US2993140A - High power phase shifter - Google Patents

Description

July 18, 1961 E. P. WESTBROOK 2,993,140

HIGH POWER PHASE SHIFTER Filed May 15, 1957 2,993,140 HIGH POWER PHASE SHIFTER Edwin P. Westbrook, Aubnrndale, Mass., assignor to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed May 13, 1957, Ser. No. 658,688 Claims. (Cl. 3155.42)

This invention relates to microwave devices and in particular to a phase shifter for microwave devices capable of handling high power. The phase shifter of the invention may be inserted in any wave guide; and so, while in the following description the invention is described with particular reference to its use in the radio frequency circuit of a microwave linear accelerator, the invention is not limited to this application.

In accordance with the invention, a dielectric rod is introduced longitudinally into the wave guide, and the amount of phase shift is determined by the extent to which the rod extends into the guide. In one embodiment of the invention, the wave guide into which the phase shifter is to be introduced is provided with an elbow, the outer surface of which is provided with a slot through which the rod is introduced longitudinally into the wave guide. In another embodiment of the invention, the rod is introduced axially into a circular wave guide into which the radio frequency power is introduced through its lateral wall and from which the radio frequency power is removed in the same manner. Since the device operates as a phase shifter, it may also be used for the purpose of adding effective length to a transmission line. The invention is not limited to any particular shape of' rod and includes the use of more than one rod, in which case the rods preferably are parallel to each other.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the use of the invention in connection with a microwave linear accelerator;

FIG. 2 is a somewhat diagrammatic view in longitudinal central section through a phase shifter embodying one form of the invention;

FIG. 3 is a side elevation of the device of FIG. 2; and

FIG. 4 is a somewhat diagrammatic view in perspective of another embodiment of the invention.

In certain constructions of microwave linear accelerators, high power output is achieved through use of two klystrons which are alternately pulsed, so that one klystron feeds power into the accelerating wave guide between pulses of the other klystron. Each pulse for each klystron may have power as high as two megawatts, with the average power being four kilowatts for each klystron. In order that the power output of each klystron may be delivered into the accelerating wave guide as efliciently as possible, the output cavity of each klystron must be coupled to the accelerating waveguide. If there is improper coupling between the klystron and the load, the load will not accept all the power available, and some of the power will be reflected. Referring to FIG. 1 of the drawing, the output from the klystrons 1, 2 is fed into a three-port junction 3 by wave guides 4 and 5 respectively, and power is fed from the three-port junction 3 to the accelerator wave guide 6 by wave guide 7. -When the first klystron 1 is pulsed, it is desired that the entire power output be delivered to the accelerator wave guide 6 and that the power which is reflected by the second klystron 2 through the wave guide 5 arrive back at the three-port junction 3 in proper phase for optimum power transfer from the first klystron 1 to the aceler-ating wave guide 6. Similarly, when the second klystron 2 is pulsed, it is the reflection of power through the Wave guide 4 from the first klystron 1 which must be properly adjusted. There- 2,993,140 Patented July 18, 1961 ice fore, in order that the junction between the wave guide 5 and the junction 3 may present the proper impedance to the power output of the first klystron 1, a phase shifter 9 must be introduced in the wave guide 5. Similarly, a phase shifter 8 is introduced in the wave guide 4. Alternatively the length of the wave guides 4, 5 from the klystrons 1, 2 might be adjusted at the factory to provide the proper phasing; but in that event each time a klystron was replaced, one might require complete a new wave guide, since the new klystron might have different characteristics. Moreover, in general, adjustments must be made after the accelerator is installed, so that the use of phase shifters is preferable. Evidently these phase shifters 8, 9 must be able to handle high power, since as noted, each klystron has a peak power output of 2 megawatts. The maximum powers involved in microwave applications during World War II were in the vicinity of 1 megawatt and high powers of 2 megawatts are a recent development.

The wave guides 4, 5 and 7 are filled with nitrogen at about 30 p.s.i.g. because the power-handling capacity of the guide is increased by pressurization or evacuation, and a pressure seal is easier to make than a vacuum seal. The pressurized region is separated from the evacuated regions within the accelerating wave guide 6 and within the klystrous 1, 2 by quartz windows 10.

Present type of phase shifters include the dielectricslab type in which a dielectric slab is supported within the wave guide by metal guide rods. Such phase shifters have not been suitable at high power since power break down has developed in the vicinity of these guide rods. Up until now high-power phase shifters have included the rotary phase shifter and also the short-slot hybrid with a pair of non-contacting shorts. The cost of such highpower phase shifters is considerable, and one of the objects of the invention is to provide an inexpensive phase shifter adapted to handle high power. For example, the cost of a short-slot hybrid phase shifter is approximately $1,800 whereas the cost of a high-power phase shifter constructed in accordance with the invention is approximately $130.

Referring now to FIGS. 2 and 3, a metal tube 11 is soldered or brazed to the outer circumferential wall of an elbow 12. An aperture is then drilled in the outer circumferential wall of the elbow 12. The metal tube 11 may conveniently be used as a jig for this purpose, so that the aperture corresponds to the inside diameter of the metal tube 11. The inner surface adjacent the junction between the metal tube 11 and the elbow 12 is then flared at 13, as by filing with hand tools, so as to present a well-rounded surface free from sharp points or projections within the elbow 12. Owing to this necessity for flaring, the metal tube 11 should have appreciable thickness. A high cylinder or bushing 14, which may be made, for example of Teflon, is slid inside the metal tube 11 against a pin 15 which enters through the wall of the metal tube 11. A dielectric rod 16 is introduced into the elbow 12 through the bushing 14 so as to lie along the axis of one of the wave guides 17 between which the elbow 12 is connected. The bushing 14 provides mechanical support for the rod 16 While insulating it from the walls of the elbow 12. The bushing 14 should be withdrawn out of the field in the elbow 12. The rod 16 may be made of any dielectric material but fused quartz is to be preferred as having the best dielectric strength and also as having low losses. The rod 16 may have anouter diameter of A of an inch and preferably is pointed with the point tapering to a diameter of A of an inch over a length of 6 inches.

A rubber hose 18 is fitted over the metal tube 11 and is held by a hose clamp '19 which is clamped tightly so as to provide a pressure seal capable of holding the pres- .encies.

may be made;

sure within the wave guide. A rubber cylinder 20 is inserted in the other end of the rubber hose 18 so as to surround the rod 16 snugly. The inner surface of the.

rubber cylinder 20 is lubricated'with a grease such as Apiezon, and the rod 16 is inserted throughthe rubber cylinder 20 and the Teflon bushing 14. A second hose clamp 21 surrounds the'rubber hose 18 at the rubber cylinder 20 but is kept loose. Any suitable bellows mechanism may be substituted for the rubber hose arrangement.

A linear movement of the rod 16 of approximately 1 foot will provide a phase shift of approximately 180 using the dimensions described above.

The smaller the radius of curvature of the elbow 12, the greater the mismatch introduced by the phase shifter because the discontinuity at the quartz rod 16 is more sudden. That is to say, there is a sudden change from the empty or pressurized guide to the dielectric-containing guide. On the other hand, the larger the-radius of curvature of the elbow 12, the greater the area of the aperture through which the rod 16 is introduced, owing to the increased angle at which the rod enters the wave guide, and therefore the harder it becomes to compensate for the mismatch introduced by the aperture. The elbow 12 should therefore have a radius of curvature which effects a good compromise between these opposing tend- The mismatch introduced by the dielectric rod 16 and its associated aperturemay be compensated by any conventional means such'as an inductive iris 22, or alternatively a wave guide transformer, a tapered wave guide section, or a stub may be used. An inductiveiris is preferable to a capacitive iris because of its greater power-handling ability.

Alternative constructions include providing two or three small rods in the elbow for better matching and in order to provide more phase shifter with less length. Power handling capabilities may be enhanced by using optimum geometry, as by selecting the proper radius of curvature for the elbow 12 as outlined above. In conjunction with additional rods, of course, additional apertures are required in the elbow.

In the embodiment of the invention shown in FIGS. 2 and 3, although the mismatch introduced by the rod 16 may be compensated, such compensation'introduces difficulties. For example, 'the introduction of an inductive iris 22 intensifies the electric field, and concentrates it so that it is more likely to break down. In an alternative embodiment of the invention, shown in FIG. 4, the mismatch introduced by the dielectric rod is com- 'pensated by the position of the short and by the provision of an appropriate taper at the end of the circular wave guide. In this embodiment a nearly perfect match Moreover, in the elbow form of the invention shown in FIGS. 2 and 3, the hollow metal tube '11 must be flared as it approaches the elbow 12 in order to avoid field concentrations.

Such-flaring is not necessary in the embodiment of the invention shown in FIG.

1 4. Moreover, the embodiment shown in 'FIG. 4 per- Referring to FIG. 4, the phase shifter includes a circular wave guide '23 laterally into whichthe high frequency power is introduced and removed through wave guides 24. The phase shifter of FIG. 4 may be introduced into a straight line wavguide 25 by providing suitable elbows '26. The dielectric rod 16 is introduced into introduced by the dielectric rod 16. Except for the features which have just been'described, the operation and 4 functioning of the embodiment of the invention shown in FIG. '4 is the same as that shown in FIGS. 2 and 3.

Having thus described the principles of the invention together with illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of-limitation, with-the scope of the invention being set forth'in the following claims.

I claim:

l. A high-power phase shifter adapted to introduce phase shift into a wave guide comprising an elbow adapted to be inserted in said wave guide, an aperture in the outer circumferential wall of said elbow, means for producing adjustable phase shift in said wave guide comprising a dielectric rod extending through said aperture into said elbow and'thence along the axis ofsaid wave guide, and means for varying the amount of penetration of said rod into said guide for phase shift.

2. Apparatus in accordance with claim 1 wherein an inductive iris is included in said elbow in order to compensate the mismatch introduced bysaid rod and said aperture.

3. A high-power phase shifter comprising a circular wave guide shorted at oneend and having a central aperture at the other end, means for producing adjustable phase shift in said wave guide comprising a dielectric rod extending through said aperture axially into said circular wave guide, and means for varying the amount of penetration of said rod into said guide for phase shift means for introducing microwave power laterally into said circular wave guide near one extremity thereof and meansfor extracting microwave power laterally out of said wave guide near the other extremity thereof.

4. Apparatus in accordance with claim 3 wherein said circular wave guide is tapered at the end having the aperture in order to compensate the mismatch introduced by said rod and said aperture.

5. Means for introducing microwave'power into a microwave linear accelerator comprising in combination: at least two high power microwave power sources, a junction, a wave guide connecting each of said power sources to said junction, a wave guide connecting said junction to the accelerating wave guide of said microwave linear accelerator, means for causing said power sources alternatively to deliver power to said junction, and a phase shifter in each of those wave guides which connect said power sources to said junction, each of said power shifters comprising-at least one dielectric rod and means for introducing said'rod axially into the wave guide associated therewith, said phase shifters being adjusted to prevent delivery 'of'power between said power sources.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 16, 1940 2,376,785 Krasik May 22, 1945 2,423,396 Linder July. 1, 1947 2,526,383 Meier'= Oct. 17, 1950 2,556,978 Pierce June12, 1951 2,576,943 Jenks Dec. 4, 1951' 2,579,327 Lund Dec. 18, 1951 2,686,901 Dicke Aug. 17, 1954 2,757,341 Lundstrom' July 31, 1956 2,774,946 McGillem Dec. 18, 1956 2,783,439 Whitehorn Feb. 26 1957 2,813,996 Chodoro-w Nov. 19, 1957 2,843,747 Ashley July 15, 1958 2,866,949 Tillotson Dec. 30, 1958 g FOREIGN PATENTS 933,841 France Jam-5, ,1948

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