US2561727A

US2561727A - Tuning of electrical resonators

Info

Publication number: US2561727A
Application number: US493778A
Authority: US
Inventors: Harold G Cooper; Howard L Schultz
Original assignee: Individual
Current assignee: Individual
Priority date: 1943-07-07
Filing date: 1943-07-07
Publication date: 1951-07-24
Anticipated expiration: 1968-07-24

Description

DIZHH'UH HUUH y 9 1951 H. G. COOPER ET AL 2,561,727

TUNING OF ELECTRICAL RESONATORS Filed July 7, 1943 E as E u E 52 55 t 52 ss 52 3| 3 5 HAROLD e. COOPER H UH HOWARD-L. SCHULTZ l abhor/wag Patented July 24, 17951 gutnvl l nvviu UNITED STATES PATENT OFFICE TUNING OF ELECTRICAL RESONATORS Harold G. Cooper, Belmont, and Howard L. Schultz, Wellesley, Mass, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application July 7, 1943, Serial No. 493,778

7 Claims. 1

This invention relate to apparatus for generating oscillations of very high frequency, especiallyoscillations of frequency of the order of 2500 mc./sec., 300 mc./sec. and higher. More particularly, the invention concerns arrangements for tuning resonator apparatus adapted for operation in' the said frequency range with vacuum tubes of the triode type, particularly vacuum tubes with close-spaced plane'elements constructed in a manner such as to bring the" grid connection outside the vacuum' tube to a ringshaped terminal surface.

For the production of stable oscillations in the above-mentioned frequency range with triode vacuum tubes, resonator apparatus has come'into use comprising three coaxial conductors, an out r cylinder connected effectively to the cathode, an inner cylinder connected effectively to the anode, and an intermediate cylinder connected to the grid. The outer and the inner cylinder electrically define a closed space and the intermediate cylinder, partially enclosin the inner cylinder in the neighborhood of the anode connection, acts as a partition in such space. The manner of operation of such resonator apparatus will be more fully explained below. Tuning of such resonator apparatus has heretofore been accomplished in various ways chiefly by introducing retractable or otherwise adjustable masses of metal into portions of the resonator having a relatively intense electric field. In genefi'al, such tuning arrangements have been limited in scope by the necessity of avoidin breakdown discharges in the resonator and it has been founddiflicult to obtain a sufiiciently wide range of vaigiation of the natural frequency of the device with any single adjustment.

It is an object of the present invention to provide improved tuning arrangements 'for resonators of the above type employed for the generation of electrical oscillations of very high frequency. 'It is a further object of the present invention to provide a resonant circuitstructure for ultra-high-frequency triode oscillators capable of j being tuned over a relatively wide range bythe: variation of a single adjustment. Other objects of this invention will be apparent from reading of, this specification.

, An oscillator circuit structure embodying the present inventionis shown in the annexed drawing, Fig. 1 being a longitudinal cross-section and Fig. 2 a transverse cross-section in the plane of the grid terminal. Inorder that the manner in which the tuning apparatus'functions may be clear, the mode of operation of the circuit struc- ,2 ture and the associated vacuum tube will be explained. 'I

In Fig. 1 the base'of a triode tube adapted for operation at the above-mentioned "frequency is shown at 5, the outer part of the base having a metal case or shell. "Although, the circuit structure or resonatoris shown in cross section. the vacuum tube is for conveniencej'of illustration shown in side elevation in Fi l. r,

The elements of the=vaciium tube are provided with their effective surfaces located in closely spaced parallelpl'ane (not shown). vThe grid electrode is continued outward from the grid plane towards the wall of the vacuum tube-to form a ring-shapedcontact surface on the outside of the tube intermediate the ends thereof, shown in the-drawing atG. The glass 'walls'of the tube appear at and'il, being cylindrical'in shape, the upper cylinder 8 having a smaller diameter than the cylinder 1, "a feature which has earned for this type of'tube the appellation or lighthouse tube. The plate connection of the tube is brought out .to a disk 9 at the upper end of the cylinder 8, the disk 9 carrying also acap or stud H! which serves as the anode connection.

of the tube. The cathode is connectedby a suitable by-pass capacitance to the metallic shell 5 which surrounds the base of the'tube, the direct connections to the cathode and its heater-being brought out to the base pins I l.

The resonator or tuner structure, likemost circuits for producing 'or' receiving oscillations at the frequency range in question, comprises a more or less developed form of cavity resonator having conducting surfaces of considerable ex tent, on which the oscillatory currents aredesigned to flow. The resonator i so connectedto the elements of the vacuum tube as to'prov'ide the necessary conditions for the generation of oscillations when the electrode voltages are provided in the usual .way.

The resonator structure comprises an outer tubular member l5 adapted to be connected to the base shell 5, which in turn is by-passed to the cathode, an inner cylindrical member l6 adapted to be connected to the anode connection Ill, which member may be. hollow if desired, and a cylinder or sleevegllj of; a diameter intermediate between the outerdiameter of the cylinder l6 and the inner diameter of the' tubular'member l5, usually less than the mean of those diameters, which is adapted to be 'mounted upon the grid terminal ring 6 and to be held in place thereon by one ormore screws l8, l8a threaded in the tubular member l5 (see Fig. 2). With the assistance of spring action resulting from slots 5I cut in the lower end of the sleeve II, a slight annular groove being cut on the inside of the slotted portion to allow the sleeve I1 to snap onto the terminal, the screws I8, I81; may provide an electrical connection between the sleeve I1 and the tubular member I5, in which case the apparatus is operated as a grounded-grid oscillator, or they may be mounted on insulating bushings in the member I5 which are adapted to isolate the screws I8, I8a electrically from the member I5 and thus to permit a grid bias or grid modulation to be applied to the grid of the vacuum tube by an electrical connection through one or both of the screws I8, I8a.

Apparently most important in the determination of the frequency of operation is the length of the sleeve I'l. Although the theoretical explanation of the operation of the apparatus in question is not as yet certain, it is believed that the sleeve I1 so divides the space between the cylinder I6 and the cylinder I5 as to provide a folded coaxial resonator one end of which is connected across the grid-plate space of the vacuum tube the other end of which is connected across the grid-cathode space of the vacuum tube. The upper end of the sleeve I1 is found to be at or very near to a voltage maximum of the resonator. In order to obtain the proper proportioning between the parts of the oscillating cavity, it is important to provide a small downward extension I9 of the cylinder II extending from the annular grid terminal 6 axially towards the base shell 5. For best results, especially for short-pulse intermittent operation, great care should be exercised in the proper design of the length of the extension I9. It appears to have some dependence upon the inter-electrode capacitances of the vacuum tube in question. For the type (EL-464 a length of about .02 wave length appears to be preferable for conditions of current short-pulse operation. For continuous operation the length of the overhang or downward extension is not particularly important. For the type (EL-464 tube and conditions of continuous operation, we prefer an overhang of about .05 wave length. In general, the overhang should be between 0 and 0.1 wave length.

It is possible that the length of the overhang extension I9 of the cylinder II, dimension a in the drawing, may exercise some control on the degree of feed-back coupling to the grid of the vacuum tube. It is to be noted, however, that great difficulties are to be encountered in the attempted explanation of oscillatory circuits of this character in terms of conventional circuit theory.

It has been observed that the electric field is particularly strong in the neighborhood of the upper end of the sleeve I1. This region is therefore adapted for the introduction of a loading capacitance the adjustment of which may be expected to have a tuning effect upon the system, and if desired an auxiliary tuning arrangement might be provided here in the form of a retractable machine screw or the like, but it is preferable to omit such arrangements because the tuning adjustment described below provided in accordance with this invention is fully effective for such tuning as is usually desired and the provision of additional tuning means would only complicate the calibration of the device.

It is also observed that an intense magnetic field exists near the base of the cylinder I! in the neighborhood of the plane of the grid terminal 6. This region of the structure is therefore appropriate for magnetic coupling to an outside circuit and accordingly the loop 23 and the coupling transmission line 24 are provided in this part of the structure as shown on the drawing. The coupling line 24 is of the coaxial conductor type having an inner conductor 25 and an outer conductor 26.

Electrostatic coupling may be provided instead by the use of a probe extending into the resonator cavity, and indeed this may be located at the same position as the loop 23, such probe being connected to the inner conductor 25 instead of the loop 23. The penetration of the probe into the cavity may be made adjustable. Although the electric field is not particularly intense at this location, this coupling is of a desirable type because of the fact that the

line

25, 26 will normally present a low impedance.

The circumferential position of the screws I8 and I8a has been known to exert an important influence upon the reliability and power obtainable in the operation of the device. A preferred location of these screws is shown in Fig. 2, which is a cross section of the apparatus of Fig. 1 in the plane of the grid. The grid terminal appears at 6, the grid mesh being shown diagrammatically at 55. The slotted portion of the cylinder I1 is shown fitted snugly about the rim of the grid terminal 6. Two of the slots are omitted in order not to interfere with the screws I8 and I8a. The points of the machine screws I8 and I8a are preferably engaged in slight conical depression in the cylinder I1, thus providing for firm contact and good mechanical conditioning.

If only one screw is to be used, the screw I8a should be omitted and the remaining screw should be located at an angular or circumferential distance from the coupling loop 23 of the order of magnitude shown in Fig. 2 (some variation being possible without appreciable change in result). Although apparatus of the type described can successfully be operated with only a single screw, such as the screw I8, we prefer the use of two screws, as shown, because of superior performance of the device when so constructed. When two screws are used, the screw I8 should be located approximately as above mentioned and the screw I8a should be located at a circumferential distance of approximately degrees from the screw I8. The exact reason why the particular arrangement of the screw or screws connecting with the cylinder I'I should be important is not known, but it is believed that some sort of an auxiliary circuit having reactive effect, perhaps having some unexplained magnetic coupling with the oscillating field of the resonator, may be con: stituted when one or more screws are introduced and brought into contact with the cylinder IT.

The length of the sleeve II, exclusive of the extension I9, as shown by the dimension D in the drawings should be from about 0.25 to about 0.4 wave length, a length of about 0.34 wave length being preferred for continuous operation with the type Gil-446 tube and a length of about 0.37 wave length being preferred for pulsed operation with the type (EL-464 tube. The variation between continuous and pulsed" operation will be pointed out more fully below.

The diameter of the cylindrical elements of the structure of the present invention does not have much effect on the frequency, but has some effect on the stability and power of the oscillations. Preferably the mean diameter of the cylinder I1 is approximately 1%; times the outer diameter of the cylinder I6 or a little larger, and at the same time about /3 of the inner diameter of the cylinder I5, or a little less. It is convenient to provide the outer diameter of the cylinder I6 approximately equal to the outer diameter of the terminal disk 9 and to provide the inner diameter of the cylinder I! at approximately the value of the outer diameter of the terminal ring 6. In order that the cylinder I! may be properly aligned on the ring 6 it is provided with a thickened portion 28 which is adapted to engage the edge of the terminal structure and with a slight annular groove (not clearly shown), immediately below the thickened portion, to provide a snap-on effect in cooperation with the slots 5 I.

- The inner diameter of the tubular member I5 is preferably substantially larger than the outer diameter of the metal shell 5, which is bypassed to the cathode of the vacuum tube. Accordingly, an end structure 29 is provided for mounting the tubular member I5 on the base shell 5. The structure 29 preferably carries a clamp ring 38 and a clamping screw 3I and the upper surface of the structure 29 is preferably rounded, as at 32, since it is believed that discontinuities in this region, such as might be provided by sharp corners, ought to be avoided.

The upper end of the resonator is effectively closed off by means of the structure 35 mounted on the sleeve I6. The structure 35 is adapted to furnish a radio-frequency short circuit between the cylinder I6 and the cylinder I5. An actual short circuit is undesirable, both because it is desired to adjust the position of the cylinder I6 and because the cylinder I6, being connected with the anode of the vacuum tube, is at high potential with respect to ground, whereas it is desirable to keep the outer parts of the structure, such as the cylinder I5, at ground potential. The cylinder I6 is insulated from the cylinder I5 by means of the insulating end structure 31. The annular structure 35 effectively provides a short circuit at radio frequencies in the range of the frequencies of operation. The length of the annular cylindrical space between the structure 35 and the cylinder I5 is made to be an electrical quarter-wave length, i. e., approximately a quarof the free-space wave length for a frequency in the middle of the desired frequency range of operation, but slightly shorter on account of end effects. The Space enclosed between the structure 35 and the cylinder I6 will likewise have an axial length of an electrical quarter-wave length. Consequently, when oscillations tend to appear across the gap 36, a high impedance will appear at the upper end of the structure 35 and consequently a verylow impedance will appear at the lower end of the structure 36. Since the clearance between the cylindrical part of the structure 35 and the cylinder I5 is much smaller than the clearance between the cylindrical part of the structure 35 and the cylinder I6, this low impedance effect at the gap 36 will take place over a range of frequencies in the neighborhood of that for which the axial length of the structure 35 is an electrical quarter-wave length.

The desired position of the structure 35 with respect to the upper end of the cylinder II has a relatively small effect, compared with the tuning effect described below, upon the frequency of the oscillation. If the structure 35 is too close or too far away from the end of the cylinder I1, the effect is essentially to introduce a discontinuity in the folded resonant transmission line, regarding the resonator as a. transmission line folded around the upper end of the cylinder I1, and such discontinuity may have to be kept within reasonable limits for proper operation of the resonator, it being understood that when the resonator operates so that this type of discontinuity is at a voltage maximum, the discontinuity has relatively little effect. The desired position of the structure 35 is best determined experimentally. With the structure 35 fixed on the cylinder I6 at a suitable position the small tuning efiect produced by moving the structure 35 is added to that tuning effect described below operated by moving the cylinder I5, and thus serves to extend slightly the tuning range. We find that for apparatus constructed as shown in the drawing, the distance between the lower end of the structure 35 and the lower end of the cylinder I6, as indicated by the dimension 0 in the drawing, should for best results lie between about 0.4 and 0.7 wave length. In apparatus designed for continuous operation with a vacuum tube of the type GL- 446, we prefer to provide this length with a value of about 0.45 wave length, and for apparatus designed for recurrent short-pulse operation with a vacuum tube of the type GL-464, we prefer a length of about 0.61 wave length.

The relatively wide range tunin of the device shown in the drawing results from the provision of an adjustable sliding anode connection betw'ee rf'the-l-ower end oftlijefc'ylindr l 6" and the anode stud"'IOI"'Whave found thatwhen the cylinder I6 is slid axially on the anode cap II) a remarkably strong effect on the resonant frequency of the device develops, through which it is possible to vary the said resonant frequency over a considerable range. With anode studs of normal size, such as about one-quarter of an inch in height, variations in wave lengths over a range of 12 mm. in the neighborhood of 10 cm. wave lengths have been found possible and it is believed that even greater variation could be achieved with longer anode studs or with other minor variations in the size and shape of the constituent .parts. In this tuning effect only about 2 mm. wave length change is contributed by the movement of the structure 35. The major tuning effect is believed to result from the unusual configuration of the electric field in the neighborhood of the gap between the lower end of the cylindrical structure I6 and the annular plug 38 on one hand and the annular terminal 9 and the stud II] on the other hand. Over a great deal of the tuning range the change in wave length is almost a linear function of the axial position of the cylinder I6.

In order that the tuning effect may be sufliciently strong to provide wide range tuning, the cylinder I6 should be considerably greater in diameter than the anode stud Ill. An annular plug 38 may conveniently be provided to engage the cylindrical surface of the anode cap II]. In order to provide a strong tuning effect, the annular gap or channel between the lower face of the plug 38 and the terminal surface 9 should have a considerable radial depth. On the other hand, the diameter of the anode cap III should not be so greatly reduced as to impede heat transfer between the cap I0 and the plug 38, since it is desired to conduct heat away from the anode through the cap I0, the plug 38 and the cylinder I5.

An important practical measure in the construction of the tuning mechanism is the reduction of the thickness of the cylinder I6 as atoms? shown at 52 and the provision of the slots 53, which also cut the plug 38 into segments. This t pe of construction of the lower end of the cylinder I6 provides a secure spring-pressed electrical contact together with sufficient mechanical flexibility to avoid breaking of the vacuum tube as a result of repeated manipulations of the cylinder I6 for purposes of tuning.

, The tuning arrangement shown in the drawing for tuning the resonator of a triode oscillator device is believed to constitute a novel tuning adjustment for cavity type resonators, and especially for coaxial conductor type resonators. In general, the principle of this arrangement provides a useful way of modifying the electrical properties of a coaxial conductor transmission line.

Although the tuning effect of axially moving the cylinder l6 with respect to the anode stud I is very considerable, this adjustment exercises relatively little influence upon the efiiciency or stability of the oscillating circuit, so that this tuning adjustment is especially useful in practice. Unlike certain other types of tunin adjustments, the tuning adjustment resulting from the axial movability of the cylinder [6 does not vary the voltage breakdown characteristics of the device. and does not reduce the clearances in the direction of the electric vector in the course of the adjustment.

In order that the above-described unusual tuning properties may be conveniently availed of, the cylinder I6 is threaded at its upper end and engaged in bushing 4| which is suitably embedded in the insulated terminating structure 31. The position of the cylinder l6 may be adjusted by turning a knob 42 mounted on the end of the cylinder I6 and preferably made of insulating material. The knob 42 is provided with an axial opening 43 through which a connection, preferably insulated, may be introduced for electrical connections to the anode of the vacuum tube. If the cylinder I6 is made hollow, as shown in the drawing, means for cooling the anode might be introduced therein, if desired. In general the cylinder It will be made of copper, which has a suflicient heat conductivity to make special cooling means unnecessary. Especially for the type (EL-464 tube, however, the connection at the lower end of the cylinder [6 to the anode stud l0 should be made in such a manner as to provide good heat conducting connections as well as good electrical connections. The cylinders l5, l5, and I! and their associated conducting structures are preferably made of copper or brass, and may be silver-plated on the surfaces carrying radio-frequency currents in order to decrease losses. For good heat conductivity the central hole usually provided in the cylinder IS in order to save weight should not be too great in diameter, nor should the reduction of the tubular wall shown at 52 be carried any farther than necessary for mechanical reasons.

A knurled nut 44 of insulating material is provided near the upper end of the conductor l6 for maintaining any desired axial adjustment of the cylinder l6. Locking pairs of

nuts

46, 41 and 48, 49 are also provided on the cylinder l6 for limiting the extent of the axial movement of the cylinder I6 so that it cannot be pushed downwardly so far as to exert undue pressure upon the disk 9 and the structure of the vacuum tube, and so that it cannot normally be retracted upward far enough to'lose contact with the anode stud l0.

A method which might be used for auxiliary variation of the operating frequency of the structure illustrated in the drawing consists in providing solid dielectric material in part of the space between the cylinder I1 and the cylinder; l6. Thus a transverse annular mass of polystyrene located in said space, preferably near its upper end, will vary the operating frequency on account of its presence by virtue of the fact that such dielectric material changes the electrical length of the coaxial line formed by the cylinders l6 and IT. The effect of the dielectric mass, moreover, varies with its axial position, probably because the intensity of oscillating electric field varies axially in the space between the cylinders l6 and II. It is therefore possible to obtain some variation of the operating frequency by axially moving in the said location an annular piece of dielectric material, such as polystyrene or the like. Because of the greater mechanical inconvenience of such an arrangement this method of varying the frequency is believed to have a limited applicability in practice but it may be useful as a factory adjust:- ment for setting the mid-tuning range frequency of the device at a desired value for a device intended to be tuned by means of a sliding anode connection.

It is to be understood that the principles of the sliding anode connection tuning arrange-' ment may be applied in a device of the type shown in the drawing by means of a sliding joint or other variable annular groove, a gap or channel constituted some other way than by a slid-.- ing anode connection. Thus the sliding joint might be provided between two portions of the cylinder It in the neighborhood of the anode stud In, an arrangement which might reduce the mechanical strain on the structure of the vacuum tube. It is believed to be desirable to provide the variable discontinuity in the resonator at positions where considerable current flows, although it is not necessary to provide it exactly at a location of current maximum, since it is likely that the tuning effect is due in a large measure to the current flowing around the groove or gap. It will be noted that the groove or channel is oriented transversely to the direction of current fiow.

With oscillator apparatus such as that herein described stable oscillations tunable over a considerable range..ma .beiobtained by providing the usual anode voltage to the circuit and providing thecathode with the desired heating potential. The "cathode terminal, which is brought out of the vacuum tube through one of the pins H may be connected directly to ground or it may be provided with a suitable biasing voltage. The oscillator may conveniently be modulated either by varying the plate voltage or by varying the grid voltage, the grid voltage being readily variable, as previously indicated, by providing the screw l8 with an insulating mount and connecting the desired grid voltage to the screw I8. Cathode modulation may also be accomplished by introducing a modulating voltage between the cathode and ground. The oscillator may also be operated to produce high intensity pulses of short duration in response to transient voltages impressed upon the circuit in any of the above modulating methods. It is to be noted, however, that for a given circuit ar rangement and a given vacuum tube the frequency of continuous operation differs from that of the oscillations occurring in high-intensity short-pulse operation, the latter frequency generally being lower. In consequence it is desirable to take this feature into account in designing the apparatus of this invention in order that this manner of behavior of the circuit may be compensated for by the design of the resonating structure.

In the above-mentioned preferred values for the length of the sleeve I1 and other dimensions of the apparatus of the present invention, those given for the type (EL-446, which tube is practicularly suitable for continuous operation, having somewhat less power output than the GL-464 and being thereby more suitable for use as a local oscillator in a superheterodyne receiver, were the dimensions referred to the wave length desired for continuous wave operation. The preferred dimensions, given for use in connection with the type (EL-464 tube, however, were referred to the wave length obtained when operation is confined to one microsecond pulse approximately 500 microseconds apart from each other. This factor accounts for the major part of the lengthening of the sleeve l1 and perhaps also for the displacement of the structure 35. The reactive loading effect of the tube on the resonator apparently is different for the two types of operation. The wave length dimensions given above refer to fractions of the so-called free space wave length for the frequency in question.

The displacement of the natural frequency of the apparatus resulting from operation in which the oscillations are produced in the form of one microsecond pulses at a recurrent rate of 2000 per second is approximately of the order of about "20 per cent toward lower frequency and reater wave length over the frequency obtained with the same resonator structure under continuous oscillation. By introducing this proportionality factor, ;the preferred dimensions for the apparatus of-the present invention for continuous oscillation in association with a type (EL-464 tube and for intermittent oscillation in association a type GIL-446 tube may readily be estimated.

What we desire to claim and obtain by Letters Patent is:

1. An oscillator for high frequencies including a three-element vacuum tube, a tunable circuit structure which includes a first cylindrical conductor slidably connected to the anode of said vacuum tube, a second cylindrical conductor substantially coaxial with and partially enclosing said first cylindrical conductor connected effectively to the cathode of said vacuum tube, a third cylindrical conductor substantially coaxial with the aforesaid conductors and located therebetween, said third conductor being connected near one end to the grid of said vacuum tube and extorimeans connected to said first cylindrical conductor for adjusting the axial position of said first cylindrical conductor relative to said second cylindrical conductor and relative also to said anode, whereby the configuration of an oscillating electrical field in the neighborhood of said 10 anode is changed, thereby varying the frequency of operation.

2. An oscillator for high frequencies including a three-element vacuum tube, a tunable circuit structure which includes first and second coaxial cylindrical conductors connected respectively to the anode and cathode of said vacuum tube, a third coaxial cylindrical conductor intermediate said first and second cylindrical conductors of a length between about 0.25 and about 0.4 wave length at a desired frequency of operation and connected at a small distance, not greater than 0.1 wave length at said frequencies from one end to the grid of said vacuum tube, also having at its other end a substantially open-circuited .terurination, and means connected to said first cylindrical conductor for adjusting the axial position thereof said adjusting means being operable to provide an annular groove of variable axial dimension in the immediate neighborhood of said anode thereby to cause tuning of said circuit structure. 1

3. An oscillator for high frequencies including a three-element vacuum tube, a tunable circuit structure which includes a first cylindrical conductor connected by an axially slidable joint to the anode of said vacuum tube, said joint being adapted to define a variable width annular groove, a second cylindrical conductor substantially coaxial with and partially enclosing said first cylindrical conductor, said second conductor being connected eifectively to the cathode of said vaccuum tube, a third cylindrical conductor substantially coaxial with said first and second conductors and located therebetween, said third conductor being connected near one end to the grid of said vacuum tube, two substantially radial metallic rod-like connections to said third conductor located near the grid of said vacuum tube at a circumferential separation of approximately from each other, said end of said third conductor farthest from said grid extending over, but spaced from, a portion of said first cylindrical conductor and extending to a substantially opencircuited termination, said first and second conductors being effectively joined together for the frequency of operation by a conducting structure located between said conductors at a point'a substantial distance beyond the said open-circuited end of said additional conductor, means connected to said first conductor for axially adjusting the position of said first cylindrical conductor relative to said second conductor and relative also to said anode, whereby said axially slidable connection may be operated and the frequency of operation thereby varied.

4. An oscillator for high frequencies including a multi-element vacuum tube, a, tunable circuit structure which includes substantially coaxial cylindrical conductors connected respectively to the anode and cathode of said vacuum tube, a third coaxial cylindrical conductor intermediate said first-named cylindrical conductors connected near one end to the grid of said vacuum tube and having at its other end a substantially opencircuited termination, and means for tuning said structure comprising an axially slidable connection between one of said cylindrical conductors and said anode for varying the configuration of the oscillator field in the neighborhood of said anode connection and means connected to that one of said first-named cylindrical conductors which is connected to said anode for adjusting the axial position thereof.

5. A generator of high-frequency electrical oscillations comprising, a multi-element vacuum tube of the lighthouse type having external ring anode, cathode, and grid connections, first, second, and third conducting surfaces of revolution respectively connected to said anode, cathode and grid, said first conducting surface being slid ably connected to said anode connection to provide an annular channel of variable axial dimension in the immediate neighborhood of said anode connection, and means connected to said first conducting surface for adjusting the axial position of said first conducting surface relative to said anode connection, thereby controlling said variable axial dimension.

6. Apparatus for tuning an oscillator of. the type which includes a multi-element vacuum tube and first, second and third coaxially disposed conducting cylinders respectively connected to the anode, cathode, and grid of said tube, said apparatus comprising a conducting annular plug inserted in one end 'of said first cylinder and contacting said anode, and means connected to said first cylinder for adjusting the axial position of said first cylinder relative to said anode,

thereby providing an annular cavity of adjustable axial dimensions surrounding said anode.

7. High frequency apparatus including a multielement vacuum tube, first, second and third coaxial cylindrical conductors connected respectively to the anode, gridand cathode of-said vacuum tube, said first conducting cylinder being slidably connected to said anode to provide an annular cavity of variable axial dimension around said anode, and means connected to said first cylinder for adjusting the axial position thereof relative to said anode thereby controlling said variable axial dimension and the frequency of said apparatus.

HAROLD G. COOPER.

HOWARD L. SCHULTZ.

REFERENCES 'crmn The followingreferences are of record in the file of this patent:

" UNITED STATES PATENTS Gurewitsch June 22, 1948 Certificate of Correction Patent No. 2,561,721 July 24, 1951 G. COOPER ET AL. I It. is hereby certified. that error appears in the printed Specification of the above numbered potent requiring correction us'followsz Column 1, line 4, for "800 mc./soc. read 8000 vim/um; column 8, line 83, before gap strike out 0,; column 9,1mo 65, for benig rend being; column 10,1ine 33, for vaoouum road vacuum; and that the said Letters Pubout should be read as corrected above, so that the some may conform to tho-record of the cage in tho Patent Oflice.

Signed and sealed this12tb day of August, A. D. 1952.--

moms F. MURPHY,

Assistant aamminibpr of Pmm.