US 2895110 A
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July 14, 1959 R. B. NELSON ET AL HIGH FREQUENCY APPARATUS Filed Aug. 16, 1956 INVENTORS Richard 8. Nelson 8 Robert S. Symons Attorney United States Patent 2,895,110 HIGH FREQUENCY APPARATUS Richard B. Nelson, Los Altos, and Robert S. Symons, Menlo Park, Calif., assignors to Varian Associates, San Carlos, Califl, a corporation of'California Application August 16, 1956, Serial No. 604,535 7 Claims. (Cl. 333-24) The present invention relates in general to high frequency apparatus and more specifically to novel improved high frequency coaxial line coupling apparatus useful, for example, in coupling high frequency energy into and out of wave propagating structures and the like.
Heretofore, coaxial lines have been utilized for coupling wave energy into high frequency apparatus but these prior art devices have had relatively narrow bandwidth characteristics. When a coaxial line is coupled into a cavity resonator by means of an inductive or magnetic coupling loop great difiiculty has been encountered in providing a uniform degree of coupling over a relatively wide band of frequencies due to the increasing inductive reactance ofthe loop with increasing frequency. In the past impedance transformers which require adjustment as the frequency is changed have been utilized external of the coaxial transmission line for canceling out the inductive reactance of the loop at higher frequencies.
The present invention provides a novel improved coaxial line coupler having a broad band vacuum seal therein and, in addition, providing a novel low Q resonant magnetic coupling loop whereby the increasing inductive reactance of the coupling loop may be canceled by a decreasing capacitive reactance whereby the coupling characteristicof the loop may be adjusted, as desired, over a relatively wide bandwidth.
The principal object of the present invention is to provide a novel improved high frequency coaxial line coupling apparatus having broad band characteristics and moderately high power handling capabilities.
One feature of the present invention is'a capacitive discontinuity means associated with the inductive coupling loop 'of'the coaxial line coupler whereby the coupling characteristics of the coupling loop may be controlled, as desired, over a relatively wide band of frequencies.
Another feature of the present invention is the provision of a novel coaxial line, gas-tight wave permeable window wherein the window is of a substantially hollow cylindrical physical configuration and disposed within the coaxial line with its side walls in spaced apart relation from the coaxial inner and outer conductors thereby minimizing reflected energy therefrom and decreasing the power dissipated therein per unit volume thus greatly enhancing its power handling capabilities.
Another feature of the present invention is a novel inductive reactance means associated with the novel coaxial wave permeable vacuum seal whereby the vacuum seal may be made to present a substantially constant impedance over a wide band of frequencies.
These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein,
Fig. 1 is a longitudinal, cross sectional view of a novel coaxial transmission line coupling means embodying the present invention,
Fig. 2 is a cross sectional view of a portion of the structure of Fig. 1 taken along line 2-2 in the direction of the arrows, and Fig. 3 is a graph of RF. gap voltage within the cavity resonator and coupling loop current versus frequency.
Referring now to Fig. lthere is shown a re-entrant type cavity. resonator 1. This cavity resonator is exemplary of one of many types of wave propagating structures which may be coupled to a coaxial transmission line 2 utilizing a coaxial connector 3 embodying the features of the present invention.
Although the coaxial connector 3 of the present invention is a reciprocal device, that is, it will transmit energy equally as well in both directions, for the sake of explanation, it will be assumed that the coaxial connector 3 is being used as the input connector for the wide range tunable cavity resonator 1.
Wave energy enters the coaxial connector 3 from the standard coaxial transmission line 2. At the input ex tremity of the standard coaxial line 2 is a female outer coaxial fitting 4 having a threaded portion on its outside diameter for mating with a similar threaded portion of a slip collar 5. The innermost wall of the female outer coaxial fitting 4 is longitudinally slotted to provide a plurality of spring-like fingers for mating with a male outer coaxial fitting 6.
A circumferential recess 7 is provided in the outside wall of the male outer coaxial fitting 6 for containing a split retaining ring 8. The split retaining ring 8 serves to hold the slip collar 5 in position and allows the slip collar 5 to freely. rotate about the male outer coaxial fitting 6.
An inner conductor 9of the coaxial transmission line 2 is provided, at its end, with a plurality of longitudinal slots running therein and, in addition, is provided with a longitudinal bore centrally placed therein thereby forming a plurality of spring-like fingers at the end .thereof. The bored and fingered extremity of the inner conductor 9 forms the female inner conductor coaxial connectorfor the coaxial transmission line 2. A male inner conductor coaxial fitting 11 comprises a length of inner conductor of smaller diameter for slidably mating with the spring fingers of the female portion of the inner coaxial conductor 9. Good electrical contact is assured between the outer and inner male and female portions of the coaxial conductors by rotation of the slip collar 5 thereby pressing together the male and female connector portions of the coaxialline. a
A circumferential inward protuberance 10 is provided a on the inside wall of the male outer coaxial fitting 6 and is disposed adjacent the mating male and female inner conductor coaxial fittings 11 and 9 to provide an impedance match at the joint of the inner conductors. .The circumferential protuberance 10 provides a capacitive reactance to offset the inductive reactance of the circumferential recess in the inner conductor 9 due to the fact that the fingers of the inner coaxial conductor of the female portion do not slide all the way down on the male portion of the coaxial inner conductor 11. r
.A circular conducting flange 12, as, of, for example, copper, is firmly affixed, as by brazing, to the inner coaxial conductor 9. A thin annular window frame member 13 as of, for example, Kovar is carried upon a trans! verse face of the inner conductor flange 12. Ahollow cylindrical wave permeable window 14. as of, for example, ceramic is carried at one end thereof by a transverse face of the annular window frame 13. A second annular window frame member 15 carries the other extremity of the dielectric window 14. A thin yieldable apertured cup member 16 of a conducting material such as, for example, copper carries the second annular window frame member 15 upon a bottom transverse surface thereof. The ceramic window 14 is held within the Kovar frame 13 by metalizing and the Kovar in turn is,
brazed to the circular copper fiange 12 and cup 16 there Patented July 14, 1959 with a portion of enlarged inside diameter in the vicinity of the dielectric window 14. In addition the inner conductor 9 in this vicinity has been formed of a decreased diameter. This portion of increased spacing between the inside and outer conductors serves to increase the inductive reactance of that portion of the coaxial transmission line to compensate for the increased capacitance due to the circular flange 12, window frames 13 and 15, dielectric window 14, and yieldable cup 16. In this manner the window portion of the transmission line is matched, in impedance, to the remaining portion of the coaxial transmission line to thereby provide an electrically flat line, that is a line without substantial standing waves. The yieldable cup 16 is carried at its outside side wall by a hollow cylindrical adapter 17.
The cylindrical adapter 17 is provided with a portion of increased inside diameter to match and mate with the portion of increased inside diameter of the male outer coaxial fitting 6. A threaded portion is provided on the outside of the cylindrical adapter 17 to threadably mate with a portion of the male coaxial fitting 6.
The cylindrical adapter 17 and the male inner conductor coaxial fitting 11 are made of a non-readily oxidizing conductive material such as, for example, cupronickel (70% copper, 30% nickel) to provide a surface which may be subjected to repeated high temperature baking in air without scaling or otherwise harming the conductive characteristics of the surface. In certain applications for the novel coaxial connector of the present invention such as, for example, where the connector forms a part of the vacuum envelope of a high frequency tube, as depicted in Fig. l, the connector must be subjected to baking in an oven at temperatures in the order of 400 C. to 600 C. to aid in removal of all harmful gases within the envelope. If the surfaces external of the vacuum are of copper and are so fired the surface will oxidize and produce a. scale which seriously impedes a good electrical contact.
A hollow conductive metallic block 18 as of, for example, copper carries the cylindrical adapter 17 upon a transverse face thereof and in alignment with a cylindrical bore 19 therein. A portion of the yieldable cup member 16 extends into the bore 19 and is fixedly secured therein substantially flush with the side walls thereof as by brazing thereby providing a gas-tight seal therebetween.
The portion of decreased diameter of inner conductor 9 extends past the dielectric window 14 and part way into the interior portion of the yieldable cup 16. This extension is provided to produce an increased inductive reactance to balance the increased capacitive reactance due to the nearness of the bottom portion of the yieldable cup 16 to the inner conductor 9. In this manner an impedance match is provided in the coaxial transmission line in the viscinity of the yieldable cup 16.
An oval bore 21 (see Figs. 1 and 2) is provided in metallic block 18. This oval bore 21 extends into the block at right angles and intersects with the cylindrical bore 19. The inner conductor 9 extends through the cylindrical bore 19 and into the oval bore 21. Within the oval bore the inner conductor 9 is provided with a right angle bend and extends out of the oval bore 21. The oval bore 21 is made of an oval cross section to familitate assembly of the inner conductor 9 within the metallic block 18. The cylindrical bore 19 or the oval bore 21 individually in combination with the center conductor 9 are designed to have the same characteristic impedance to prevent energy reflection at the intersection thereof. However, for certain application a change in characteristic impedance at the intersection of the bores may be desirable for producing certain coupling characteristics.
The inner conductor is pre-formed with its loop and right angle bend therein and then assembled within the metallic block 18. It would not be possible to assemble the pre-formed center conductor 9 if the oval bore 21 were of a cylindrical configuration of the same dimensions as bore 19. The portion of the inner conductor 9 extending out of the oval bore 21 is provided with a reverse bend and is connected at its extremity to the metallic block 18. The reverse bend portion of the inner conductor 9, that extends external of the metallic block 18, forms an inductive coupling loop 23.
A capacitive loading slug 24 as of, for example, copper is fixedly carried upon the inner conductor 9 and is disposed within close proximity to the magnetic coupling loop 23. The purpose of the capacitive loading slug 24 is to provide a capacitive reactance associated with the inductive reactance of the magnetic coupling loop 23 whereby the inductive reactance of the coupling loop may be compensated for over a broad band of frequencies as of, for example, 45% of the center tunable frequency of the cavity resonator, as desired, to produce various coupling characteristics.
The size, disposition and shape of the capacitive loading slug 24 may be varied, as desired, to produce the wanted capacitive reactance. The ultimate dimensions are normally found utilizing empirical methods.
The metallic block 18 is provided with an annular shoulder at 25 for securing to the cavity resonator 1 or other wave propagating structure, as desired. When the metallic block 18 is fixedly secured to the wave propagating structure the magnetic coupling loop 23 extends into the interior portion of the wave propagating structure to provide a wave energy magnetic coupling between the coaxial transmission line and the wave propagating structure.
In operation electromagnetic wave energy enters the coaxial connector via a coaxial transmission line 2 having a center conductor 9. The electromagnetic field configuration, for the dominant mode, of the transmission line is such that the electric field vectors extend radially between the inner conductor 9 to the outer conductor of the transmission line 2.
In the vicinity of the vacuum sealed dielectric window 14 it will be noted that the electric field vectors are substantially at right angles to the thin dimension of the dielectric window 14. With the window 14 disposed in this manner the capacitive discontinuity induced by the ceramic window 14 is minimized, thereby facilitating an easy impedance match to the coaxial transmission line on both sides of the ceramic window 14. In addition, due to the capacitive divider action of the air, vacuum, and dielectric materials in the space between the inner conductor 9 and the outer conductor of the coaxial transmission line, the electric field and hence the power losses in the dielectric window are minimized thereby increasing the power handling capabilities of the dielectric window.
Upon passing through the dielectric window the wave energy is propagated through the cylindrical bore portion 19 of the transmission line and thence through the oval bore portion 21 of the transmission line and into the cavity resonator 1 or other type of wave propagating structure.
The capacitive loading slug 24 and the inductive coupling loop 23 electrically form a parallel resonant circuit, having a resonant frequency f (see Fig. 3). It will be noted that the resonant circuit formed thereby will have a very low Q due to the loading of this resonant circuit caused by the resistive losses of the cavity resonator and the resistive losses of the coaxial transmission line. By the proper choice of the resonant frequency i of the magnetic coupling circuit with relation to the operating frequency at the wave propagating structure one is able to achieve flexibility, as desired, in the coupling characteristics between the coaxial transmission line and the wave propagating structure over a broad band of frequencies without the necessity of adjusting the parameters of the coupling apparatus.
An example of this coupling flexibility is illustrated in Fig. 3 wherein it was desired to obtain a power coupling characteristic, to the cavity resonator 1, which increased with frequency. More specifically, the cavity resonator 1 was tunable over a range of frequencies indicated by the arrows of Fig. 3, and it was desired to achieve increased coupling to the cavity with frequency because the characteristics of the cavity were such that the coupling to the beam decreased with increased frequency. To achieve this desired coupling characteristic the resonant frequency of the coupling loop f was selected to be slightly higher than the expected range of the cavity resonant frequency f In this manner the coupling current and thus the energy coupled into the cavity resonator was made to increase with a corresponding increase in frequency. In Fig. 3 the curve labeled V having a center frequency f represents the RF. gap voltage within the cavity resonator and the curve marked I represents the coupling current flowing through the coupling loop in the cavity resonator.
A flat coupling characteristic may be achieved by selecting the coupling loop resonant frequency h, to correspond with the center frequency f of the tuning range of the wave propagating structure. A coupling characteristic wherein the coupling decreases with increased frequency may be achieved by selecting the coupling loop resonant frequency 3, to be on the low frequency side of the center frequency f of the tuning range of the wave propagating structure.
Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, 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.
What is claimed is:
1. In 'an evacuated coaxial coupling apparatus for coupling wave energy from one wave propagating structure to another a conductive metallic block forming the outer conductor of the coaxial coupling apparatus, said conductive metallic block having a cylindrical and an oval bore therein intersecting at substantially right angles to each other, and a unitary center conductor means threaded through the bores of said metallic block and spatially separated from the side walls thereof and having a portion extending externally of the block formed with a reverse shape therein and connected to said metallic block thereby forming an inductive coupling loop externally of the metallic block, said oval bore provided to make possible the insertion of said pro-formed unitary center conductor means.
2. Apparatus according to claim 1 including an apertured, yieldable cup member carried in alignment with the cylindrical bore and by said metallic block, a cylindrical wave permeable window means carried by a transverse surface of said yieldab'le cup member, and said cylindrical wave permeable window means disposed with its longitudinal axis substantially in parallelism with and concentrically of the longitudinal axis of said (unitary center conductor means and spatially separated therefrom for substantially the entire length of said window means whereby said wave permeable window means presents a relatively thin portion thereof to the electric field lines thereby greatly enhancing the power handling capabilities of the wave permeable window and minimizing energy reflections therefrom, in use.
3. A gas tight wave permeable window assembly for coaxial transmission lines comprising, coaxially disposed inner and outer conductors, a cylindrical wave permeable window member coaxially disposed with respect to said inner and outer conductors and disposed therebetween in spaced apart relation for substantially the entire length of said window, and metallic end sealing members vacuum sealing the ends of said window member to said inner and outer conductors respectively whereby the power handling capacity of the wave permeable window assembly is greatly enhanced.
4. The apparatus according to claim 3 including, a portion of said outer conductor of the coaxial transmission line having an enlarged inside diameter, and said portion of enlarged outer conductor extending the entire length of said cylindrical wave permeable window and extending a substantial longitudinal overlapping distance opposite said end sealing member sealing said window and said inner conductor whereby the impedance of said window is matched over a broad band of frequencies.
5. In an apparatus according to claim 3 wherein a portion of said inner conductor of the coaxial transmission line is provided with a decreased outside diameter, and said portion of said inner conductor having the decreased outside diameter extending the entire length of said wave permeable window and having substantial overlapping longitudinal end portions opposite the ends of said wave permeable window member to minimize wave reflections from said window member over a broad band of frequencies.
6. A wide band vacuum sealed coaxial coupling apparatus for transmitting high frequency electromagnetic wave energy between an evacuated tunable cavity resonator and a wave propagating structure including, means forming an inductive coupling loop coupled to the high frequency magnetic fields of the evacuated cavity resonator, means forming a broad band impedance match for matching the impedance of the cavity resonator to the impedance of the wave propagating structure, said impedance matching means comprising substantially solely a fixed slug carried by the center conductor of the coaxial coupling apparatus and forming a capacitive discontinuity physically closely spaced to said inductive coupling loop, a gas tight wave permeable window member having a substantially hollow cylindrical physical configuration vacuum sealed between and spaced apart from the center and outer conductors of the coaxial coupling apparatus for substantially the entire length of said window member, and said cylindrical wave permeable window disposed with its thin side wall portions substantially at right angles to the electric field lines between the inner and outer conductors of the coaxial coupling apparatus for substantially the entire length of said window, and metallic end sealing members vacuum sealing the ends of said window member to the center and outer conductors respectively of the coaxial coupling apparatus whereby a broad band, high power capacity window is obtained.
7. A high frequency electromagnetic wave energy coupling apparatus for translating wave energy from a tun able cavity resonator to a coaxial transmission line over a broad band of frequencies including, means forming an inductive coupling loop coupled to the magnetic fields of the cavity resonator, means forming a broad band impedance match for matching the impedance of the cavity resonator to the impedance of the coaxial line, and said impedance matching means consisting essentially of a slug forming a capacitive discontinuity physically closely spaced to said inductive coupling means whereby the inductive reactance of said inductive coupling means is independently balanced out over a broad band of frequencies.
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