US3048803A

US3048803A - Temperature compensated resonant cavity

Info

Publication number: US3048803A
Application number: US799773A
Authority: US
Inventors: William A Schanbacher
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1959-03-16
Filing date: 1959-03-16
Publication date: 1962-08-07
Anticipated expiration: 1979-08-07

Description

Aug. 7, 1962 w. A. SCHANBACHER 3,048,803

TEMPERATURE COMPENSATED RESONANT CAVITY Filed March 16. 1959' 2 Sheets-Sheet 2 n Tillllllf ut mama,

ilnited States Fatent 3,048,803 TEMPERATURE CGMPENSATED RESONANT CAVITY William A. Schanbacher, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Mar. 16, 1959, Ser. No. 799,773 9 Claims. (Cl. 333-83) The present invention relates to microwave resonant cavity structures and, more particularly, to such cavity structures having elements providing automatic temperature compensation to prevent changes in the resonant frequency of the cavity within the structure.

Numerous temperature compensation schemes for microwave cavities have been devised to provide a constant operating frequency; however, in general, such schemes do not provide the degree of accuracy over the wide range of temperature changes required by present day applications. This has principally led to the use of materials having a low coefiicient of the thermal expansion for the entire cavity, thereby rendering other temperature compensation means unnecessary. Such materials are generally diflicult to machine and, additionally, are heavier and more expensive than standard waveguide materials, such as brass or aluminum.

It is, therefore, an object of the present invention to provide a resonant cavity having a temperature compensation structure permitting use of standard waveguide materials for the major portion of the cavity.

Another object is to provide a resonant cavity, princi pally of standard waveguide materials, with automatic linear adjustment of the length of the cavity for variations in the diameter due to temperature changes.

Still another object of the invention is to provide a temperature compensating mechanism adaptable for automatically and linearly adjusting the length of a resonant cavity in response to diameter changes due to temperature variations.

In accordance with the invention a resonant cavity is provided with structure automatically and linearly adjusting the length of the cavity as the diameter varies because of temperature changes, and the like, to maintain the resonant frequency at a substantially constant value. Such structure includes at least two elements having an angular contacting surface therebetween. One of the elements is of the same conventional material as the cavity wall material, such as brass or aluminum; however, the material of the other element is one having a different coeflicient of thermal expansion, such as lnvar. The sum of the angles of the contacting surfaces, where more than one is utilized, is established to provide axial movement between the elements in response to temperature changes and establish a substantially linear change in length for a change in diameter of the cavity. The

value of the resonant frequency of the cavity then remains substantially constant for variations in temperature.

Other objects and advantages will be apparent from the following description considered together with the accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of a resonant cavity assembled in accordance with the present I invention;

of FIG. 2;

FIG. 4 is a perspective view of a second embodiment of the present invention;

"ice

FIG. 5 is a partially exploded perspective view, partly in section, of the embodiment of FIG. 4; and

FIG. 6 is a perspective longitudinal cross section of the plunger of FIG. 5.

Referring to FIG. 1 in detail, there is shown a cylindrical resonant cavity structure 11 having a separate upper portion 12 and lower portion 13 suitably held together by a plurality of tension springs 14, extended between holes 16 in the two portions. For propagating microwave energy to and from the cavity structure 11, waveguides such as rectangular waveguides 17 and 18 are respectively mounted on end plates 19 and 21 of the two portions 12 and 13.

When energy is to be propagated through waveguides 1'7 and 18 in a dominant, plane polarized, mode and the cavity of structure 11 is excited in a circularly polarized mode, the broadwalls of each of the waveguides 17 and 18 are extended parallel to the end plates 19 and 21 with electromagnetic coupling provided therebetween by coupling apertures 22 and 23, respectively. Such apertures 22 and 23 are disposed centrally through the end plates 19 and 21 and through the respective adjacent broadwall of the waveguides 17 and 13 between the longitudinal center line and one side thereof. For convenience and rigidity of the mechanical connection between the end plates 19 and 21 and the waveguides 17 and 18, respectively, the latter are suitably secured in grooves 24 of the former.

in accordance with the invention the upper portion 12 of the cavity structure 11 comprises several different elements adapted for assembly into a unit. Thus, there is provided an inner cylinder 26 having an annular flange 27 at the uppermost portion. A first ring 31 having an inside diameter substantially the same as the outside diameter of the inner cylinder 26 is provided as a slip fit for alignment purposes. The outer peripheral surface of the first ring 31 is provided with threads 33 to engage similar threads on the inner peripheral surface of a second ring 34, which serves as a spacer between the first ring and a third ring 36. So that the relative positions of the three

rings

31, 34, and 36 may be readily adjusted, matching threads 37 are also provided on the outer peripheral surface of the second ring 34 and on the inner surface of the third ring 36. The threads 37 have a different pitch than the threads 33 for adjustment purposes, as will be set forth hereinafter.

The third ring 36 also has an internal shoulder 38 and a second internally threaded portion 39 of greater diameter for receiving a threaded retaining ring 41. The upper end plate 19 has a ridge 42 on the upper surface, against which the retaining ring 41 abuts as it is threaded into threaded portion 39 of the third ring 36. A first recessed portion 43 is included in the under surface of the upper end plate 19 and the outermost edge thereof rests against a thin annular diaphragm 44 of conductive material supported by the shoulder 38. Such annular diaphragm is flexible and has an inner diameter equal to the inside diameter of inner cylinder 26.

For establishing electrical properties of the cavity within the structure 11 the diaphragm 44 is electrically connected to flange 27, as by suitable soldering procedures, or the diaphragm may be unitary with the flange of inner cylinder 26. Thus, in assembly a narrow opening 46 (see FIG. 3) remains between the diaphragm 44 and the adjacent wall of recessed portion 43 of the upper end plate 19, and this opening is limited in depth to a distance substantially equal to a half wavelength at the resonant frequency of the cavity (see FIG. 3). Also, upper end plate 19 is provided with a second re"- cesse'd portion '47, having a diameter substantially equal to the inner diameter of inner cylinder 26, which extends in depth for a distance substantially equal to a quarter wavelength at the resonant frequency of the cavity from the center of the opening 46. This latter quarter-wave dimension of the second recessed portion 47 establishes a current null of the microwave excitation at the mouth of the opening 46 and the former half-wave dimension of the depth of the opening establishes a low value of impedance at the mouth to minimize electrical losses at the junction of the upper end plate 19 and the diaphragm 44.

Now to assemble the upper portion 12 of the cavity structure 11, the following procedure may be readily used. The three

rings

31, 34, and 36 may be threaded together in such order. The inner cylinder 26 is then inserted into the ring assembly until the diaphragm 44 is suitably disposed to contact the internal shoulder 38 of the third ring 36. Finally, the upper end plate 19 is fitted to engage the diaphragm 44 and the retaining ring 4-1 is threaded into the third ring 36 to thereby provide a unitary upper portion 12 for the cavity structure 11. Thus, axial movement between the upper end plate 19 and the inner cylinder 26 is provided by distortion of the diaphragm 44. Also, the length of this portion 12 of the cavity is adjustable by the

differential thread arrangement

33 and 37 of the three

rings

31, 34, and 36 as by turning the second ring 24.

The lower portion 13 for the cavity structure 11 may be readily fabricated as a unit, as illustrated in FIG. 2, to provide lower end plate 21 with a substantially thinwalled cup portion 51. The lowermost internal portion 52 of the cup 51 has a smooth surface and an inside diameter substantially the same as that of the inner cylinder 26. The cup 51 is also provided with internal threads 53 to engage external threads 54 on the outer surface of the inner cylinder 26, such that a continuous internal surface is provided between the lowermost portion 52 of the cup and the inner surface of the cylinder when the two are threaded together. This junction occurs at a quarter wavelength distance from the bottom of the cavity where 'a current null exists. The uppermost portion of the cup 51 has a conical section 56 similar to a conical section 57 of the-lowermost portion of the first ring 3 1 and these two sections are in opposing alignment. Conical sections 56 and 57 will be referenced as cones here inafter. Prior to assembly of the cavity structure 11, a ring 61 is inserted between the two cones 56 and 57. Such ring 61, which will be referenced as ring hereinafter, is illustrated in FIG. 2 to have two tapered portions 58'and 59, which respectively engage the two cones 56 and 57.

With the upper and lower portions 12 and 13 threaded together in the manner set forth in the foregoing, by means of threads 53 and 54 with the ring 61 disposed between the cones 56 and 57 and with the tension springs *14 in place, there is provided a complete resonant cavity structure 11. When the entire structure is made of one material, such as brass or aluminum, with the exception of the ring 61, which is made of a material such as Invar having a substantially lower value of coefficient of expansion, linear temperature compensation is possible to maintain the 'resonant frequency of the cavity substantially constant. Such linear compensation for temperature changes is based upon the fact that there is, in general, a relationship whereby the resonant frequency of a cavity is a function of the cavity length and the cavity diameter and that for higher order dimensional changes, such as those caused by thermal expansion, a very good linear approximation between the change in length and the change in diameter is possible to maintain the frequency of the cavity constant. In accordance with the present invention, the approximation is embodied 1 in the slope of the tapers 58 and 59 of the ring 61 and the matching cones 56 and 57, and this will be set forth in greater detail hereinafter.

Where it is desired to operate the cavity structure 11 with a vacuum condition in the cavity, a tube 62 may be inserted through the upper end plate 19 to communicate with the cavity through the opening 46 and adapted for connection to a vacuum pump (not shown). Such operating condition also requires that conventional dielectric plugs 63 and 64 be respectively mounted within the two coupling apertures 22 and 23, so that the cavity of structure 11 is sealed with respect to the input and output microwave system.

Now, with the cavity structure 11 suitably connected into a microwave system, an increase in the temperature causes the diametrical dimensions of the cavity structure 11 to expand. The Invar ring 61 having a lower coefjcient of thermal expansion remains substantially the same. The result is that the upper and lower portions 12 and 13 move toward each other by virtue of the angular sliding engagement between the ring 61 and cones 56 and 57 and the tension of the springs 14, so that the length of the cavity decreases by suitable selection of the slopes of the angular sliding elements. The decrease in length distorts the diphragm 44 without altering the electrical properties of the cavity and can be made substantially linear with respect to the increase in diameter, as previously stated. It is readily apparent that the converse result occurs should the temperature of the cavity structure decrease.

A somewhat different structural arrangement utilizing the same principles is illustrated in FIG. 4 of the drawings and reference is made thereto. In the arrangement of such figure, a cylindrical resonant cavity structure 81 is provided with one coupling aperture 82 (see FIG. 5) disposed centrally in the closed lower end of the cavity structure for electromagnetically coupling energy between the cavity and a rectangular waveguide 83 extended along a continuation of the longitudinal axis of the cavity. A second coupling aperture 84 extends through the peripheral wall of the cavity structure 81 to electromagnetically couple energy between the cavity and a second rectangular waveguide 86 (see FIG. 4) disposed with the broad walls thereof parallel to the longitudinal axis of the cavity.

The uppermost portion of the cavity structure 81 is open-ended and has internal threads 87 extended between the upper end thereof and an intermediately disposed shoulder 83. To support a plunger 91 within the cavity structure 81, a circular plate 92 having peripheral threads 93 engaging the threads 87 of the cavity structure is provided with a central tubular extension 94 having internal threads 96. Such extension 94 has a slightly converging taper and a plurality of longitudinal slots 97, so that a threaded nut 98 applies holding pressure to an object within the extension when threaded on external threads 99 of the extension.

In accordance with this form of the invention the plunger 91 comprises several elements combined as a unit to provide suitable temperature compensation for maintaining the resonant frequency of the cavity substantially constant. Thus, as illustrated in FIG. 6, a circular cup-shaped element 101 having an outer diameter substantially the same as the inner diameter of the cavity structure 81 is provided with a central post 102 extended in the same direction as rim 103 of the cup. The dimension of such rim 103 is equal to a quarter wavelength to provide a low impedance point (substantially a short circuit) between the cavity wall and the lowermost portion of the plunger 91. A hollow tube 104 coaxially receives the post 102 and is provided with a circular flange 106 at the lower end and such flange has a diameter substantially equal to the inside diameter of the cup-shaped element 101.

The upper end of the tube 104 has external threads 107 which match the internal threads 96 of extension 94 of plate 92 and an enlarged internal bore 108 to receive a plug 109. By suitably securing one end of a tension spring 111 to the plug 109 and the other end to the post 102, when the post is disposed within the tube 104, the two elements are suitably held together. To facilitate assembly and secure the spring 111 to the post 102, such post is provided with a threaded bore 112 which receives a threaded plug 113 to which the other end of the sprlng is suitably secured. Two annular plates 116 and 117 having peripheral bevels 118 and 119, respectively, are disposed under circular flange 1% within the cup-shaped element 131 with the bevels facing each other in radial divergence. Between the two plates 116 and 117, there is disposed a third plate 121 having an enlarged peripheral rim 122 with

angular tapers

123 and 124 on the respective innermost surfaces to slidably engage the respective bevels 118 and 119 of the two plates. Hereinafter, the plates 116 and 117 Will be referenced as cones 116 and 117 and the rim 122 as ring 122.

With the foregoing elements of the plunger 91 assem bled to have the stated relationships, the tension of spring 111 maintains the cones 116 and 117 in sliding contact with the ring 122. When the cones 116 and 117 are of conventional waveguide material, such as brass or aluminum, and the intermediate ring 122 is of a material having a lower coeflicient of thermal expansion, such as Invar, the relative vertical or axial position of the three plates 116, 117, and 121 varies with changes 1n tempera; ture. By suitable selection of the angles of the cones 116 and 117 and the ring 122, the overall longitudinal dimension of the plunger 91 varies linearly with respect to variations of the diameter of the cones, and this fea ture will be described more fully hereinafter.

The plunger 91 is supported in the cavity structure 81 by plate 92 and nut 98 to provide the desired cavity dimensions. A cover 126 having threads 127 engag ng threads 37 of the cavity structure 81 suitably seals the upper open end of the structure. To provide evacuation of the interior of the cavity structure 81 conventional microwave iris windows 131 (only one being shown) are provided at coupling flanges 133 and 134, respectively, of the waveguides 83 and 86 and one of the coupling flanges 133 is provided with a port 136 communicating with the interior of the waveguide 83 for coupling to a vacuum pump (not shown).

The operation of the cavity structure 81 is substantially the same as that previously set forth with respect to the structure 11 described in connection with the embodiment of FIG. 1. Thus, in this latter embodiment, a change in temperature results in a change in the diameter of the cones 116 and 117 of the plunger 91 and such change in dimension causes a corresponding change in the length of the plunger to maintain the resonant frequency substantially constant. In the first instance the longitudinal dimension of the cavity within the structure 11 is linearly changed by an automatic adjustment of the respective aligned positions of the two separate portions 12 and 13 of the structure. In the second instance, the longitudinal dimension of the cavity within structure 81 is linearly changed by an automatic adjustment of the length of the plunger 91 within the cavity. In both instances it is to be realized that the automatic adjustments are carried out in response to temperature changes and the like, which might otherwise alter the resonant frequency of the cavity.

As has been stated previously, the resonant frequency, F of a microwave cavity as a function of the cavity length, L and the cavity diameter, D and that, for higher order dimensional changes such as those caused by thermal expansion and the like, a very good linear approximation between the change in length and the change in diameter can be found such that the cavity frequency remains constant. The realization of the required change in cavity length is accomplished by transforming the differential radial expansion of the combination of elements having differing coefficients of thermal expansion into an axial motion via the sloping surfaces.

It follows mathematically from the foregoing that when the change in length, dL equals a constant, C, times the change in diameter, dD then the change in the frequency, F,, is equal to zero. For an example of the mathematical development of the foregoing, reference is made to the plunger 91 of FIG. 6 for illustration of certain angles, Z, and dimension, D. As the two similar cones 116 and 117 expand, the ring 122 maintains substantially its same diametrical dimension. Therefore, the two cones 1116 and 117 are forced apart axially along the angle formed by the matching surfaces of the cones and the ring 122 and this axial movement is dependent upon the size of the angle. For a more comprehensive expression, the total number of degrees of the angle, or angles where more than one compensating combination is provided, must fill the following equation:

where:

C:=constant from AF =O, when dL =C-dD N=number of angles of matching surfaces C =coeflicient of thermal expansion D =diameter of the cavity D'=etfective minor diameter of ring 122 (see FIG. 5)

Z =angle of ith matching surface measure from a perpendicular to the longitudinal axis of plunger 91 (sec FIGS) While the foregoing analysis has been set forth in detail with respect to the compensating combination of elements for the plunger 91 of the cavity structure 81, illustrated in FIGS. 4-6, the same analysis holds for the compensating combination of elements for the cavity structure 11 of FIGS. l-3. Thus, there has been described in detail a combination of similar elements as applied to two structurally different resonant cavity structures 1 1 and 8 1 to provide automatic compensation of the dimensions of the cavity Within the structure and thereby maintain a substantially constant value of resonant frequency for the cavity under varying temperature conditions. It is to be noted that the choice of materials discussed for various elements has been illustrative only and that other combinations of materials are within the scope of the invention. Further, it has been indicated in the foregoing that more than one combination of compensating elements having sloping surfaces may be readily utilized so long as the equation set forth above is satisfied.

Additionally, the compensating structure has been described and illustrated with respect to cones and rings for both embodiments and these elements have matching angular or tapered portions, which are not to be limiting in any manner because many other contact arrangements between the elements is feasible to provide the desired linear relation between changes in diameter and changes in length. Thus, where the ring is suitably tapered the cones may have a sliding contact therewith such as by a rounded or pointed ridge, and vice versa. It will therefore be apparent that the controlling factors are the provision of at least two elements having at least one angular surface along which the other slides to provide a linearly related axial movement between the two elements.

While the salient features of the present invention have been described in detail with respect to two embodiments, it will be readily apparent that numerous changes may be made within the spirit and scope of the invention and it is, therefore, not desired to limit the invention to the exact details shown except insofar as they may be set forth in the following claims.

I claim:

1. Temperature compensation structure for -a resonant cavity having a cylinder and a pair of parallel end walls separated by a predetermined length, said structure comprising at least one ring element disposed normal to the axis of said cylinder and having a first coefficient of thermal expansion, at least one annular element having a surface thereof in sliding contact with a surface on said ring element and having a second coefiicient of thermal expansion dilfering from that of said ring element, at least one of said surfaces having a conical shape with the angle tihereof being proportional to the ratio of diametr-ical thermal expansion and axial movement between said ring and cone elements with said expansion and movement being linearly related.

2. Temperature compensation structure for a resonant cavity comprising at least one ring disposed normal to the axis of said cavity and having a first coefiicient of thermal expansion, and a pair of cones respectively contacting said ring with sliding engagements and having a second coeflic-ient of thermalexpansion, said first coefficient being different than said second coefiicient, said sliding engagements being conical with the sum of the angles being proportional to the ratio of diametrical thermal expansion and axial movement between said ring and cones with said expansion and movement linearly related.

3. In a temperature compensated resonant cavity, the combination comprising a cylindrical resonant cavity structure having a cavity of predetermined diameter and length and means included in said structure for controlling said length in response to thermal changes in said diameter, said means including at least one ring disposed normal to the axis of said cavity and having a first coefficient of thermal expansion and at least one cone mounted in contact with said ring and having a second coefiicient of thermal expansion, said first coefficient being different than said second coefiicient, said cone and ring having an angular contacting .surface with the angle of such surface being proportional to a constant equal to the ratio of changes in diameter and changes in length of said cavity.

4. In a temperature compensated resonant cavity, the combination comprising a hollow metallic cylinder and a plunger mounted in said cylinder to provide a resonant cavity, said plunger including elements of different coefficients of expansion with conical contacting surfaces therebetween, the sum of the angles of such surfaces being proportional to a constant equal to the ratio of changes in diameter and changes in length of said cavity.

5. In a temperature compensated resonant cavity, the

combination comprising a hollow metallic cylinder and a plunger mounted in said cylinder to define a cavity, said plunger having two circular members movably mounted with respect to each other with a combination of a ring element disposed between two cone elements mounted between said circular members, said ring and cone elements having different coefficients of thermal expansion and angular sliding contact surfaces therebetween, the sum of the sliding angles of said surfaces being proportional to a constant equal to the ratio of changes in diameter and changes in length of said cavity.

6. In a temperature compensated resonant cavity, the combination comprising a hollow metallic cylinder, a plunger having parallel upper and lower circular members mounted for axial movement of the lower member with respect to the upper member, means for mounting said plunger within said cylinder with a fixed relation between said upper circular member and said cylinder to define a cavity, and a ring element mounted between said upper and lower circular members With cone elements respectively disposed on either side of such ring element in contact with said ring element and adjacent circular member, said ring element having a different coefficient of thermal expansion than said cone elements and having angular sliding contact with said cone elements with the sum of the angles of said sliding contacts being proportional to a constant equal to the ratio of changes in diameter and changes in length of said cavity.

7. In a temperature compensated resonant cavity, the combination comprising a cylindrical resonant cavity structure having separate upper and lower portions and a first coefficient of thermal expansion, a ring having a second different coefiioient of thermal expansion disposed between said upper and lower portions with at least one angular sliding contact surface therebetween, the angle of said surface being proportional to a constant equal to the ratio of changes in diameter and changes in length, and means disposed between said upper and lower portions securing the portions together for axial adjustment by said angular surface.

8. In a temperature compensated resonant cavity, the combination comprising a cylindrical resonant cavity structure having separate upper and lower portions and a first coefficient of thermal expansion, said upper portion including adjustable annular elements flexibly supporting an inner cylinder of substantially the same inside diameter as that of said lower portion, a ring having a second different coefficient of thermal expansion disposed between one of said annular elements and said lower portion with at least one angular sliding contact surface therebetween, the angle of said surface being proportional to a constant equal to the ratio of changes in diameter and changes in length, and means disposed between said upper and lower portions securing said portions together for axial adjustment by said angular surface with said inner cylinder secured within said lower portion.

9. In a temperature compensated resonant cavity, the combination comprising a cylindrical resonant cavity structure having separate upper and lower portions and a first coeflicient of thermal expansion, said upper portion including adjustable annular elements, an inner cylinder having an inside diameter substantially the same as the inside diameter of said lower portion, a thin annular diaphragm mounted between said annular elements and said inner cylinder for flexible support, at least one ring disposed between one of said annular elements and said lower portion with at least one angular sliding contact surface therebetween, the sum of the angles of said contact surfaces being proportional to the ratio of diametrical thermal expansion and axial movement between said portions with said expansion and movement being linearly related, and spring means disposed between said upper and lower portions together with said inner cylinder secured within said lower portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,439,908 Rigrod Apr. 20, 1948 2,600,225 Ehrenfried June 10, 1952 2,824,258 Snow et al. Feb. 18, 1958 2,880,357 Snow et al. Mar. 31, 1959