US3364383A - Waveguide impedance transformers - Google Patents

Description

Jan. 16, 1968 B. F. COOPER 3,364,383

WAVEGUIDE IMPEDANCE TRANSFORMERS Filed Aug, 19, 1963 2 Sheets-Sheet 1 PRIOR ART I E P 4 2 4 Li H 'NV-EN-TOE ATTORNEYS Jan. 16, 1968 B. F. COOPER 3,364,383

WAVEGUIDE IMPEDANCE TRANSFORMERS Filed Aug. 19, 1963 2 Sheets-Sheet 2 INVENTOR 64m We @071 BY 6am 4% w,

ATTOQN EY- 3,364,383 WAVEGUIDE IMPEDANCE TRANSFORMERS Brian Frederick Cooper, Chelmsford, Essex, England, as-

signor to English Electric Valve Company Limited, London, England, a British company Filed Aug. 19, 1963, Ser. No. 303,054 Claims priority, application Great Britain, Oct. 19, 1962, 39,631/ 62 9 Claims. (Cl. 315-39) This invention relates to waveguide impedance transformers such as are often used, in microwave technique, for such purposes as coupling the output resonator of a microwave oscillator, such for example as a magnetron, to its output waveguide. More specifically the invention relates to waveguide impedance transformers of the quarter-Wave type.

The invention is illustrated in and explained in connection with the accompanying drawings in which:

FIGURE 1 is a diagrammatic representation of the prior art quarter-wave type of impedance transformers to which this invention relates;

FIGURE 2 is an outer end view of a rectangular waveguide having a circular flange within which is mounted one illustrative waveguide impedance transformer of this invention;

FIGURE 3 is an elevation view of the waveguide and waveguide impedance transformer of FIGURE 2 partially in section;

FIGURE 4 is an inner end view of the waveguide and waveguide impedance transformer of FIGURE 2;

FIGURE 5 is an elevation view of a slotted block employed in the waveguide impedance transformer of this invention;

FIGURE 6 is an end view of the slotted block of FIGURE 5;

FIGURE 7 is a plan view of the slotted block of FIGURES 5 and 7;

FIGURE 8 is a perspective view partially in section showing the embodiment of FIGURES 2, 3, and 4 attached to the output of a microwave oscillator; and

FIGURE 9 is an equivalent diagrammatic representation of the embodiment of FIGURES 2, 3, and 4.

Referring to FIG. 1 the output resonator of a microwave oscillator, for example a magnetron, is represented by the block 1, the output of the resonator being indicated by dots. A load (not shown) is fed through a main waveguide 2 via an intermediate quarter-wave length of waveguide-commonly termed a transformer waveguide 3. The external Q value of the resonator 1 is determined by the impedance Z presented by the load at the resonator output. If Z is the characteristic impedance of the transformer waveguide 3, which is one quarter of a wavelength long, and if the main waveguide 2 is matched and is of characteristic impedance Z the value of Z at the resonator output is given by:

Since, therefore, the external Q value of the resonator depends on the value of Z it is important, for eflicient and satisfactory operating, to pre-determine this value accurately. It. is also desirable to allow adjustment in coupling to compensate for other dimensional variations. In known practice the waveguide 3 is accurately machined to determine its dimensions with precision. This is difficult and expensive to do, having regard to the high degree of accuracy commonly required. The present invention seeks to provide improved waveguide impedance transformers of the quarter-wave type without this disadvantage and which shall be such that the transformer waveguide which forms part of the transformer can be i United States Patent 0 easily adjusted, after manufacture. In this way cost of manufacture is materially reduced.

According to this invention a quarter-wave type waveguide impedance transformer includes .a transformer waveguide which is deformable to permit alteration of the characteristic impedance thereof to adjust the same accurately to a desired pre-determined value.

Referring to FIGS. 2 to 8 inclusive, these shown an embodiment of this invention wherein the main waveguide 2 of characteristic impedance Z is a rectangular waveguide one broad wall of which appears in FIG. 3 in which the guide is shown broken away. The said main guide terminates in a rectangularly apertured circular flange member 4 which is adapted to be fitted to the output resonator of a magnetron or other microwave device. Normally the space in the resonator forms part of the evacuated space of the device and, in order to preserve the vacuum, a glass or other suitable vacuum-tight window (not shown) may be sealed across the cross-section of the guide 2 near the flange member 4 in accordance with customary practice. In the end of the guide 2 where it enters the flange member 4 are fitted two similar rectangular metal blocks 5. One of these blocks is shown separately in FIGS. 5 to 7. In the construction shown they are made separately from the guide 2 and flange member 4 but are fitted in the mouth of the guide 2 and brazed to the opposite broad walls thereof so as to be, structurally speaking, part of the guide. One external dimension x of each block 5 is the same as the larger dimension of the interior cross-section of the guide 2; a second dimension y of each block is equal to one quarter of the intended working wave length; and the third imension z is of such value that 2z is less than the smaller dimension of the interior cross-section of the guide 2. Each block 5 has a slot 6 of length less than one quarter of a guided wave length cut in it parallel to and close to one of the two faces which are of dimensions x by y, the slot running the full length of the dimension x. The blocks 5 are fixed in the mouth of the guide 2 with their slotted faces facing into the guide 2 and the slots 6 near the space which is left between the blocks. The arrangement is best shown in FIGS. 4 and 8. The quarter-wave transformer waveguide 3 is constituted by the space between the blocks 5 and it will be seen that, in the construction shown, the cross-section of the guide hasone dimension x and the other dimension, which will hereinafter be termed the height dimension, equal to the separation between the adjacent faces of the blocks 5. This dimension, the height dimension, can be readily adjusted over a range which is adequate for practical purposes, by inserting a suitable tool into the waveguide 2 (before, of course, the window-if anyis fitted therein) and widening the slots by inserting the tool therein or, if desired, narrowing the slots by inserting the tool between the blocks 5. This results in flexing or bending the thin portions 5a of the blocks left between the slots 6 and the adjacent faces parallel thereto, thereby altering the separation of the said faces and with it the height of the transformer waveguide and thereforethe value Of Z1.

Preferably the adjustment is made such that the height of the transformer guide 3 is constant (at any cross section) over the dimension x. This requires that the slots 6 shall extend for the full length x as is the case in the illustrated embodiment. This is, however, not essential and the slots can be shorter giving adjustment over a shorter length and non-constant height for the transformer waveguide 3 over a given cross section. As will be seen from the drawings the slots form two short-circuited waveguides which are not cut off at the transformer design frequency. The design must, of course, be such that they are of acceptably small electrical effect on the performance of the transformer. This requirement, and how it may be satisfied, will be better understood from FIG. 9, which is an approximately equivalent diagrammatic representation of the illustrated embodiment.

Referring to FIG. 9 this equivalent diagram comprises the resonator 1, the transformer waveguide 3, the main waveguide 2 and two branch guides 66 constituted by the slots 6. If the branch waveguide has a low attenuation constant and its characteristic impedance is Z then Z the impedance presented by the series branch waveguide at its mouth (see FIG. 9) is given by:

Z =Z tan a 1 where on is the phase constant and 1 is the dimension shown in FIG. 9 (2) If a and 11 are, respectively, the broad and narrow cross sectional dimensions of the main guide 2 and a and 11 are respectively, the broad and narrow crosssectional dimensions of the branch guide 66.

If the branch guide is approximately one quarter of a wave length long and a (approx.)=a

Z (approx.)=Z,Z:

It will be seen therefore, that if the narrow cross sectional dimensions of the branch wave guide-i.e. the width of the slot 6-is kept small in relation to the narrow cross-sectional dimension of the main guide 2, the normalised impedance at the mouth of the branch waveguide will be small and the said branch waveguide will have only small effect on the impedance presented by the load (feed via the guide 2) at the terminals of the resonator 1.

While one form of the invention has been shown for purposes of illustration, it is to be understood that various changes in the details of construction and arrangement of parts may .be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. A quarter-wave impedance transformer including a quarter-wave length of rectangularly sectioned guide having two opposite broad faces of thin metal formed of thicker metal portions incompletely slotted to a depth of substantially one-quarter of a wave length and defining said thin metal faces supported by bendable necks of metal carrying said thin metal faces.

2. A transformer according to claim 1 wherein said broad face forming thicker metal portions each comprise a metal block, said broad faces comprising broad faces of said blocks, the slotting of said thicker metal portions being parallel to said broad faces and extending less than fully through each block, said bendable neck being defined by the unslotted portion of said blocks adjacent the termination of the slotting within said blocks.

3. A transformer according to claim 1 wherein the slotting of said thicker metal portions extends fully along one dimension of said thicker metal portions.

4. A transformer according to claim 1 for use in association with a main waveguide in communication therewith, the width of the slotting of said tricker metal portions being substantially less than the width of the main waveguide.

5. A magnetron structure comprising in combination a magnetron and a quarter-wave impedance output transformer as defined in claim 1.

6. A magnetron structure comprising in combination a magnetron and a quarter-wave impedance output transformer as defined in claim 4.

7. A quarter-wave impedance transformer comprising a quarter-wave length of rectangularly cross-sectioned wave guide, a pair of metal portions having opposed broad face portions comprising opposed broad faces of said quarter wave length of wave guide, each of said metal portions having a slot therein in proximity to said face portions to define relatively thin Walls intermediate said broad faces and said slots, and neck portions interconnecting said thin walls and the remainder of said metal portions, said thin Walls being adjustable toward and away from each other for adjustment of the characteristic impedance of said quarter-wave impedance transformer.

8. A quarter-wave impedance transformer according to claim '7 wherein each of said slots comprises a short circuited wave guide.

9. A transformer according to claim 7 wherein the depth of said slot is substantially one-quarter of a Wave length.

References Cited UNITED STATES PATENTS 2,411,534 11/1946 Fox 333-35 2,432,093 12/1947 Fox 333--35 2,466,922 4/1949 Wax 315-39 2,526,399 10/1950 Ohress 333-35 2,531,437 11/1950 Johnson 333-35 2,576,186 11/1951 Malter 33335 2,607,849 8/1952 Purcell et al 33335 2,666,869 1/1954 Clogston et a1. 3l5-39 2,701,343 2/1955 Lange 333-83 2,805,337 9/1957 Dunsmuir 315-3953 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

Claims (1)

1. A QUARTER-WAVE IMPEDANCE TRANSFORMER INCLUDING A QUARTER-WAVE LENGTH OF RECTANGULARLY SECTIONED GUIDE HAVING TWO OPPOSITE BROAD FACES OF THIN METAL FORMED OF THICKER METAL PORTIONS INCOMPLETELY SLOTTED TO A DEPTH OF SUBSTANTIALLY ONE-QUARTER OF A WAVE LENGTH AND DEFINING SAID THIN METAL FACES SUPPORTED BY BENDABLE NECKS OF METAL CARRYING SAID THIN METAL FACES.

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