US3521115A

US3521115A - Helix coupled impedance transformer and tubes using same

Info

Publication number: US3521115A
Application number: US688929A
Authority: US
Inventors: Hunter L Mcdowell
Original assignee: SFD LAB Inc
Current assignee: S F D LAB Inc; SFD LAB Inc
Priority date: 1967-12-07
Filing date: 1967-12-07
Publication date: 1970-07-21
Anticipated expiration: 1987-07-21
Also published as: FR2021280A5

Description

Ju y 1970 H. L. M DOWELL 3,521,115

HELIX COUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Filed Dec. 7. 1967' 2 sheets-sneer 1 FIG I +2 6 I 7 9 & 5g INVENTOR HQ 4 v I HUNTER L. M0 DOWELL BYdfvj/maki TORNEY 1 July 21, 1970 H. L. MCDOWELL 3,521,115

HELIX COUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Filed D430. 7, 1967 2 Sheets-Sheet 2 FIG. 5

INVENTOR.

HUNTER LfMC DOWELL BY MM QT NEY United States Patent 3 521,115 HELIX COUPLED IMPEDANCE TRANSFORMER AND TUBES USING SAME Hunter L. McDowell, Chatham, N.J., assignor to S-F-D Laboratories, Inc., Union, NJ., a corporation of New Jersey Filed Dec. 7, 1967, Ser. No. 688,929 Int. Cl. H01 25/36; H03h 7/38 US. Cl. 3153.5 9 Claims ABSTRACT OF THE DISCLOSURE A helix coupled periodic circuit broadband impedance transformer is disclosed which includes an array of conductive elements such as parallel conductive bars, with successive Ones of the bars being interconnected by a helical structure and disposed over a ground plane such that the impedance transformer circuit forms a two wire line. The bars form periodic shunt capacitive elements, whereas the helical turns between successive bars form periodic series inductance in one of the lines of the circuit. The transformers are dimensioned such that the ratio of periodic inductance to periodic capacitance changes widely from one end of the transformer circuit to the other in a continuous manner. On the other hand, the product of period inductance times periodic capacitance is caused to remain substantially constant throughout the transformer circuit such that the high frequency cutoff for the transformer remains relatively constant.

DESCRIPTION OF THE PRIOR ART Heretofore, impedance transformers had been proposed for matching transmission lines to helix coupled bar circuits in microwave tubes. In these prior transformers, the circuit consisted of a helical conductor in combination with a horn shaped ground plane member which flared from a relatively low input impedance to a relatively high output impedance of about 200 ohms. While such a transformer may provide a transformation in impedance with a ratio of 4 to 1, the transformer becomes impracticable for transforming with higher ratios such as 1 to 8, i.e., from 50 ohms to 400 ohms. Another common way of making a very broad band impedance transformation is by use of an exponentially tapered length of transmission line. For example, a foot length of tapered strip line may be employed to match a 50 ohm coaxial line to a 300 ohm iterative impedance helix coupled bar circuit. Such an impedance transformer has been shown to have a VSWR of about 2 to 1 over a substantial band of frequencies in the low VHF range. However, the 5 foot length of the transformer section is excessively long to be practical for most tube applications. Also, the desired impedance transformation is from approximately 50 ohms to 400 ohms and, thus, the length of such a previously proposed tapered strip line would have to be substantially in excess of 5 feet.

Thus, a need exists for a relatively small impedance transformer capable of providing impedance transformation ratios on the order of 4 or more and providing a relatively wide band refiectionless match.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved helix coupled impedance transformer capable of providing relatively high impedance transforming ratios over relatively wide bands of frequencies.

One feature of the present invention is the provision of an impedance transformer comprising an array of conductive elements disposed over a ground plane with suc- 3,521,115 Patented July 21, 1970 cessive ones of the conductive elements being interconnected by a helix structure and the helix structure and the conductive elements being dimensioned such that the periodic inductance to periodic capacitance ratio changes substantially from one end of the impedance transformer to the other end.

Another feature of the present invention is the same as the preceding feature wherein the product of periodic inductance times periodic capacitance of the impedance transformer circuit does not differ by more than 30% over the length of the circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the transformer includes first and second circuit stages with a first circuit stage having less turns of the helix structure interconnected between successive conductive elements than in the second stage.

Another feature of the present invention is the same as any one or more of the preceding features wherein the impedance transformer is provided in combination with a high frequency tube for matching the impedance of a helix coupled bar type slow wave circuit to the impedance of an input or output transmission line, such transformer being preferably disposed externally of the tubes vacuum envelope.

Other features and advantages of the present invention will become apparent upon approval of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic, linearized, longitudinal sectional view of a helix coupled slow wave circuit employed in a microwave tube,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 2--2 in the direction of the arrows,

FIG. 3 is a view of the structure of FIG. 1 taken along line 33 in the direction of the arrows,

FIG. 4 is a simplified equivalent circuit for the slow wave circuits of FIGS. 13,

FIG. 5 is a plan view of a portion of the structure of FIG. 2 taken along line 5-5 in the direction of the arrows,

FIG. 6 is a schematic plan view, partly broken away, of a microwave tube employing features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-3, there is shown a helix coupled bar type slow wave circuit and a microwave tube using same. The helix coupled bar type slow wave interaction circuit 1 includes an array of conductive bars 2 with successive bars 2 being interconnected by a helix structure 3. Conductive leads 4 are tapped off of the helix 3 at suitable periodic intervals as of one or two turns. The leads 4 then connect to the center of the bars 2. The helix 3 is supported by a ceramic bar 5 from the inside wall of the vacuum envelope 6 of the tube.

The bars 2 are supported above one conductive ground plane member 7 via the intermediary of a plurality of ceramic bars 8 which are brazed on one side of the conductive bars 2 as of copper, and brazed on the other side to the conductive ground plane 7 of the tube envelope structure 6. A suitable insulative ceramic 8 is beryllia which has relatively high thermal conductivity and, thus, provides a good thermally conducting path from the bars 2 to the tube envelope 6 for removing heat from the bars 2 in use. Also, the bars 2 are disposed over a second conductive ground plane member formed by the cathode electrode structure 9. The ceramic bars 8 which serve to insulate the conductive bars 2 from the conductive ground plane 7 preferably have a width less than the width of the conductive bars 2 such that the bars serve to effectively shield r shadow the ceramic insulators 8 from sputtered cathode material that could otherwise deposit over these insulating members to short out the radio frequency fields thereon.

The cathode electrode structure 9 may be of the emitting or non-emitting type. If it is of the non-emitting sole type, the electrons for interaction with the slow wave circuit are injected into the region between the bars 2 and the sole 9. In case the cathode electrode 9 is of the emitting type, it is typically fabricated of a high secondary emitting ratio material such as beryllia copper, or aluminum oxide. The helix coupled bar type slow Wave interaction circuit 1 is typically operated at ground potential to form the anode electrode and a power supply 11 typically operates the cathode electrode 9 at a potential substantially negative with respect to the anode potential. A magnet structure, not shown, produces an axially directed magnetic field B, which is directed through the magnetron interaction region 12 defined by the space between the conductive bars 2 and the cathode electrode 9. p

A helix coupled impedance transformer structure 15, more fully described below, is carried externally of the tube envelope structure 6 for transforming the relatively low impedance, as of 50 ohms, of a typical coaxial transmission line to the relatively high iterative impedance (a characteristic impedance) as of 400 ohms of the helix coupled bar type slow wave interaction circuit 1. Impedance transformer 15 includes a printed circuit board 16 which is mounted to the tube envelope 6 via a channel member 17. The impedance transformer 15 is connected to the slow wave circuit 1 via a conductive lead 18 which passes axially through bore 19 in the envelope 6 and ceramic window 21 which serves to provide a gas tight wave permeable window structure sealing off the bore 19.

Referring now to FIG. 4, there is shown the equivalent circuit for the slow wave structure 1. Slow wave structure 1 forms a two conductor transmission line. One of the conductors is a ground plane member formed by

ground plane structures

7 and 9. The other conductor is the helix coupled bar circuit with the conductive bars 2 providing periodic shunt capacitive elements 'between the helix and the

ground plane members

7 and 9. That portion of the helix structure between adjacent tapping wires 4 forms a periodic series inductance of the slow wave circuit. The capacitance between the conductive bars 2 provides a periodic capacitance in series and with the slow wave circuit, such periodic capacitance being parallel connected with the periodic inductance of the helix 3. Typically, the capacitance between adjacent conductive bars 2 is relatively small compared to the shunt capacitance between the bars 2 and the

ground planes

7 and 9 and, thus, the slow wave circuit has a typical low pass, forward wave fundamental dispersion characteristic. The circuit is capable of operating down to DC because the helix coupled bars, forming one conductor of the circuit, are isolated by the insulative members 8 from the

ground plane members

7 and 9 and, therefore, the circuit is capable of transmitting DC power.

Referring now to FIG. 5, there is shown the impedance transformer structure 15. The impedance transformer 15 comprises a two stage helix coupled bar circuit. More specifically, an array of conductive bars 31, as of copper, formed on an insulative printed circuit board 32 is affixed over a ground plane member 33 such as a copper plate. A helix structure 34 interconnects the conductive bars 31 of the array of bars. The transformer 15 is a two stage transformer having a first stage 35 and a second stage 36.

In the first stage 35, successive bars 31 are interconnected by only one turn of the helix 34, whereas in the second stage 36 successive bars 31 are interconnected by two turns of the helix 34. The transformer 15 has an equivalent circuit substantially the same as that of FIG. 4 where the bars 31 provide periodic shunt capacitive elements to the ground plane 33, thus, producing periodic shunt capacity in a two conductor circuit. Periodic inductance is provided by those portions of the helix structure 34 interconnecting adjacent bars 31 to provide periodic inductance in series with one of the conductors of the two conductor circuit.

The transformer circuit 15 is arranged such that the iterative characteristic impedance of the circuit 15 continuously increases from the low impedance end of the circuit to the high impedance end of the circuit. The iterative characteristic impedance for the transformer circuit 15 is defined approximately by the relation:

where L is the periodic series inductance of the helix circuit 34 and C is the periodic shunt capacity of bars 31 to the ground plane 33. The transformer 15 is arranged such that the ratio of L/ C continuously increases from the low impedance end of the matching transformer circuit 15 to the high impedance end thereof. In the transformer 15 of FIG. 5, the low impedance end is at 37 and the high impedance end is at 38. When the transformer 15 is employed for matching the helix coupled bar circuit 1, as depicted in FIGS. l-3, the iterative characteristic impedance of the transformer 15 at the high impedance end 38 is preferably equal to the iterative characteristic impedance of the slow wave circuit 1 which is about 400 ohms. Typically, the low impedance end 37 of the transformer forms an impedance match to a conventional power transmission line, such as, coaxial line 39 having a characteristic impedance of 50 ohms. Thus, at the low impedance end 37 of the circuit 15, the series periodic L is relatively low by providing only a single small turn of the helix 34 between adjacent bars 31 and the bars 31 are relatively long to provide a relatively high shunt capacity. At the high impedance end of the first stage 35, the turns of the helix 34 are relatively large in diameter and the bars 31 are relatively short. Likewise, in the second stage of the impedance transformer 36 the low impedance end of the second stage gives relatively small inductance 'by providing two small diameter turns of the helix structure between adjacent bars 31 and the bars are relatively long. At the high impedance end 38 of the second impedance transformer stage 36 the periodic inductance is relatively high by providing two large turns of the helix between successive bars 31 and the bars are relatively short to reduce the shunt capacity.

The terminal bars 31 at the ends of the impedance transformer sections 35 and 36 are dimensioned to be half length, i.e., half the length they would have if they were to be progressively longer or shorter than their adjacent bar as determined :by the tapered length of the bars. These half length bar sections come about from filter theory. An alternative configuration for the terminal ends of the transformer sections 35 and 36 would be to employ terminal bars 31 which are equal to the length they would have to be if they were progressively longer and shorted according to the tapered lengths, and then employ terminal inductors L (turns of helix 34) which have half the inductance L they would have if they were to be progressively larger or smaller. The difference be tween these alternative ways of terminating sections 35 and 36 amounts to that between terminating a filter network in the mid-series or mid-shunt arms.

Both the length of the bars 31 and the diameter of the helix 34 are tapered because the cut-01f frequency for the transformer is proportional to the inverse of the square root of the L-C product. To obtain the desired transformer bandwidth, it is essential that the high frequency cut-off be kept at approximately the same frequency throughout the length of the transformer 15. By approximately the same frequency throughout, it is meant that the L-C product should not vary by more than 30% over the length of the transformer circuit 15.

In addition to the periodic series inductance L, the mutual inductance between the turns of the helix 34 is having an effect on the transformation ratio. The existence of mutual inductance is not necessary to make the circuit work. However, it appears that this mutual inductance is what may be responsible for giving a larger impedance transformation than calculated originally for an experimental transformer section. This comment does not affect the truth, at least roughly, of the necessity for maintaining the L/ C ratio constant while transforming the ratio of L/C.

Several advantages accrue from placing the impedance transformer 15 externally of the tube envelope 6. One advantage is that it can be readily adjusted to prevent undesired reflections from the transformer. "In addition, tune mode absorbers may be afiixed to the transformer for absorbing energy from certain undesired modes of oscillation within the tube. In the transformer 15 of FIG. 5, the circuit has been simplified for the sake of explanation and clarity in the drawings. Typically, each of the transformer sections 35 and 36 would comprise more bars and more turns of the helix 34. For example, each stage would typically include approximately 17 bars, tapered in the manner as indicated in FIG. 15. Each section would be approximately 3 inches in length such that the overall length of the complete transformer 15 for transforming 50 ohms to 400 ohms is approximately 6 inches. This compares quite favorably with the prior art tapered strip lines which were over 5 feet in length for smaller impedance transforming ratios.

Referring now to FIG. 6, there is shown a microwave tube employing the transformer 15 of the present invention. The tube includes a cylindrical vacuum envelope 6 surrounding a cylindrical non-emissive cathode electrode 9. Slow wave structure 1 curves around the cathode electrode 9 in a part circular arc to define a severed nonreentrant slow wave circuit, and an annular magnetrontype interaction region in the space between the conductive bars 2 and the cathode sole electrode 9. A conductive circuit sever 41 is disposed between the input end of the slow wave circuit 1 and the output end to provide a non-reentrant slow wave circuit. A pair of two stage impedance transformers 15 are provided for transforming from 50 ohm input and output

coaxial cables

42 and 43, respectively, to the slow wave structure 1. The transformers 15 are disposed on the end of the tube externally of the vacuum envelope 6.

In operation, signal wave energy to be amplified is applied via input coaxial line 42 and impedance transformer 15 to the input end of the slow wave circuit 1. Electromagnetic waves excited on the slow wave circuit 1 interact with a circulating stream of electrons in the magnetron interaction region between the sole 9 and the bars 2 to produce an amplified output signal on the slow wave circuit 1. The output signal is extracted from the output end of the slow Wave circuit 1 via impedance transformer 15 and fed to a suitable load via output coaxial line 43.

The tube of FIG. 6 is especially useful for providing wide band amplification at relatively low microwave frequencies; such as those in the VHF frequency range. The microwave tube preferably employs an axially injected electron beam which moves axially through the magnetron interaction region to a separate collector electrode, such that noise in the electron stream is not appreciably coupled to the slow wave circuit, thereby extending the dynamic range of the amplifier into the low signal regime.

The impedance transformer 15 and helix coupled bar type slow wave circuit 1 are useful in tunable oscillator tubes and amplifier tubes of either linear or circular geometry.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can 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 a high frequency apparatus, means forming an impedance transformer circuit for connecting a first cir cuit to a second circuit and for matching the impedance of the first and second circuits, the improvement wherein, said impedance transforming circuit includes means forming a ground plane conductor, means forming an array of conductive 'bars disposed overlying said ground plane, means for insulatively supporting said bars relative to said ground plane, means forming a conductive helix structure interconnecting successive ones of said conductive elements of said array, said conducting elements providing predominantly a periodic shunt capacitance for said impedance transforming circuit, said helix structure providing predominantly a periodic series inductance interconnecting successive capacitive elements, said helix structure and said conductive elements being dimensioned such that the ratio of periodic inductance to periodic capacitance changes from one end of said impedance transforming circuit to the other end thereof to obtain the impedance transformation.

2. The. apparatus of claim 1 wherein the product of the periodic inductance and the periodic capacitance at one end of said impedance transforming circuit does not differ by more than 30% from that product taken anywhere along said impedance transforming circuit.

3. The apparatus in claim 1 wherein the ratio of periodic inductance to periodic capacitance changes in a continuous manner from one end of said matching transformer circuit to the other end thereof.

4. The apparatus of claim 1 wherein said impedance transforming circuit includes a first and second circuit stage, said first circuit stage having less turns of said helix structure connected in between successive bars than in said second stage.

5. The apparatus of claim 1 including, means forming a helix coupled bar type slow wave interaction circuit, said slow wave circuit including an array of parallel conductive bars, means forming a helix structure interconnecting successive ones of said bars, and said impedance transforming circuits being coupled at one end to one end of said slow wave interaction circuit.

6. The apparatus of claim 5 wherein said slow wave interaction circuit includes an array of thermally conductive ceramic members bonded to said bars of said slow wave interaction circuit, means forming a metallic support structure, and said ceramic members being bonded to said support structure to provide a thermally conductive path from said bars of said interaction circuit through said ceramic member to said metallic support for cooling said slow wave circuit.

7. The apparatus of claim 6 including means forming an evacuated envelope structure containing said slow wave interaction circuit, and wherein said impedance matching circuit is disposed outside of said evacuated envelope structure.

8. The apparatus of claim 6 including means forming a cylindrical cathode electrode structure, said bars of said slow wave interaction circuit being disposed concentrically with and adjacent to said cathode electrode structure to define a curved interaction region therebetween to contain a stream of electrons for electronic interaction between the stream of electrons and radio frequency energy on said slow wave circuit to produce an output signal.

9. The apparatus of claim 7 wherein said bars are 7 8 Wider than said ceramic support members, whereby said 3,020,498 2/1962 Ash et a1. 333-34 X bars serve as sputter shields for said ceramic members 3,414,756 12/1968 Farney 333--34 X to inhibit coating of said ceramic insulators.

HERMAN KARL SAALBACH, Primary Examiner References Cited 5 S. CHATMON, JR., Assistant Examiner UNITED STATES PATENTS 2,588,832 3/1952 Hansell 315 3.5 X 2,828,440 3/1958 Dodds et a1 3153.6 315-3.6; 333-33, 35 2,987,644 6/1961 Anderson 3153.5