US2501052A

US2501052A - High-frequency transmission system

Info

Publication number: US2501052A
Application number: US616364A
Authority: US
Inventors: Melvin A Herlin
Original assignee: US SEC WAR
Current assignee: United States, WAR, Secretary of; US SEC WAR
Priority date: 1945-09-14
Filing date: 1945-09-14
Publication date: 1950-03-21
Anticipated expiration: 1967-03-21

Description

March 21, 1950 M. A. HERLIN HIGH-FREQUENCY TRANSMISSION SYSTEM Filed Sept. 14, 1945 2 Sheets-Sheet l 32 LIN DISSIPATIVE LOAD 36 CAVITY 30 MAGNETRON STEP DOWN TRANSFORMER INVENTOR.

MELVIN A. HERLIN ATTORNEY ?aiented Mare 21, 1959 STATE Application September 14, 1945, Serial No. v616,364

H E o 6 filaims. i This invention relates in general to magnetrons and more particularly to means for stabilizing the operating frequency thereof, especially with regard to plural cavity magnetrons.

ments in frequency constancy with regard to changes in operating conditions. Any magnetron is stable to some degree because there is associated with it a resonant circuit which stores elec- The conditions which affect the magnetron r; tlical energy at its resonant q cy- It c operating frequency include load conditions, term be shown that the degree to which a sys m tends perature of th magnetron elements, d to operate near this frequency is a function of the pushing amount of energy stored in the resonant circuit.

The effect of load conditions on magnetron I p v f fr q n y constancy, r f r operating frequency may be expressed in terms in may be looked for from a device Which Stores of the pulling figure (see Microwave Magnetrons, additional en r y a the desired frequency of Collins, pp. 182-3, 642-3, 770-1). One definition p r i n. Wi h r rd o h yp f magnetron for pulling figure is the value of the maximum vi ts anode Structure Comprising a number change of frequency experienced by a magnetron of cavity resonators, which shall be called a feeding an output line having a voltage standing l5 p a Cavity magnetron, the device usually takes Wave ratio of 1.5 due to a change in load impedhe m Of a e t Cav ty coup in 801118 ance sufficient to shift the standing Wave 360 a ner to the C cu t 1 the magnetronelectrical degrees along the line. Reduction in In the discussion of stabilization, i i5 @011- pulling figure may be obtained by decoupling venient to define the stabilization factor S: the output line from the magnetron to a greater t t 1 d extent. This procedure is undesirable in that o energy Store m System the circuit efficiency is reduced in this operation energy stored m unaltered magnetron and, further, that the electronic efiiciency of the hai; frequency changes due to varying operating magnetron falls off as the load is reducedconditions are reduced by this factor (provided he e f c f temperature on magnetron Operthe frequency of the stabilizing cavity is not ating frequency is Ordinarily given by multiplying affected by these changes) can be seen from the magnetron fr q n y by the c ffi of Equation2in the book by E. A. Guillamin entitled thermal expansion of the material of which the Communication t k volume 11 page 229, magnetron resonant members are constructed, and t explanation connected therewith the result being, Say, the number of megacycles Among the objects of the present invention, per degree of temperature shift. The severity therefore with which the pulling figure affects the magne- To provide a magnetron m t for use tron frequency as compared to the effect of tem- 1; extremely high frequencies; perature depends in the main on the operating To provide Such an t t circuit far t frequency magnetron that a high degree of stabilization Pushing, which is the effect of the parameters may be obtained; and within the magnetron on the operating frequency, 3 To provide means in the output circuit for depends upon the indivldual magnetron In lessening the effects of undesirable modes of oscilgeneral, the effect of pushing is quite small with lation of t magnetron respect to the effects of pulling and temperature 0 In accordance with the present invention there Changeis provided a plural cavity magnetron which is Another dlfficulty encountered 111 e h e a tightly coupled to a high-Q cavity resonator magnetron at predetermined Operatm? through a wave guide, the cavity being connected quency is the tendency for such a tube to osc1llate to t branch line of a Wave guide T junction in Gthel modes than the desired- That A dissipative load consisting of a length of a post due to the large number of degrees of freedom of powdered iron dispersed in a ceramic, which that a p al c v y magnetron possess, it is 1 substance will be referred to as polyiron, is placed sible for the electric and magnetic fields within substantially parallel to the electric field in the the cat d Space to assume Several differwave guide and off center therefrom in the branch ent configurations. Each of these field patterns wave guide. A capacitive conducting post is corresponds to a different frequency of oscillaplaced adjacent to the polyiron post to balance tion of the magnetron. These configurations are out the susceptance effects thereof. This polycalled modes of oscillation or, more simply, modes. iron post is so placed that undesired modes of The term stabilization, as used in connection the magnetron will be appreciably loaded down With magnetron operation, refers to improveby it, so that these modes have extreme dilficulty building up. The desired mode, however, will be virtually unaffected.

A line stretcher is placed in the main wave guide, to which the magnetron is coupled, to correct for small variances in the effective length of the line connecting the magnetron and the cavity resonator which may occur during manufacture. In order not to load the magnetron too heavily, a capacitive screw is placed in th portion of the main wave guide to which the load is connected, in order to increase the mismatch looking toward the load at this point. The line stretcher, capacitive screw and inductive post are more fully explained later in this specification.

This invention will best be undersood by reference to the drawings, in which:

Fig. 1 is an illustration of the magnetron and stabilization means according to the present invention; and t t Fig. 2 is an approximate equivalent circuit diagram of the device of Fig. 1. c, t

Fig. 3 is a diagrammatic section in plan, of the wave guide T-section and stabilizing cavity resonator taken on the line B- -B Fig. 4.

Fig. 4 is a diagrammatic section of the main wave guide and magnetron anode block in elevation, taken along the line A--A Fig. 3.

Fig. 5 is a sectional View of the main wave guide l4 taken through elements l! and 18 on the line D-D Fig. 3. M v N Fig. 6 is a sectional Vi$\Y of the branch wave guide 26 taken through

elements

22 and 23 on the line C- -C Fig. 3. I

Referring now to adescription of the invention and to the drawings, Fig. 1 shows a plural cavit magnetron in of a type of construction sometimes called the fbathtu construction, because of the shape of the magnetron case (see Microwave Magnetrons, Collins, pages 770-4, Figs. 19.37 and 19.38). The resonant cavities are positioned substantially symmetrically with respect to the 'bathtub shaped outer case of the magnetron unit, with the long a'xes'of the cavities being essentially perpendicular to the long face H of the case as indicateddiagrammatically -at 4 I, Fig. 4. The two cathode leads, not shown,are brought into the magnetron through the unit 2. The magnetron output is coupled to one of 'thecavity resonators bya coupling loop ill Fig. 4 formed from the center conductor 43 of a coaxial line 13 which passes through the anode block 42 of the magnetron, the outer conductor 13 of which maybe 'seen in Fig. l. Thisoutput energy is in turn coupled into a wave guide l4 by causing the center conductor 53 of the coaxial line to extendbeyond the outer conductor i3 and to form a probe 44 Fig. 5, which is positioned substantially parallel tothe electric field E Fig. 4 inside the wave guide, thelguide preferably operating in the TEo,1 mode. The electric field is thus substantially parallel to one end plate i9 of the guide. Two permanent magnet pole pieces i5 and I6 are shown providing the longitudinal magnetic field necessary for the operation of the magnetron.

The susceptance looking into the wave guide hi from the output end 21 as shown in Fig. 6 is made variable by inserting an inductive conducting post I! and a capacitive tuning screw is inside the guide !4 and at essentially the same cross section therein. The post I! is a small diameter vconducting post which is firmly welded to the top and to the bottom oi the wave guide it, and whose axis is substantiall parallel to the electric field inside the guide. The screw 18 makes very good contact with the top of the guide, and the distance of the bottom face of the screw from the bottom wall of the guide may be varied by turning the screw.

The function of the conducting post i"? is to create an increased effective inductance at the point in the wave guide at which the post is placed. Currents flow through such a post from one plate of the wave guide to the other, thus affecting the main magnetic held in the guide, which field is substantially perpendicular to the post, in such a manner that the effective inductance at this point is increased.

The function of the screw I8 is to create an increased effective capacitance at the point in the wave guide at which the screw is placed. The efiect is to increase the electric field in the guide at this point due to the shortening of the electric lines, thus increasing the eifective capacitance.

A branch wave guide couples the main wave guide to a high-Q cavity resonator 2! by means of a wave guide T-junction, the efiective length of line from the magnetron to the cavity resonator being made substantially equal to an integral number of half wavelengths at the operating frequency of the system. The branch guide 20 is butted against the main guide It in such a manner that corresponding faces of both guides are substantially parallel. The coupling hole cut in the side of the main guide I is of the same area as the area of the inside of a section cut perpendicular to the long axis of the branch guide 20. A large wave guide iris, 25 Figs. 3 and 4, c0uples the branch guide 2% to the stabilizing cavity resonator 2!. An iris is a small hole through which the electromagnetic energy 'must pass, the geometry of the hole determining the magnitude and phase angle of the effective impedance presented by the iris. The load 22 which damps out the undesirable modes is placed in the branch wave guide 20. This load 22 is a post of material of powdered iron dispersed in a ceramic. When used for dissipating high-frequency electromagnetic energy in this manner, this substance is called polyiron. This post 22 is placed on. center in the guide 29 substantiall parallel to the electric field therein, and is firmly connected to the top and the bottom of the guide. Along the same cross section of the branch guide as the polyiron post 22 is a conducting capacitive post 23, which is connected to either the top or the bottom of the guide and extends a portion of the way between top and bottom. An arrangement for varying the volume of the cavit resonator 2!, and thus its resonant frequency, is by means of a rod 25 and screw 26, which is made to vary the position of the end 26 of the cavity. 7 The output of the system is taken from the end 2? of the main wave guide It. A capacitive screw 28, similar to screw I8, is placed in the main wave guide in substantially the position shown.

An approximate equivalent circuit diagram of the apparatus of Fig. 1 is shown in Fig. 2. The magnetron is represented here as a parallel reso" nant circuit consisting of inductance iii": and capacitance 3 i. The wave guide line connectin the cavity resonator and the magnetron is repre sented here as a series resonant circuit of variable capacitance 32 and inductance 33, the line being substantially an integral number of half-wavelengths long. The resistance 34 represents the efiect of the polyiron post 22. The'stabilizing cavity resonator is represented by the parallel resonant circuit of inductanceEE and capacitance of the line connecting the magnetron and the cavity be an integral number of half-wavelengths at the frequency in. The desired mode in, therefore, causes the line to act as a comparatively small impedance and the cavity to act as a com-.

paratively large impedance, both of these impedances being resistances under hypothetical ideal operating conditions. Since the resistance of the dissipative load 34 is small compared to the impedance offered by the cavity at resonance, very little voltage is available across the load 34 to dissipate energy therein, and a large amount of the power leaving the magnetron is coupled to the main load 31, this power being cut down somewhat by the deliberate mismatch caused by capacitance 38. It can furthermore be seen that the magnetron may be tuned by varying the resonant frequency of the cavity.

Electromagnetic waves of a frequency difiering from f0 cause the wave guide line to act as a higher impedance than in the previous case, and cause the cavity resonator to act as a lower impedance. There is thus a lower value of voltage built up across the branch containing load 31, and of this voltage, a comparatively large fraction appears across the dissipative load 34. This undesired mode therefore is loaded down, or damped, by the polyiron post 22. The reason this stabilizing means can be applied only to comparatively low power magnetrons is that the dissipative elements are positioned within the wave guide, thus allowing only a limited amount of heat dissipation due to the confined space present.

The output coupling loop 40 of the magnetron is made comparatively large, in order that the coupling to the cavity resonator be tight. The lengths of the branch wave guide 20 and the main wave guide 14 to the T-junction are such that the load is coupled to a point in the wave guide where the cavity resonator and magnetron appear as effective parallel resonant circuits in parallel.

The lossy polyiron element is placed at a voltage minimum or nodal point for optimum effect. The polyiron post 22 presents an inductive susceptance to the energy flow down the branch guide 20, so a capacitive susceptance conducting post 23 is used at the same cross section to balance out this inductive susceptance. Since the polyiron is in the role of a conductance across the wave guide, it is placed in a position where the magnetron and cavity appear as series resonant circuits in series to provide the proper mode damping action. This position is substantially a quarter-wave-length from the T- junction.

It is desirable to tightly couple the stabilizing cavity to the magnetron so that the former has the optimum effect, but this tends to load down the magnetron considerably. The capacitive screw 23 therefore, which acts as a load step-down transformer, is necessary in order not to load down the magnetron too heavily. At the same time, use is made of this tuning screw to balance out an undesirable susceptance characteristic of the T-J'unction.

Due to variances in magnetron production, it

is difficult to predict the effective length of theline connecting the magnetron and the cavity when the two are assembled in the apparatus. To cancel out these efiective line length changes which may occur, the line length compensating susceptances I1 and I8 are used. The inductive post I! is fixed, and the geometry of the capacitive screw I8 is such that inductive and capacitive resultant susceptances may be provided over the range of the turning of the screw I8. Since a change in line length results in a change in the value and phase of the susceptance the line presents to the circuits coupled thereto, this variable susceptance unit is a means of varying the effective line length. It is sometimes called a line stretcher.

Thus there is provided a highly flexible stabilized magnetron for low power use. understood that this invention is not limited to use with the type of magnetron or to the design of circuit shown. It may be applied to any type of low power magnetron, the only limitation on the circuit design being that the apparatus adheres to the basic principles set forth.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A stabilized magnetron circuit comprising a plural cavity magnetron, a variable frequency stabilizing cavity resonator a wave guide T- junction section closed at one end connecting said cavity magnetron to said cavity resonator, a coaxial line connecting one of the cavities of said magnetron to the closed end of said wave guide, probe, probe iris means connecting said stabilizin cavity resonator to a branch of said wave guide T-J'unction, a polyiron energy absorbing post positioned unsymmetrically in said branch guide a capacitive conducting post positioned in substantially the same cross sectional plane of said branch guide as said polyiron post for substantially balancing out the inductive susceptance of polyiron post, said cross sectional plane positioned at a positioned node at normal frequency whereby said polyiron rod absorbs electromagnetic energy of frequencies differing from the operating frequency of said magnetron, a variable impedance positioned in said main wave guide comprising a conducting inductive post and a first capacitive variable screw in substantially the same cross sectional plane of the wave guide as said inductive post, and a second capacitive variable screw placed in the output end of said main wave guide to provide a mismatch in the wave guide beyond the branch of said T-junction.

2. A stabilized magnetron circuit comprising a plural cavity magnetron, a cavity resonator, a wave guide connecting said magnetron to said cavity resonator, a polyiron energy absorbing post mounted at a potential node at the operating frequency in said wave guide, a capacitive conducting post positioned in substantially the same cross sectional plane in said wave guide as said polyiron post for effectively cancelling out the inductive susceptance of said polyiron post, a variable impedance comprising an inductive post positioned in said wave guide, and a It is to be capacitive variable screw positioned in substan tially the same cross sectional plane in said wave guide as said inductive post, and-wave guidemeans for coupling the output of said magnetron circuit into a load.

3. A stabilized magnetron circuit comprising a plural cavity magnetron, a cavity resonator, a

wave guide connecting said magnetron to said cavity resonator, a polyiron dissipative element mounted in said Wave guideat a potentialnode at operating frequency, whereby said polyironelectromagnetic dissipative element absorbs energy of frequencies difiering from the operating frequency of said magnetron, and wave guide: means for coupling the output of said magnetron.

circuit into a load.

4. A stabilized magnetron circuit comprising a plural cavity, magnetron, a cavity resonator, a

wave guide connecting said magnetron-to said cavity resonator, dissipative load means mounted at a potential node in said Wave guidemeans for coupling the output of said magnetron circuit into a load.

5. A stabilized vacuum tube oscillator circuit,

comprising a vacuum tube oscillator, a cavity resonator, a wave guide connecting said oscillator to said cavity resonator, dissipative load means mounted at a potential node in said wave 0 guide connection to said. cavity resonator, whereby-said dissipative load partially absorbs electromagnetic energy of frequencies differing from the operating frequency of said oscillator, and wave guide means for coupling the output of said oscillator circuit into a load.

6. A stabilized magnetron circuit comprising a magnetron, a cavity resonator with a wave guide connection having a potential node constituting an efiective parallel resonant circuit, an energy dissipative means located at said node, a wave guide connecting said magnetron to said effective parallel resonant circuit, whereby said dissipative means absorbs electromagnetic energy associated with frequencies difiering from the operating frequency of said magnetron, and wave guide transmission means for coupling the output of said magnetron circuit into a load.

MELVIN A. HERLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,088,749 King Aug. 3, 1937 2,153,131 Bohme Apr. 4, 1939 2,406,402 Ring Aug. 27, 1946 2,408,055 Fiske Sept. 24, 1946