Cavity resonator

April 25, 1961 m. _c. Thompson, jr„ etal 2,981,908

CAVITY RESONATOR

Filed Dec. 15, 1958

Sheets-Sheet 1

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April 25, 1961

M. C. THOMPSON, JR., ETAL

CAVITY RESONATOR

2,981,908

Filed Dec. 15, 1958

Sheets-Sheet 2

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United States Patent Office _p, ,. _Δ²’?⁸¹;⁹®⁸

Patented Apr. 25, 1961

2,981,908

CAVITY RESONATOR

Moody C. Thompson, Jr., Boulder, Frank E. Freethey, Brighton, and Donald M. Waters, Boulder, Colo., assignors to the United States of America as represented by the Secretary of Commerce

Filed Dec.15,1958, Ser. No. 780,646

Claims. (CI. 333—83)

The present invention relates to cavity resonators such as are employed at radio frequencies in the microwave region and particularly contemplates an improved resonator which has an extremely low frequency drift with temperature change.

There are numerous occasions in electronic instrumentation when a cavity resonator having a very low temperature coefficient- is essential. For example, when using a cavity resonator as a frequency stabilizing element in a klystron oscillator or as- the resonant cavity in connection with a microwave refractometer it is important to maintain a fixed frequency of oscillation despite temperature^ changes which may affect the size of a cavity resonator.

Presently known procedures to achieve such , result in volve the fabrication of the resonant cavity from materials which are relatively temperature insensitive such as Invar in combination with a conducting medium such as brass or steel. By properly proportioning the relative; dimensions of the cavity when constructed of such material, it is possible to obtain compensation to a high degree, for example, about one part in 10 million per degrees centigrade, as an over-all frequency-temperature coefficient. However, a variety of dynamic heating effects- can. result from such, a composite cavity which may invalidate, for certain applications, such otherwise satisfactory long-term coefficient. .

In accordance with the principles of the present invention the cavity is fabricated from a temperature-insensitive ceramic: material -the surface of which is provided with an electrically conductive coating. . Since certain refractory materials such as solid solutions of the /3-spodumene system of lithia-alumina silicates have expansion coefficients very close to zero over limited temperature ranges, the physical dimensions and hence the resonant frequency of the cavity can be held constant despite temperature variations. The specific types of ceramic, material employed and the method of fabricating them will be further detailed as the description proceeds.

; It is accordingly an . immediate object of the present invention.to provide an improved cavity resonator which has a very low temperature coefficient of frequency variation., Another object of this invention is to provide an improved cavity resonator which is particularly adaptable for use in connection with a microwave refractometer for measuring refractive indexes of gases,

Still another object of the present invention is to provide a cavity resonator which is highly stable and relatively insensitive to temperature effects. ?

A still further object of this invention is to provide a cavity . resonator of high .thermal stability which can be produced easily and economically.

Other uses and advantages of the invention will become apparent upon reference to the specification and drawings, in which _:Fig. 1 is a disassembled view of one form of cavity resonator assembly embodying the principles of the present invention;

Fig. 2 is a view of the body portion of the cavity resonator of Fig. 1;

Fig. 3 is a view of the ceramic moulding employed in making a cavity resonator;

Fig. 4 is a detailed sectional view showing the construction of one modified iris;

Fig. 4(a) is a detailed Sectional view showing a con10 struction of a second modified iris;

Fig. 5 is an isometric view of a modified form of cavity resonator particularly suitable for use in connection with a microwave refractometer;

Fig. 6 illustrates an assembly technique employed in 15 producing the structure of Fig. 5, and

Fig. 7 shows the modified cavity resonator of Fig. 5 assembled in a mounting.

In general in accordance with the principles of the present invention the cavity resonator is fabricated from 20 ceramic material by first forming a particular selected ceramic into a generally cylindrical or other hollow shape and curing it by well-known refractory methods to form a moulding approximating the dimensions of the desired cavity resonator. For convenience, one end 25 wall of said cylinder or moulding may be formed integrally with the cylinder, a closure member being provided for the other end wall to permit mechanical operations on- the internal surfaces of the cavity. Alternately, the cavity can be fabricated with open end walls ³⁰. or the cavity may be formed of two hollow sections which form a cavity when joined. The interior surface of the cavity is then carefully lined with conducting material in the form of a coating or film and suitable irises are provided for . the transmission of the microwave. The 35 specific manner in which such outlined procedure is implemented will be further detailed in the particular examples enumerated below.

Fig. 1 shows a Completed microwave cavity resonator constructed in accordance with the principles of the ⁴⁰ present invention.. Fig,. 1- shows the cavity with the cover plate removed to provide a view of the interior of the . cavity and with one of the waveguides detached in order to show the construction of one form of the .iris. .

⁴³ As shown in Fig. 1, the cavity resonator comprises a main body 1 formed of ceramic material. The interior of -the body is provided in the illustrated embodiment with a cylindrical cavity 2 which may be finish-machined or otherwise formed to exacting dimensions. Portions ⁵⁰ of the outside of the body are suitably flattened to provide bosses- 3 which are adapted, to receive'the flanges 4 of the waveguides. 5. The region defined by the bosses 3 are further provided with suitable microwave irises 6 which may be formed either by boring through, the.wall.

⁵⁵ of a cavity or which may be left solid in accordance With the principles subsequently to be described.

The interior surface of the cavity 2 is provided with a surface of electrically conducting material which may be brushed, sprayed, electrodeposited, or otherwise applied . θθ to the interior surface of the cavity. The waveguides 5 are conventional and require no further description with the . exception that the flange 4 thereof is preferably made of Invar or other material-having a low temperature coefficient of expansion. Suitable receseses 3a for fastening 65 elements are provided in the body of the cavity 'and. these recesses, are provided with inserts 7 for. threadingly receiving suitable fasteners as will be described. The in-serts as well as. the fasteners are made of temperature...: insensitive material as will - be described' to' inhibit ? the 70 effect of temperatures on the geometrical size and, consequently, the frequency of the cavity. Alternately as will be described, the cover member 8 which is shown detached

2,981,908 above-described manner. Such treatment resulted in a firm bonding.

As indicated in Figs. 1 and 2, irises for the transmission of microwave energy may be provided in the body 5 or wall portion of the cavity by boring suitable recesses 6a and holes 6 through the cavity wall.

Fig. 4 shows an alternate embodiment singularly adapted to the present invention for forming microwave irises without the necessity of boring an opening through 10 the wall of the ceramic body. As indicated in Fig. 4 a portion 6b of the electrically-conductive surface corresponding to the size and location of the iris is removed either by scraping or by initially masking such area so that the electrically-conductive surface applied in the 15 previously-described manner will not adhere to such portion (6b) of the wall of the ceramic body. Such “opening” provides an excellent microwave conducting port despite the fact that the body of the ceramic material is not removed. It was found that a wall thickness of ap20 proximately 0.030” was sufficient. Such wall thickness not only preserves the strength of the cavity but readily permits the transmission of microwave energy. In effect, such method provides a microwave coupling iris which literally is filled with ceramic instead of air.

Alternately as shown in Fig. 1 the irises are made merely by boring small holes 6 in the end walls of the cavity and lining the holes with the conducting material in the above-described manner.

Specific examples of cavities made in accordance with 30 the above-outlined procedures will follow.

Example 1.—A cylindrical ceramic body 1 together with a cover plate 8 as shown in Fig. 3 were fabricated by usual ceramic moulding techniques using the above-described ceramic material corresponding to McDanel L35 53A and 581-G as above identified. The material was first formed into a cylindrical cup 1 (Fig. 3) having a closed bottom and an open top together with a disk 8 approximately the diameter of the cup to serve as a cover. The interior cylindrical surface of the cup was 40 carefully machined and ground as by centerless grinding techniques to provide a cavity of desired dimension commensurate with the desired resonant frequency of the cavity. The bottom of the cup was also finished as was the flat face of the cover member.

.,. Diametrically opposed portions of the outside cylin⁰ drical portion of the cup were then flattened to provide the bosses 3 described in connection with Fig. 1 by grinding. A portion 6a on each of such bosses 3 was then further reduced by counter boring to provide a ceramic g₀ wall thickness of approximately 0.030. Such counterbores were provided on each boss 3 in alignment with each other. A small iris hole was then drilled through the wall of the cup within the counterbore 6a to provide a microwave iris. The size of holes employed _5g ranged from Vs to 14 in diameter. The entire surface of the cup together with the cover plate was then carefully coated with Du Pont silver paint as above identified.

The bore of each iris was also coated with such con₆₀ ducting paint. Both the cup and the cover plate were then placed in an electric oven and treated in the manner described above and such treatment was then followed by the application of sufficient number of additional coats of silver paint until a uniform conducting ₆g surface was obtained on the cup and cover plate. Suitable holes were then drilled in both the cover plate 8 and in the rim of the cup as shown in Fig. 1 corresponding to the position of each of the inserts 7. Threaded inserts made of Invar alloys such as identified above 70 were then fabricated, inserted in the previously drilled holes and cemented in place by using a paste made of pulverized L-53A- ceramic and sodium silicate. Similarly holes were drilled in each of the bosses 3 corresponding to the like holes provided in the flange 4 of 75 the waveguide. The holes in the bosses were then filled from the cavity in Fig. 1 may be cemented or soldered to the cavity without the use of fastenings to further reduce the possibility of temperature effects consequent to the use of metallic fastenings.

Fig. 2 further details the construction of the body portion of the cavity resonator and more clearly shows the construction of the iris made in accordance with one modification of the invention to be described.

Fig. 3 shows the initial construction of the cavity together with its closure member 8. Such cavity as will be apparent can readily be constructed by conventional moulding techniques which are commonly employed in the ceramic art. Specifically, materials such as ceramics identified as McDanel L-53A and 581-G made by the McDanel Refractory Porcelain Company of Beaver Falls, Pennsylvania, may be employed. Such ceramic material is formed in the shape of a right circular cylindrical cup as shown in Fig. 3 together with a flat circular disc forming a closure member therefor. The cups are initially fabricated by moulding techniques to provide a cavity having a geometry conforming to the particular size of resonant cavity desired.

The ceramic material after moulding and curing is easily and readily worked by conventional mechanical means such as grinding and machining to provide the necessary precise final dimensions. Tolerances of a thousandth of an inch are readily obtainable, care being exercised to hold the ceramic materials during the process of machining without fracturing them.

Although several methods are available for obtaining the necessary electrically-conducting surface (such as evaporating, sputtering, etc.) the use of silver paint is considered the most practicable and economical. Various types of conducting paint are available for such purpose. Specifically, Du Pont “Type F” silver paste No. 6449 and 6296 are particularly suitable. Such pastes are thinned to brushing consistency using butyl Cellosolve and the paste is then applied to the critical surfaces of the cavity by brushing, spraying, or dipping and then airdried for about an hour. Subsequently, the cavity is fired in an electric furnace through cycles recommended by the manufacturer of the conducting paste. For Du Pont No. 6296 the procedure entails raising the furnace within which the ceramic has been placed to a temperature from ambient to about 1385° F., soaking the ceramic at such temperature for about 3 or 4 minutes then shutting off the furnace and allowing it to cool slowly to room temperature before removing the ceramic. When applying No. 6449 conducting paint the furnace is fired in the same manner except that a peak temperature of only about 1050 to 1100° F. is employed. In either instance, the first and second coats of the conducting paste are absorbed to a considerable degree by the relatively porous ceramic and usually three coats are provided on the cavity in the above-described manner in order to obtain a layer of adequate thickness and uniformity.

The ceramic cup and cover after being treated with conducting material in the above-described manner, are then machined in order to provide fastenings for attaching the waveguides 5 and end plate 8. As indicated in Fig. 1, suitable holes 3a are drilled in the ceramic body where necessary to provide fastenings for the cover Opiate 8 and the flanges 4 of the waveguides. Fitted inserts 7 made from commercial “Nilvar” an Invar alloy made by the Driver-Harris Corporation are then cemented into each of these holes. The material for cementing the inserts in the holes comprises a paste made from pulverized McDanel L-53A ceramic combined with sodium silicate. .

In addition, another method of fastening was found satisfactory. Such method consisted of coating the ceramic surfaces to be joined with a thick coat of the above-identified conducting paint. The surfaces to be joined such as the cover plate 8 were then clamped .firmly. to the body 1 of the ceramic cavity and fired in the

2,0 with Invar inserts in the same manner. Such cavity was found to be resonant at 9319 me. and had a Q of 11,740,

Example 2.—A cavity was constructed in accordance with the procedure detailed in Example 1 except that the cavity was made from the ceramic corresponding to McDanel 581-G ceramic. The cavity was provided with an inside diameter of about 1.72 and an interior length or height of about 1.36 such cavity upon test was found to have a resonant frequency of 9,304 me. and a Q of 10 about 13,650.

Example 3.—A cavity was fabricated in accordance with the steps outlined in connection with Examples 1 and 2 except no through holes were bored to provide irises. Specifically, the portion of the wall of the ceramic 15 at the bottom of each of the counter-bores 6a were carefully masked during the coating operation to provide an area 6b (Fig. 4) approximately corresponding to that of the bored irises. The resulting construction was a ceramic cup having solid walls in which the irises were 20 “ceramic filled” instead of being open. Such cavity having solid irises was found to have a resonant frequency of response of 9,306 me. and a Q of 4400. Various sizes of such unpainted “holes” for. the irises were tried ranging down to Vs diameter on the inside and Ai ²³diameter on the outside. The Q did not appreciably change but the transmission loss was found to be higher than that of the open type of iris.

Example 3A.—A still further modification of the iris construction similar to Example 3 was made by boring a ³⁰hole through the wall of the ceramic body such as the hole 6a, Fig. 1, and lining the hole with conductive material, as described in connection with Example 1, The hole was then filled with ceramic paste (Fig. 4A) made of pulverized L-53A ceramic and sodium silicate. The ³⁵ceramic body was then provided with a conducting surface, the iris being formed by masking as described in connection with Example 3. The transmission loss was. substantially reduced from that of Example 3.

In all of the above examples, the end plate 8 was se- ⁴⁰cured to the cup of body 1 by means of machine screws made of “Nilvar” which passed through the holes in,the plate 8 into the Nilvar threaded sleeves or inserts 7 provided in the body portion of the cavity 1. The waveguide flanges 4 were also specially made of Nilvar and ⁴³were about 14 thick. By virtue of their thickness and the dimension of the flat 3 formed in the body, of the cup, the flange 4 of the transmission line corresponded generally to the outside dimensions of the cup. . The flange of the waveguide was secured to the body 1 by ⁵⁰means of machine screws fitting into Nilvar threaded insert sleeves.

Figs, 5-7 illustrate how the principles of the present invention can. be implemented to. produce cavities of rela- tively complex shape. The cavity shown in Fig. 5 and ⁸⁵Fig. 7 has been formed to be particularly applicable for use in microwave refractometers’where the test gas of . continuously vafyirig refractivity, may be introduced .into .·' the test cavity rapidly and with, little flow resistance, and its effect upon the resonant frequency of the cavity being ⁰⁰recorded, with minimum error due to temperature variations altering the size or shape: of the cavity.

. Specifically, the·,body la of the cavity is made fol·/ lowing the procedure outlined in connection with Exam- „ _ pies 1 arid 2. , · The front and back portions of the cavity ⁰⁰are left open:: Suitable bosses 3b are’milled in the body to-provide mountirig surfaces for the wave-guides 5. · / Orifices in the form of cylindrical Tings are. then formed of ceramic material and they are then inserted, γθ concentrically with : the cylindrical body la by means of Struts 9 which are also made of ceramic strips,' // / .

.. Fig._: 6 shows brie method ’bf assembling the:-.’cavity shown in Figs. 5 and 7. Slots 10 may be milled in the' _:periphery of the cavity 1«. The struts 9 are then in- 75 ,908 serted in the slots 10 and the ring 8 is then mounted in the nest formed by the inner ends of the struts. The struts are “soldered” into the slots in the cavity wall and the ring to the strut ends with the Du Pont silver paint used to coat the cavity surfaces, applied rather heavily and fired according to the cycle previously described. If desired, the cavity may optimally be provided with a protective jacket such as the metallic casing 11 shown in Fig. 7.

Such described embodiment illustrates the ease with which complex shaped cavities can be fabricated in accordance with the principles of the present invention.

It is apparent from the above description that in accordance with the principles of the present invention resonant cavities having a high degree of temperature stability can readily be constructed cheaply and in large quantities. While the ceramic bodies 1 in connection with the present case were moulded by conventional ceramic moulding techniques, followed by careful machining of the interior surfaces after curing, it is obvious that by use of precision dies; cavities having accurate dimensions to produce the desired degree of resonance can be directly moulded followed by very little or no machining operations.

Moreover, as above noted, fastening of the cover plate and waveguide to the body 1 is readily accomplished by using the conducting surfacing paste as a cement, or alternately, by soldering the conducting surfaces together. Also other cements may be employed for bonding. The use of the Invar fastenings is thereby dispensed with.

It will be apparent that. in. accordance with the principles of the present invention the need for expensive temperature-insensitive materials is either minimized or dispensed with. Moreover, the labor and material costs incident to the manufacture of cavity resonators out of metallic material and particularly out of temperatureinsensitive alloys is materially reduced.

While particular ceramic materials and electrically conducting coatings have been specificaly identified as examples to enable construction and practice of the present invention it will be readily apparent that many varieties of commercially available ceramic materials and electrically conducting coatings can readily be employed in accordance with the principles of the present invention.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of invention as defined in the appended claims.