US3846720A

US3846720A - Compact microwave termination and uses thereof

Info

Publication number: US3846720A
Application number: US00416036A
Authority: US
Inventors: T Mohr
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1973-11-15
Filing date: 1973-11-15
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05
Also published as: CA1000815A

Abstract

There is disclosed a compact termination for enclosed wave propagation media, particularly waveguide, in which the propagation medium is loaded with an absorptive material throughout a section terminating in a reflective member. The absorptive material or loading material is shaped into first and second portions having impedances lower than the characteristic impedance of the propagation medium and respective lengths in the direction of wave propagation, these impedances and lengths together, providing negligible reflection into the unloaded medium. The loaded section is rendered shorter than one-quarter wavelength by making the portion that is next to the reflective end member have a value of impedance intermediate the impedance of the other portion and the characteristic impedance of the unloaded medium. The direction of stepping of the impedances in this termination is the reverse of that of prior art stepped terminations. The idea is extended to compact waveguide terminations that permit better heat-sinking and to a termination for a coaxial cable.

Description

United States Patent 1 Mohr [ Nov. 5, 1974 1 i COMPACT MICROWAVE TERMINATION AND USES THEREOF [75] Inventor: Theodore Warren Mohr, Whitehall,

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, Berkeley l3 CIRCULATOR ISQZATOR HEAT SINK CORE 23 @IEBMI NATION Y Primary Examiner-Paul L. Gensler [57] ABSTRACT There is disclosed a compact termination for enclosed wave propagation media, particularly waveguide, in which the propagation medium is loaded with an absorptive material throughout a section terminating in a reflective member. The absorptive material or loading material is shaped into first and second portions having impedances lower than the characteristic impedance of the propagation medium and respective lengths in the direction of wave propagation, these impedances and lengths together, providing negligible reflection into the unloaded medium. The loaded section is rendered shorter than one-quarter wavelength by making the portion that is next to the reflective end member have a value of impedance intermediate the impedance of the other portion and the characteristic impedance of the unloaded medium. The direction of stepping of the impedances in this termination is the reverse of that of prior art stepped terminations. The idea is extended to compact waveguide terminations that permit better heat-sinking and to a termination for a coaxial cable.

12 Claims, 11 Drawing Figures 71 TERMINATION WAVE PROPAGATlON DlSSlPATIVE 1 MAT7ER1AL HEAT SINK a5 DISSIPATIVE COAXIAL CABLE STUB PATENTEUxnv 51914 11846320 I3 CIRCULATOR HE g S INKJ I-\-;

ffw l2 TERMINATION I l ISOLATOR PATENTEUHUV 51914 3.846320 sum w T? PAIENTEDHUII 5' I914 3.846720 SIIEEI 701' 7 FIG. /0

7I TERMINATION WAVE PROPAGATION 73 F -77 DISSIPAT MATER F/G. II

DISSIPATIVE MATERIAL 82 8| TERMINATION WAVE PROPAGATION COMPACT MICROWAVE TERMINATION AND USES THEREOF BACKGROUND OF THE INVENTION This invention relates to apparatus for terminating enclosed wave propagation media, such as waveguide and coaxial cable.

While a great variety of terminations are known for waveguides, the commonly used terminations are at least one-quarter wavelength long in order to provide negligible reflection into the guide and to assure good bandwidth.

Nevertheless, in some applications for terminations, compactness may be much more desirable than broad bandwidth. For example, in some parts of microwave communication systems carrying only relatively narrow bands of frequencies, it is desired to provide isolation between components in a relatively compact space. Typically heretofore so-called resonant isolators have been used in these instances.

ln modernizing such systems, it is now frequently desirable to replace resonant isolators with terminated circulators, since the circulator structures are less costly when manufactured in quantity, yield higher performance in low power applications, and have other uses as well. Some improvement in bandwidth over that of a resonant isolator is also desired. The termination of the circulator that makes it an isolator must be compact to permit replacement of isolators in existing systems and reduction of volume of new systems.

SUMMARY OF THE INVENTION According to my invention, a compact termination is provided for an enclosed wave propagation medium such as a waveguide or coaxial cable by loading a section of the medium extending to a reflective end memher with absorptive material shaped into flrst and second portions having impedances lower than the characteristic impedance of the unloaded medium, the loading being accomplished in less than a quarter wavelength in that the second portion is intermediate the first portion and the reflective end member and has a value of impedance intermediate between the impedance of the first portion and the characteristic impedance of the unloaded medium.

Subsidiary features of the invention relate to techniques for proportioning respective impedances and electrical lengths of the portions to provide negligible reflection into the unloaded medium, relate to means for providing superior heat-sinking and further relate to application to coaxial cable terminations.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of my invention will become apparent from the following detailed description taken together with the drawings in which:

FIG. I is a pictorial cross-sectional view of a terminated circulator or isolator using my invention;

FIGS. 2 and 3 show different views of the termination itself;

FIG. 4 schematically indicates the various impedances and lengths pertinent to the proportionings of the absorptive material of the termination;

FIG. 5 through 9 show polar admittance diagrams, known as Smith charts, for various proportionings of the parameters indicated in FIG. 4;

FIG. 10 shows a modification of a microwave termination for waveguides having superior power handling capabilities; and

FIG. 11 is a pictorial cross-sectional view of a coaxial cable termination according to my invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT In FIG. 1 the microwave waveguideisolator II includes the circulator I3 and the termination 12. While the circulator shown is a symmetrical three port, the subsequent discussions apply equally to any three-part circulator. The wave propagation path through the circulator 13 is generally from top to bottom through the input port 14 past the core element 19 and pole pieces 18, then out the output port 15. The circulator 13 is completed by a third port 16, to which termination 12 is attached.

The termination 12 that makes the circulator 13 into an isolator 11 must be fairly compact in order to minimize the lateral protrusion of the apparatus from the wave propagation path. In the present instance, the termination 12 is a section of loaded waveguide less than one-quarter wavelength long. It includes the waveguide sidewalls 24 and reflective end plate 25, as well as the absorptive element 20 which provides the loading.

Absorptive element 20 has its smallest diameter portion 22 adjacent reflective end plate 25 and its largest diameter portion 21 farthest therefrom. It typically has a copper core 23 which may include the means for fastening it to reflective end plate 25. Alternatively, the core may be deleted and the fastening may be by adhesive. It will be noted that the direction of stepping of the diameter of reflective element 20 is the reverse of that of broadband microwave terminations. This fact is the key to the compactness of element 20.

Element 20 is shown in FIG. 2 demounte'd from circulator 13 so that it may be appreciated that it could also be used to terminate any waveguide or any waveguide stub. Such a termination would be relatively narrowband, but would have at least the bandwidth of current or present microwave communications bands. The view of termination 12 as shown in FIG. 3 simply illus trates that it has advantageously a cylindrical crosssection in both parts, even in rectangular waveguide. Nevertheless, termination 12 could also have a rectangular or other non-cylindrical cross-section.

For a discussion of the proportions of element 20, reference is made to the schematic diagram of FIG. 4 in which the characteristic impedance of the unloaded waveguide is designated Z the impedance Z is assigned to the length of guide occupied by portion 21 and Z to the length of guide occupied by portion 22. For a central element 20, impedance is inversely related to diameter. Z, is the lowest of the three impedances; and Z is intermediate the values of Z and Z Z and Z are actually loaded waveguide impedances associated with the respective portions of element 20. Since the polar admittance diagrams start at the reflective end plate 25, which has infinite admittance, the length 1 is assigned to element 22 and the length 1 is assigned to element 21.

Referring now to the polar admittance diagram of FIG. 5, one sees that conductive values of the ratio of admittance to characteristic admittance at the point in question are plotted on the vertical diameter of the chart with a unity admittance ratio (match) at the center. Capacitive admittance ratios are to the right in the chart and inductive admittance ratios are to the left. Reflective end plate 25 of FIG. 1 is represented by the point at the bottom of the vertical diameter of the diagram, since it represents infinite admittance.

Let us assume that the ratio of Z,to Z is one-half and that the ratio of Z 'to Z is four. These are logical assumptions since the largest portion 21 will have the lowest impedance. Consider now that an electromagnetic wave is propagating from reflective end plate 25 along the axis of the guide along element 22 toward element 21. The dissipative material of element 22 has a constant loss per unit length in terms of dB at a given wavelength. This is represented in the Smith chart of FIG. 5 by the so-called return loss curve B which is illustr'atively characterized by a 24 l/)\ dB loss. The admittance at any given point as we move away from end plate 25 lies at a point on this curve. When we reach element 21 after traversing distance I, subtending an angle 720 I /A we encounter a discontinuity. On the diagram, this discontinuity is represented by the segment 32. The beginning and end points of segment 32 are the admittances at the discontinuity normalized on impedances Z 2 and Z. respectively. Since Z,/Z /2, the normalized values of conductance and susceptance at the end point of segment 32 will be just one-half of the values at its beginning point.

As the electromagnetic wave moves along element 21 toward the unloaded guide, the impedance at any point is represented by curve segment 33, which lies on another constant loss per unit length curve that could be interpolated between curves A and B.

Finally at the outer edge of element 21, an impedance discontinuity is encountered which requires renormalization from impedance value Z, to the impedance value Z as indicated by straight line segment 34. In other words, the normalized conductance and susceptance at the beginning of segment 34 are multiplied by four which is the ratio Z /Z, to get the coordinates of the end point of segment 34.

If we have properly proportioned the impedances and the lengths l and we will have a match and the end point of 34 will fall at the origin of the chart. If we have not, the final segment 34 willterminate at a point away from the origin indicating a mismatch. In general the ideal proportions depend upon frequency thus limnew termination and tends to indicate in part the range through which some of the parameters can be varied. From FIG. to FIG. 6, the length 1, is illustratively increased without changing the ratio ofZ, to Z which illustratively still remains one-half. Thus the line segment 41 covers a greater arc length now, about 0.16 of a wavelength, and the line segment 42 representing the discontinuity between

portions

21 and 22 terminates on normalized values of conductance and susceptance which are about one-half the values of the previous example. The length 1 corresponding to the are that the line segment 43 spans is now reduced so that the admittance at its outer edge is purely conductive. The final line segment 44 represents the discontinuity between impedance Z, and the characteristic impedance Z, of

the unloaded guide. In this case, in order for the termination to be matched, the impedance ratio Z /Z must be somewhat greater than 5:1.

It will be seen that for any set of reasonable ratios 2, to Z and Z to 2 where z, Z Z lengths 1 and 1 can be chosen to provide a purely resistive impedance of termination 12 with a sum of I, and 1 that is less than one-quarter of a wavelength (180around the polar impedance diagram), since the remaining part of the required lrotation through the Smith chart will always be spanned electrically by the discontinuity represented by the line segment 42. In other words, that discontinuity has effective electrical length but not physical length along the guide. The reason for the shortness of the termination of the present invention can be explained in terms of the polar impedance diagram by noting that prior art tapered or stepped terminations plotted on the polar admittance diagram involve an ever-tightening spiral about the origin or successive rotations and admittance transformations on the real axis (no angular change). Termination polar admittance plot always spans just half the impedance chart, the inductive half, and if properly designed, the element 20 has a plot that ends at the center of the chart, unity admittance ratio, indicating a proper match.

In the above discussion the admittance of the abrupt discontinuities of the waveguide or load cross-section (end effects) have not been taken into account. Since these admittances have a significant effect on the design and since they make determination of the-loaded guide impedances difficult, the design is carried out by trial and error modification of the lengths and the diameters of

sections

21 and 22. The number of trials re quired to obtain the final dimensions may be significantly reduced through the use of Smith chart deviation experiments of the type indicated in FIGS. 7, 8 and 9. These experiments show the qualitative effects of variations in the various parameters and serve as a guide to what parameter should be adjusted and in what direction the adjustment should be made.

As a specific example, one termination has been built and tested for use in terminating a circulator as in FIG. 1. The waveguide is of a type known as WR l 59, atypical 6 gigahertz waveguide having internal dimensions 1.590 X 0.795 inches, the absorptive material of element 20 is a resistive resin-type polymer including some iron commercially available as Emerson and Cummings MFl 17. The diameter of smaller portion 22 is 0.037 inches, the larger diameter is 0.629 inches, is 0.125 inches and I is 0.175 inches. The operating frequency range is 5.865 to 6.425 gigahertz. Therefore, the total termination length of 0.300 inches is only oneeighth of a guide wavelength. The measured return loss of the termination over this 9 percent band is greater than 27 dB. When the circulator is tuned to match the termination the isolation is greater than 30 dB. A termination return loss of 25 dB over a 12-15 percent band is feasible.

In the higher power termination of FIG. 10 the termination 71 may be viewed simply as a loaded section of the waveguide 75 in which the lossy material is applied to the sidewalls of

sections

73 and 74 of the guide. The section 73 has the lowest impedance because of the smallest lateral separations from the guide axis and is followed by a section 74 next to the reflective end plate which has an intermediate impedance value because of intermediate separations of the guide walls from the guide axis. As in FIG. 1 the intermediate impedance portion 74 is intermediate in position between the reflective end plate 77 and the other portion 73.

The heat-sink 76 is then placed in thermal contact with walls 75 about the loaded section of the guide in order to carry away the heat generated by dissipation of microwave energy in the

lossy sections

73 and 74. Note that in this case the lossy body or termination load is in two separated

sections

73 and 74.

In FIG. 11 the principles of the present invention are extended to termination 81 for coaxial cable. The coaxial cable includes the outer conductor 85 and an inner conductor 86. The absorbing element 82 is a cylinder of dissipative material in contact with the outer conductor 85

portions

83 and 84 of and extending over the combined lengths of the enlarged center conductor 86. The portion 83 isenlarged more than portion 84, which is intermediate portion 83 and the end of the guide so that the coaxial cable section including portion 84 will have a value of impedance intermediate the impedance of the unloaded guide and the impedance of the coaxial cable section including portion 83. The principles of operation are identical to those explained above; and termination characteristics can be similarly plotted on a Smith chart. Such a coaxial cable termination can be used, for instance, to terminate one part of a coaxial circulator used as an isolator or to terminate an unused part of a four part directional coupler. The relatively large contact area between the dissipative element 82 and the outer conductor 85 provides for effective conduction of heat out of the dissipative material. For higher power applications a heat sink is easily applied, or the substantial heat-sinking capability already present can be augmented by forced air or water flow over the exposed surface of the outer conductor.

I claim:

1. Apparatus for terminating an enclosed wave propagation medium having a characteristic impedance, comprising a section of said medium, a reflective end member for said section, and means for loading said section, including a bodycomposed of material absorptive for the propagating waves and shaped into first and second portions providing said loaded section with respectively associated impedances lower than said characteristic impedance and having respective lengths in the direction of wave propagation presenting resistive impedance at the input to said section, the body being rendered shorter than a quarter wavelength in that the Second a r ic" interm d e sa xstiqo ti nd. said end member and provides said loaded section with a respectively associated impedance intermediate the impedance provided by said first portion and said characteristic impedance.

2. Apparatus according to claim 1 including a circulator structure having a loss port, the loaded section of wave propagation medium being coupled to said loss port, whereby said apparatus comprises an isolator.

3. Apparatus according to claim 1 in which the impedances and lengths of the first and second portions of the loaded section are proportioned to make the resistive impedance presented at the input of the loaded section equal to the characteristic impedance of the wave propagation medium.

4. Apparatus according to claim 1 in which the section of enclosed wave propagation medium comprises a section of hollow metallic waveguide and the body composed of absorptive material is positioned in contact with the end member and spaced from the sides of the metallic waveguide, the second portion of said body having smaller transverse dimensions than the first portion of said body.

5. Apparatus according to claim 4 in which the transverse dimensions of the body are stepped from said first portion to said second portion.

6. Apparatus according to claim 4 in which the body has a heat-sink core contacting the end member.

7. Apparatus for terminating an enclosed wave propagation medium having a characteristic impedance, comprising a section of said medium, a reflective end member for said section, and means for loading said section, including first and second hollow bodies composed of material absorptive for the propagating waves and shaped to provide said loaded section with first and second respectively associated impedances lower than said characteristic impedance, said bodies having respective lengths in the direction of wave propagation presenting resistive impedance at the input to said section, the loaded section being rendered shorter than a quarter wavelength in that the second body is intermediate the first body and the end member and provides said loaded section with larger lateral dimensions than does the first body and a respectively associated impedance intermediate the respectively associated impedance provided said section by said first body and the characteristic impedance.

8. Apparatus according to claim 7 including a heat sink thermally coupled to said bodies through both the sides of said section and the reflective end member.

9. Apparatus for terminating a coaxial line wave propagation medium having a center conductor and a characteristic impedance, comprising a section of said coaxial line wave propagation medium, a reflective end member for said section, and means for loading said section, including first and second portions of the center conductor having respective first and second diameters larger than the diameter associated with said characteristic impedance and a body of absorptive material coaxially formed about said first and second portions, the second portion of the center conductor being between said first portion and the reflective end members and having smaller lateral dimensions than said first portion.

10. Apparatus according to claim 9 in which the body composed of absorptive material has substantial thermal coupling to the sidewalls and to the reflective end member.

11. Apparatus according to claim 9 in which the body of absorptive material comprises a cylindrical annulus of the absorptive material in contact with the outer conductor of the coaxial line medium and extending over the combined lengths of the first and second portions of the center conductor.

12. Apparatus according to claim 11 in which the outer conductor provides significant heat-sinking capability in the vicinity of the absorptive cylindrical annulus.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 8Ll6 ,720 Dated November 5, 197

Inventor(s) Theodore w Mohr It is certified that error appears in the above-identified patent and that said Letters Patent are hereby' corrected as shown below:

Column 3, line50 z 2 A0.19T should read z w W .l9

Column line l, restore "z" (twice) to larger size.

Signed and sealed this 7th day of January 1975.

(SEAL) Attest: I

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69' i u.s. GOVERNMENT PRINTING OFFICE: I9l9 0-306-334.

FORM PO-IOSO (10-69)

Claims

1. Apparatus for terminating an enclosed wave propagation medium having a characteristic impedance, comprising a section of said medium, a reflective end member for said section, and means for loading said section, including a body composed of material absorptive for the propagating waves and shaped into first and second portions providing said loaded section with respectively associated impedances lower than said characteristic impedance and having respective lengths in the direction of wave propagation presenting resistive impedance at the input to said section, the body being rendered shorter than a quarter wavelength in that the second portion is intermediate said first portion and said end member and provides said loaded section with a respectively associated impedance intermediate the impedance provided by said first portion and said characteristic impedance.