US3118120A

US3118120A - Thermally compensatable tunable cavity

Info

Publication number: US3118120A
Application number: US32992A
Authority: US
Inventors: Shapiro Henry; Robert F Romero
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1960-05-31
Filing date: 1960-05-31
Publication date: 1964-01-14
Anticipated expiration: 1981-01-14

Description

Jan. 14, 1964 H. SHAPIRO ETAL 3,118,120

THERMALLY COMPENSATABLE TUNABLE CAVITY Filed May 51, 1960 IN VE NTORS 4 54/27 .swa/uea United States Patent 3,113,120 THERE/EARLY GMPENSATAELE TUNABELE QAVETY Help, Shapiro, New Rochelle, and Robert F. Romero, llaznaroneclr, N.Y., assignors, by assignments, to lunch Systems, mo, Beverly Hills, Califi, a corpo- Maryland Filed May 31, 196%, Ser. No. 32,992 2 Claims. (@l. 333-433) This invention relates to the art of resonant units, more particularly of the temperature compensatable tunable cavity type for high frequency transmission.

As conducive to an understanding of the invention, it is noted that where a resonant unit is of the type such as illustratively shown in Patent No. 2,918,636, dated December 22, 1959, that includes an anode cavity which is tunable by moving a plunger controlled by rods extending from the cavity, since the effective length of the anode cavity substantially deter-mines the operating frequency of the device, where once the plunger has been set by movement of its control rods, changes in ambient and/ or operating temperatures cause a change in the efiective length of the anode cavity, the operating frequency of the device will be varied.

Where attempts have been made to make the resonant unit temperature stable by constructing the unit with materials with low thermal expansion coefficients, the high cost of such materials renders the units impracticable for ordinary commercial purposes. Furthermore, since materials suitable for resonant units that have a negative thermal coefficient of expansion are not available, it has not been possible intentionally to overcompensate a resonant unit to take care of imperfect temperature compensation elsewhere in elements coupled to the resonant unit.

it is accordingly among the objects of the invention to provide a resonant unit that may readily be fabricated and is strong and sturdy and is not likely to become deranged, and which has an anode cavity that may readily be tuned to a desired operating frequency, and which will automatically adjust the length of the anode cavity with increases in ambient temperature to compensate for thermal variations in the materials used in the construction of the resonant unit or related units.

According to the invention, the resonant unit cornprises a cavity in which a plunger is slidably mounted and controlled by rods extending from one end of the cavity, the plunger position substantially determining the resonant 0 frequency of said cavity. The rods which are locked in fixed position when the cavity is tuned are of material that has a suitable thermal coefiicient of linear expansion relative to that of the cavity, so that with an increase in ambient temperature, for example, the resonant frequency can be decreased, decreased or held substantially constant.

in the accompanying drawings in which is shown one of various possible embodiments of the several features of the invention,

The single FIGURE is a fragmentary longitudinal sectional view of a resonant unit according to the invention, with parts broken away.

Referring now to the drawings, the temperature compensatable device shown to illustrate the invention is of the type shown in Patent No. 2,918,636, the description and drawings of which are included herein by reference for a more complete showing of the environment in which the present invention is incorporated, and the unit herein shown will omy be described to the extent necessary for an understanding of the present invention. Thus iii) ilhjzh Patented Jan. 14, 196

such device comprises a metal sleeve 10 which defines the outer wall of the anode cavity 11.

The lower end 41 of sleeve 10 is seated in a circular recess 42 in a bottom plate and is at'tixed in said recess as by soldering. The plate '43 has a central opening 44 coaxial with sleeve 10 and a pair of diametrically aligned openings 45, the rear edge of plate 43 having a mounting flange 47. Aflixed to plate 43 is a disc 51 which has an outstanding axial hub 52 coaxial with the opening 44 in plate 43 and a pair of openings 5! aligned with opening 45.

Extending through hub 52 into sleeve 10 and aifixed to the hub as at 53 is a metal sleeve 54 which defines the inner wall of the anode cavity 11 and the outer wall of a grid-cathode cavity 55 more fully described in said Patent No. 2,918,636.

The inner end 58 of sleeve 54 mounts a contact ring 59 which has a plurality of resilient contact fingers 61 designed to make an effective electrical and mechanical connection with the grid ring 62 of tube 34-.

Slidably mounted on sleeve 54 in sleeve 10 is a metal plunger 64 by means of which the anode cavity 11 is tuned. The plunger 64 on its inner and outer peripheries has resilient contact fingers 6-5 which engage the opposed surfaces of sleeves it and 54 to provide a good electrical connection and also serve to retain the plunger 64 in set position, the volume of sleeve 11 between plunger '64 and disc 17 defining the active portion of the anode cavity.

To adjust the position of plunger 64 to tune the anode cavity 11, a sleeve 181 is slidably positioned in each of the aligned pairs of openings 45, Si in bottom plate 43 and disc 51. Each of the sleeves 181 extends through a hub 182 secured to the outer surface of disc 51 and is secured in any desired position by set screws 183.

A rod 66 extends through each of the sleeves 181 and is locked in desired position as by set screws 13 extending through the lower end of the associated sleeve 181.

Thus, referring to the drawing, with the sleeves 181 pushed upwardly as far as possible and locked in position by set screws 183, the effective length of the rods 66 will be from the center line of set screws 184 to the bottom of the plunger 64. With the sleeves 131 moved downwardly, it is apparent that the distance between the set screws 184- and the bottom of the plunger will be increased, thereby materially increasing the effective length of the rods 66 and hence increasing their eifectiveness as temperature compensating elements.

According to the invention, the rods 66' connected to plunger 64 are of material such as to have a coetlicient of linear thermal expansion with respect to the sleeve 16 as to effect a movement of the plunger 64 with changes in ambient temperature that will increase, decrease or hold substantially constant the resonant frequency of the cavity 19.

Thus, assuming that the cavity is of brass, which has a coefiicient of linear thermal expansion of approximately 17.7 to 212x10" per degree centigrade, with a temperature range of from 25 to 300 degrees C., the rods 66 could be of polystyrene, which has a coeflicient of linear thermal expansion of approximately l0 per degree 0., or phenol-formaldehyde, which has a coefficient of 3G to 40x10 per degree C.

In both illustrations, the coeficient of linear thermal expansion of the rods 66 is greater than that of the sleeve 10. Hence, based upon the distance of the plunger 64 from the location at which the rods are secured by set screws 184, which depends on the length of the rods and the material of such rods, the position of the plunger in the cavity ill will vary with changes in ambient tempera- '5 3 ture so that the resonant frequency of the cavity may be increased, decreased or held substantially constant.

in practice, the type of material to be used for the rod is determined empirically. based on the efiective length of the rods 65 by checking the resonant frequency at difierent temperatures and with different rod materials, and selecting the particular rods which give the desired compensation.

The specific mathematical relations which illustrate the compensation are as follows: Referring to the drawing, let

K1 =coeificient of linear thermal expansion of sleeve 10 K2=coefiicient of linear thermal expansion of sleeve 54 K3=coefiicient of linear thermal expansion of plunger 64 K4=coetficient of linear thermal expansion of plate 43 K5=coefiicient of linear thermal expansion of block 182 K6=coefiicient of linear thermal expansion of rods 66 Dl=inner diameter of sleeve 10 D2=outer diameter of sleeve 54 C =eifective capacity of plate-grid section of tube 34 L length of sleeve 14 from point of contact of plate of tube 34 to plate 43 A=thickness of plate 43 B=distance from top face of block 73 to center line of set screws 184 W=thickness of plunger 64 X =distance from point of contact of sleeve ltl to plate of tube 34 to top of plunger 64- C=distance from bottom face of plunger 64- to center line of set screws 184 V=velocity of light f resonant frequency of cavity 11 M=mode of resonance (n O, quarter wave; n= 1, three quarter wave, etc.)

Ignoring the detuning sheet of the output '75, the resonant frequency of the plate-grid coaxial cavity 11 is derived as follows:

The characteristic impedance of a coaxial cavity is:

The Equation 3 shows the dependence of the resonant frequency f on the plunger position X; the two diarncters D1, D2, and the effective tube capacity C Thus, if the plunger'position X can be made'to vary as desired, as the ambient and/or operating temperatures change, the resonant frequency can be made to remain stationary or vary in a prescribed manner.

. Referring to the drawing, plunger position X is given by: t

' 'X=L+A+B-C W For any temperature T, the plunger position X is:

in the Equation 5 it is understood that L, A, B, C and W are measured at temperature T degrees. The change in plunger position X caused by a change in temperature from T to T degrees is given by: r

Referring to Equation 6, it is clear that by a proper (a) L6=% (b) Ks (c) K6 With the relatively simple constructions above described, the effective length of the rods s5 may be appreciably changed independently of the position of the plunger to provide accurate compensation without changing the rod material and it is apparent that with tunable cavities of given physical characteristics, once the physical characteristics of the rods have been determined, production runs of such cavities may be made which will have the desired compensation.

As many changes could be made in the above equipment, and many apparently widely different embodiments of th s invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

l. A resonant cavity tunable over a broad band of frequencies, comprising a cylindrical sleeve having at least one end wall, a plunger slidaoly mounted in said sleeve, at least one rod secured at one end to said plunger and extending parallel to the longitudinal axis of said sleeve through said end wall, said rod having a thermal coefiicient of linear expansion differing from that of said sleeve to provide a predetermined variation of the position of said plunger in said sleeve with changes in temperature, a member secured to the outer surface of said end Wall adjacent said rod, a movable sleeve extending through said end wall and said member, said rod extending through said movable sleeve, means extending through said member to lock said movable sleeve in a predetermined position, and means extending through the outer end of said movable sleeve to lock said rod in a predetermined position, whereby the effective length of said rod will extend from said second locking means to said plunger.

2. A device of the character described tunable over a broad band of frequencies where the maximum frequency is many times the minimum frequency, comprising three coaxial metal sleeves, the outermost sleeve being shorter than the other two sleeves, said outermost sleeve and the second sleeve adjacent thereto defining an anode cavity, said second sleeve and the third sleeve defining a gridcathode cavity, means to tune said cavities, said tuning means for at least the anode cavity comprising a plunger slidably mounted in said cavity between the outermost sleeve and the second sleeve adjacent thereto, said anode cavity having an end wall, a member secured to the outer surface of said end wall, a movable sleeve extending through said end wall and said member, a rod secured. at one end to said plunger and extending parallel to the longitudinal axis of said anode cavity through said movable sleeve, means extending through said member to lock said movable sleeve in a predetermined position, means extending through the outer end of said movable sleeve to lock said rod in a predetermined position, whereby the effective'length of said rod vdl-l extend from said second locking means to said plunger, said rod having a thermal coefficient of linear expansion differing from that of said outermost sleeve to provide a predetermined variation of the position of said plunger in said anode cavity with changes in temperature, said second sleeve having an elongated longitudinal slot therethrough in the portion thereof extending beyond the outenrnost sleeve, a slidable member mounted on the portion of said second sleeve extending beyond the outermost sleeve, an input carried by said slidable member, a contact member electrically connected to said input, carried by said slidable member and insulated therefrom and from said second sleeve, said contact member extending through said slot and resiliently and slidably engaging said third sleeve to feed the input signal into the grid-cathode cavity, the lowest operating frequency of said device being determined by the effective maximum length of the anode cavity, the far end of the slot in the second sleeve being at a distance from the inner end of the second sleeve substantially equal to an odd multiple of one quarter Wave length which is at least the next larger odd multiple of the number of quarter Wave lengths used in the anode circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,109,880 DOW Mar. 1, 1938 2,456,770 Dearing Dec. 21, 1948 2,790,151 Riblet Apr. 23, 1957 2,918,636 Shapiro Dec. 22, 1959

Claims

1. A RESONANT CAVITY TUNABLE OVER A BROAD BAND OF FREQUENCIES, COMPRISING A CYLINDRICAL SLEEVE HAVING AT LEAST ONE END WALL, A PLUNGER SLIDABLY MOUNTED IN SAID SLEEVE, AT LEAST ONE ROD SECURED AT ONE END TO SAID PLUNGER AND EXTENDING PARALLEL TO THE LONGITUDINAL AXIS OF SAID SLEEVE THROUGH SAID END WALL, SAID ROD HAVING A THERMAL COEFFICIENT OF LINEAR EXPANSION DIFFERING FROM THAT OF SAID SLEEVE TO PROVIDE A PREDETERMINED VARIATION OF THE POSITION OF SAID PLUNGER IN SAID SLEEVE WITH CHANGES IN TEMPERATURE, A MEMBER SECURED TO THE OUTER SURFACE OF SAID END WALL ADJACENT SAID ROD, A MOVABLE SLEEVE EXTENDING THROUGH SAID END WALL AND SAID MEMBER, SAID ROD EXTENDING THROUGH SAID MOVABLE SLEEVE, MEANS EXTENDING THROUGH SAID MEMBER TO LOCK SAID MOVABLE SLEEVE IN A PREDETERMINED POSITION, AND MEANS EXTENDING THROUGH THE OUTER END OF SAID MOVABLE SLEEVE TO LOCK SAID ROD IN A PREDETERMINED POSITION, WHEREBY THE EFFECTIVE LENGTH OF SAID ROD WILL EXTEND FROM SAID SECOND LOCKING MEANS TO SAID PLUNGER.