US2981908A - Cavity resonator - Google Patents

Description

April 25, 1961 M. c. THOMPSON, JR, ETAL 2,981,908

CAVITY RESONATOR 2 Sheets-Sheet 1 Filed Dec. 15, 1958 m M w w m mmgm m wd A fifi 65M J Z? %%M CONDUCT/V5 C'OIQT/A/G April 25, 1961 M. c. THOMPSON, JR, ETAL 2,981,908

CAVITY RESONATOR 2 Sheets-Sheet 2 Filed Dec. 15, 1958 INVENTORG M004] 6 7/20mps0/M Frank EFPee we Dona/0' M Wafers M ATTORNEY Stats z,9 s1,90s

CAVITY RESONATOR Moody C. Thompson, Jr., Boulder, Frank E. Freethey,

Brighton, and Donald M. Waters, Boulder, Colo., assigno'rstothe United States. of America as represented by the Secretary of Commerce 1 Filed na. 15 1958,.Ser. No.7s0,646

2 Claims. (Cl. 333-83 The present invention relates to cavity resonators such as are employed at radio frequencies in the microwaveregion and particularly contemplates an improved resonatorwhich 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 amicrowave. refractometer it is important to maintain a fixed frequency of oscillation despite temperature. changeswhich may afiect the size of'a cavity resonator. V

Presently known procedures to achieve suchresult in volve. the fabricationof the resonant cavity from mate- 'vrials which are relatively temperatureinsensitive such as Invar in combination with aconductingmedium such' as brass or steel. By properly proportioning the relative dimensions of the cavity-when constructed oflsuch material, it is possible -to obtain compensation to ahigh' degree, for example, about one part in million per degrees Centigrade, as an over-all frequency-temperature coefficient. Howeyer, a variety of dynamic heating effects can result from such a composite cavitywhich may ing them will be further detailed as the description proceeds. r h It isaccordingly an immediate" object of the present inventionto provide an improved cavity resonator which tion. r

Another object ofthisinvention is to provide an improved. cavity resonator which is particularly adaptable The-specific types of 2,981,908 Patented Apr. 25, 196 1 '2 resonator assembly embodying the principles of themesent 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 construction ofasecond 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 producing the structure ofyFig. 5, and V t 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 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 wall of said cylinder or moulding maybe formed integrally with the cylinder, a closure member being provided for the other end wall to permitmechanical op- .erations on the internal surfaces of the cavity. Alterspecific 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 accordancewith the principles of the present invention. Fig. 1 shows the cavity with the cover plate removed to provide a view of the interior of the. cavity and withone of the. waveguides detached in order. to show the construction of one form of the,

' iris.

with a cylindrical cavity 2 which maybe finish-machined .or otherwiseformed to exacting dimensions.

Portions of the outside of the body are suitably flattened to provide bosses- 3 which are adapted. to receiveithe flanges' 4 of the waveguidesw 5; The region defined by the bosses 3 are further provided with suitable microwave irises 6 has a verylow'temperature coeflicient of frequency variafor, use in connection with a microwavefrefractometer for measuring'refractive indexes of gases,

Still another ;object of the present invention is to providea cavity resonator. which is highly stable and rela tively insensitive to temperature effects;

cavity resonator of. high thermal stability which can; be

produced easilyand economically;

A San further object of this invention isf/to sm a which may be formed either bybcring through. thewall.

.' of a cavity or which maybe leftsolid in accordance with the principles subsequently to be described.

The interior-surface of the cavity'Z is provided with ,a-

surface of electrically conducting material which may be brushed, sprayed, electrodeposited, or :otherwise'applied-. to the interior surface of the cavity. The waveguides 5 are conventional and require no furtherdescr'iption with the" exception thatthe flange 4 thereof is preferably made of Invar or other material having a lowtemperature co- 1 eflicient'of'expansion. {Suitable receseses 3a.for fastening elements are provided in the y of the cavii land. these recesses. are provided with inserts 7 for, th'readingly s A receiving suitable fasteners as will be described; [The in- Other uses and; advantages of the iinventioni will be- "come. apparent upon reference to the fspecification and drawings, in which I Fig. '1 is" a disassembled ViewQQf ne ofcavity.

sert's as well-as. the fasteners are made of temperature- I:

insensitive material "as will-be described l to inhibit lithe effect [of temperati res ion the geometrical size and, con j sequently; the frequency of the cavity; Alternat'ely asiwill 1 be described, the cover fnember8 whichis showndetached The interior ing a closure member therefor.

isatis factory. ceramic surfaces to be joined-with a thickcoat of the above-identified conducting paint. 'The surfaces to be r 3 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.

gether with its closure member 8. Such cavity as will be apparent can readily be constructed by conventional moulding techniques whichare commonly employed in the ceramic art. Specifically, materials such as ceramics identified as McDanelL-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 form- 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 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} 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 I then machined in order to provide fastenings for attach- .ing 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 {plate 8 andthe flanges 4 of the waveguides. I Fitted inserts 7 made from'commercial Nilvar an Invar alloy made byth e Driver-Harris Corporation are then cemented into each of these holes. .the inserts in the holes comprises a paste made from pul- The material forcementing verized McDanel L-53A ceramic combined with sodium silicates In addition, another method of fastening was found Such method consisted .of coating the joined such as the cover plate S were then clamped firm- 4 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 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 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 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 ap 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 amicrowave 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 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 L- 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 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 aswas the fiat face of the cover member.

Diametrically opposed portions of the outside cylindrical 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 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 ranged from /8" to A" 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. v

The bore of each iris was also coated with such conducting 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 suflicient number of additional coats of silver paint until a uniform conducting surface was obtained on the cup and cover plate. Suitable holes were then drilled in both thecover 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 the waveguide. T he holes in the bosses were then filled r with Invar inserts in the same manner. Such cavity was found to be resonant at 9319 me. and had a Q of 11,740, a 1 I Example 2.-A cavity-was constructed in accordance with the procedure detailedin Example 1 except that the '7 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 about 13,650. 7

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 irisesr Specifically, theportion of the wall of the ceramic 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 solidwalls in which the irises were ceramic filled instead of being open. Such cavityhaving 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 forthe irises were tried ranging down to diameter on the inside and Mt" diameter on the outside. The Q did ,notappreciably 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 hole through the wall of the ceramic body such as the hole 6a, Fig. l, 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 ceramic body was then provided with a conducting surface, the iris being formed by-rnasking 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 secured 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 orinserts 7 provided in the body portion of the cavity 1. The wave- 6 serted in the slots and the ring 8 is then mounted in the nest formed by the inner ends of the struts. The

. struts are soldere 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 withthe 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 invention it will be readily apparent that many varieties 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 beemployed for bonding. The use of the Invar fastenings is thereby dispensed with.

a It will be apparent thatin accordance with the principles of the present invention the need-for expensive temperature-insensitive materials is either minimized or disinsensitive 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 of commercially available ceramic materials and electhe 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 thecup; The

flange of the Waveguide wassecured to the body 1 by means of machine screws fitting into Nilvar threaded use in' microwave refractometersfwhere the test gas of continuously-varying r'efractivity. may; be introduced into I the test cavity rapidly and withlittle flow resistance, and its eifect upon the resonant frequency of the "cavity being recorded, with'rninimum error due to temperature vari? 'ationsaltering the size or shapelof the cavity.

trically conducting coatings can readily be employed in accordance with the principles of the present invention. It will beapparent that the embodimentsshown are only exemplary and that various modifications can be made in construction and arrangement within the scope of invention as defined in theappended claims.

What is claimed is: l. A resonator having a desired resonant frequency comprising a body of material having a low coefficient of expansion and containing a hollow volume bounded by at least one surface on allexcept a first and second side, .the volume having a configuration and size dependent upon said resonant frequency and said first and second side allowing the free passage of a gas through said volume, a coating of electrically conductive material bonded to said surface, a-first and second hollow ring, means Specifically, the-,bodyla of thec'avity is master lowing the procedure outlined in connection with Examples 1 and 2. The front aud back'por'tions of the cavity are leftopen'. Sui-tablebosses 3b are milled iirxthe body: to provide mounting surfaces for the waveguides 5.

" Orific'es in the form of cylindrical rings are then formed of ceramic material ari d theyare then 'inserted for mounting said, first and second ring in said first and second side, respectively, said last mentioned means permitting-the passage of a gas through said first and second side,- and means for coupling electromagnetic energy through said surface and through said volume. a 2.. resonator having a desired resonant frequency comprising .a bodyof 'material having a low coefiicient ,of expansion and containing ahollow volume bounded .by at least one surface on all excepta first and second side, theyolume having a configuration and sized deconcentricallywiththe cylindricalbody In by means of struts 9"which are also made of ceramic strips.

Figiy6 sho'ws onemethod bfassembling .th'ej cavity 7 shown in..Figs-. 5fand 17 Slots 10 maybe-,milledjinlthe" periphery of the cavity 1a. QThe struts' 9 arethen inpendent upon said resonant frequency and said first and second side allowing thefreepassage of a gas through said-volume, a coating ofelect'rically conductive material ,bonded tosaid surface, a first and second hollowring, at

.least a firststrut mounting said first ring in said first side,

atjl'east a second strut mountingsaid second ring in said 7 second side, and means for coupling electromagnetic energy through said surface and through said volume.

References Cited in the file of this patent UNITED STATES PATENTS 2,281,247 Peterson Apr. 28, 1942 8 White Sept. 26, 1950 Rosencrans' Mar. 22, 1955 Foster June 10, 1958 McArthur Nov. 11, 1958 Ashbaugh July 28, 1959

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