than the distance between the divergent ends of the output ramps or the ridges in the output guide. Consequently the quarter wave length waveguide chokes, taken by themselves, insert a series inductance in the output transmission line. When considered simultaneously with the electrical eflect of the output window, however, this inductive effect may be used to compensate for the capacitive reactance of the output window, and thus provide a substantially continuous and non-reactive transmission line between the exponential ramps and the ridges in the output waveguide.

In practice it has been found that the foregoing result may be achieved by making the length of the remote portions of the chokes equal to one quarter the wavelength in free space of the center frequency of the band over which the coupler is to operate, as is customarily done in waveguide chokes, and to make the radial distance from the end of the ramps and ridges to the remote portions of the chokes approximately twenty or thirty percent longer than a quarter wave length at the center frequency. Thus from an electrical point of view, the output window appears to be supported by a notched section in the transmission line formed by ramps 14 and 16 and the ridged output waveguide.

It has been found that the utilization of the basic concepts herein set forth provide output couplers which are capable of operating at relatively high power levels and over frequency ranges greatly exceeding what has heretofore been achieved by any known prior art waveguide output couplers.

While the output coupler of the invention has been described with reference to only one particular embodiment, it will be understood that various modifications could be made in the construction thereof without departing from the spirit and scope of the invention. Accordingly, it is to be expressly understood that the foregoing description shall be interpreted only as illustrative of the invention, and that the appended claims be accorded as broad and interpretation as is consistent with the basic concepts herein taught.

What is claimed as new is:

1. In a microwave vacuum tube including a high frequency generating structure, a broadband output coupler comprising: a pair of divergent output ramps for extracting energy from the high frequency generating structure, said ramps being spaced from each other by a predetermined distance at their divergent ends; an output horn surrounding said output ramps and forming a part of the vacuum envelope of the vacuum tube; a first annular member sealed at one end to the mouth of said horn extending back over said horn, said annular member being flared outwardly at its other end and having an axial length substantially equal to one quarter of a wavelength at the center frequency of the frequency band over which the vacuum tube is to operate; a cuplike second annular member having an apertured bottom and an open end, the open end of said cuplike member being sealed to the flared end of said first annular member; an output window heremetically sealed in the aperture in the bottom of said second annular member and positioned adjacent the di vergent end of said ramps whereby said first and second annular members form a quarter wavelength waveguide choke for reflecting a short circuit between said ramps and said output window; a ridged waveguide having first and second ridges spaced from each other by substantially said predetermined distance; and means for mounting said waveguide adjacent said output window opposite said ramps with said ridges coplanar with said ramps.

2. The broadband output coupler defined in claim 1 wherein said ridged waveguide has a substantially cylindrical external periphery which is notched annularly to provide an end region adjacent said output window having a diameter substantially equal to the diameter of said first annular member and a central region remote from said output window having a diameter substantially equal to the diameter of said second annular member, and a third annular member having a plurality of spring fingers at one end thereof, slidably engageable with the external periphery of said second annular member, said third annular member being aflixed at its other end to the central region of said ridged waveguide whereby said third annular member and the notch in the periphery of said waveguide form a second quarter wavelength choke for reflecting a short circuit between said output window and the ridges in said waveguide.

3. A broadband output coupler for extracting electrical energy from the high frequency structure within a microwave vacuum tube, said output coupler comprising: an output horn constituting a portion of the vacuum envelope of said tube; a pair of exponentially divergent ramps 6 t mounted in said horn on opposite sides thereof and operable as an impedance transformer for transmitting electrical energy from the high frequency structure through said horn, the divergent ends of said ramps being spaced from each other by a predetermined distance; an output window disposed adjacent the divergent ends of said ramps; a ridged waveguide disposed adjacent said window on the opposite side thereof, the ridges in said waveguide being spaced from each other by substantially said predetermined distance and being aligned with said ramps within said horn; first means for hermetically sea-ling said output window to said horn and forming a first quarter wavelength choke to reflect an apparent electrical short circuit between said window and said ramps; and second means for inter-coupling said output horn and said ridged waveguide, said second means forming a second quarter waveguide choke to reflect an apparent electrical short circuit between said window and the ridges in said ridged guide, the short circuits reflected by said first and second chokes forming a notch in the transmission line formed by said ramps and the ridges in said ridged guide to thereby insert a series inductance in the transmission line to compensate for the shunt capacitance eifect introduced by said output window.

4. A mechanical and electrical connection between two sections of a broadband output coupler for providing a substantially continuous electrical path for transmission of energy from a high frequency structure within a microwave vacuum tube, said connection comprising: a. first degassed section of said coupler including an output horn constituting a portion of the vacuum envelope of said tube; a pair of exponentially divergent ramps mounted in said horn on opposite sides thereof and operable as an impedance transformer for transmitting electrical energy from the high frequency structure through said horn, the divergent ends of said ramp being spaced from each other by a predetermined distance; an output window disposed adjacent the divergent ends of said ramps; a second section open to atmospheric pressure including a ridged waveguide disposed adjacent said window opposite said ramps, the ridges in said waveguide being spaced from each other by substantially said predetermined distance and being aligned with said ramps within said horn; first means for hermetically sealing said output window to said horn and forming a first quarter wavelength choke to reflect an apparent electrical short circuit between said window and said ramps, said first means including a cup-like annular member in the bottom of which said window is sealed, and a second annular member having an outwardly flared upper end for interconnecting said cup-like member and said output horn with space relation of a second predetermined distance; and second means for intercoupling said output horn and said ridged waveguide, said second means forming a second quarter wavelength choke to reflect an apparent electrical short circuit between said window and the ridges in said ridged guide, said second means including an annular spring contact member slidably engageable over the central region of said cup-like member and forming with said ridged waveguide said second quarter wavelength choke, affixed at their lower ends to the central region the short circuits reflected by said first and second chokes forming a notch in the transmission line formed by said ramps and the ridges in said ridged guide to thereby insert a series inductance in the transmission line to compensate for the shunt capacitance efiect introduced by said output window.

References Cited in the file of this patent UNITED STATES PATENTS 2,591,350 Gorn Apr. 1, 1952 2,765,423 Crapuchettes Oct. 2, 1953

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