Broadband Circulator Having Multiple Resonance Modes

Abstract

A junction circulator in which the usual magnetically biased gyromagnetic post is divided into two parts of different size so that each part is capable of supporting a resonance mode respectively spaced from the other in frequency within the intended broadband of the circulator to extend the range in which the mode phase relationship required for circulation is extended. Conductive cores may be located in one or both of the parts or the parts may be located in separately formed conductive cavities to introduce other mode resonances to further extend the band.

Description

BACKGROUND OF THE INVENTION

This invention relates to symmetrical coupling devices for electromagnetic wave energy and, more particularly, to very broadband waveguide Y-junction circulators.

The basic Y-junction circulator comprises a conductively bounded junction of three waveguides having a magnetically biased gyromagnetic body extending along the axis of symmetry of the junction. Numerous variations of this basic structure, principally having to do with the size and shape of the gyromagnetic body and with means for matching its impedance to the remainder of the structure, have been proposed to improve one or another of the operating characteristics of the circulator.

It is now clearly understood that circulator action depends upon the relationship between the responses of the junction to three modes supported in the junction, namely, as in-phase mode and two counter-rotating modes, the reflection coefficients of which must be mutually displaced in phase by 120.degree.. The differences in bandwidth of various forms of circulators depend upon the degree to which it is possible in a particular structure to maintain this phase relation as frequency is changed.

In the copending application of the inventor Owen hereof, Ser. No. 7,872, filed Feb. 2, 1970, now U.S. Pat. No. 3,617,946 granted Nov. 2, 1971, there is disclosed a circulator of improved performance wherein the usual magnetically biased gyromagnetic, cylindrically-shaped post extending along the axis of symmetry of the junction is foreshortened to create a dielectric discontinuity between one conductive boundary of the junction and one end of the post. At the same time a conductive core is extended from one conductive boundary of the junction, part of the way along the axis of the post. In general the dielectric gap causes counter-rotating electric fields to be induced in the gyromagnetic cylinder normal to the magnetic bias thereby exciting dielectric waveguide modes for which the gyromagnetic body acts as a tuned resonant structure. The net phase shifts for these modes with the gyromagnetic material unmagnetized are identical and are determined by the electrical length of the cylinder. Magnetizing the cylinder, however, increases and decreases the path lengths of the counter-rotating modes respectively, and by adjusting the biasing field and the length of the cylinder, these modes can be separated by 120.degree. from each other as required for circulator action. The conductive core, on the other hand, tunes the in-phase mode. These three modes can thus be tuned independently in a way which results in their reflection coefficients being displaced by 120.degree. over a band. The width of this band depends upon the extent over which the reflection coefficients versus frequency for the modes can be made to run parallel to each other, i.e., have equal and constant slopes. Since these characteristics are, however, basically resonant characteristics, they include steep slopes in the vicinity of the resonance and near zero slopes at frequencies removed from the resonant frequency so the bandwidth of circulation is finite.

Summary of the Invention

In accordance with the present invention is has been discovered that the bandwidth of the foregoing circulator can be extended several times the prior bandwidth by introducing multiple resonances so spaced in the frequency spectrum from each other that the characteristics of reflection coefficients versus frequency merge to form a continuum of constant slope across the broadband. More particularly, two separate ferrite cylinders are included in the junction, each providing a different electrical path length for each of the counter-rotating modes that is respectively resonant at a different one of a pair of spaced frequencies. In accordance with a further embodiment a thin conductive pin is located axially within each cylinder, each having a different axial length to provide double resonances for the in-phase mode. In a further embodiment it will be shown that the transition region between the separate characteristics can be smoothed by use of a broadly resonant conductive platform adjacent to one conductive boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the prior art junction circulator as disclosed in the above-mentioned copending application;

FIG. 2 is a diagram of reflection coefficients versus frequency as found for the modes in the prior art, and for comparison, those modes as found in several embodiments according to the invention;

FIG. 3 is a cross section taken through the junction region and showing the modified gyromagnetic structure and tuning pin in accordance with the invention;

FIGS. 4 through 7 are cross sections showing arrangements of gyromagnetic bodies and tuning pins alternative to that of FIG. 3;

FIG. 8 illustrates a further improvement of the invention including a broadly resonant transformer structure; and

FIG. 9 is a diagram of reflection coefficients versus frequency for the embodiment of FIG. 8.

DETAILED DESCRIPTION

Referring more particularly to FIG. 1, the prior art circulator according to the above-mentioned copending application is shown comprising three rectangular waveguides 20, 21 and 22 intersecting in a Y at angles 120.degree. in an H-plane junction (the plane of the guide broad dimension) to form a conductively bounded common region from which the waveguide branches symmetrically extend. Extending coaxially with the axis of symmetry of the Y within the junction is a cylinder 25 of gyromagnetic material, such as yttrium iron garnet or ferrite. Cylinder 25 is biased along the axis of symmetry by being permanently magnetically polarized or polarized by the use of external magnetics as represented schematically by the vector HDC. Cylinder 25 has a small hole 24 drilled along its axis. The top end of cylinder 25 is contiguous to the top conductive boundary 23 of the common region and a gap 27 filled either by air, or by a suitable non-magnetic dielectric material having dielectric constant close to that of air or at least substantially different from that of cylinder 25, forms a space between the lower end of cylinder 25 and the lower conductive boundary of the junction. A thin conductive pin 28 is located axially within hole 24 and is conductively connected to the top conductive boundary. A conductive platform 26 raises the lower conductive boundary, shortens gap 27 and acts as an impedance matching transformer.

Operation of such a circulator is usually explained by dividing the excitation of one port of the junction into three excitations each involving excitation of all three ports. The three excitations correspond to the eigenvectors for the scattering matrix for the junction. A first excitation excites all three ports equally and in phase while the remaining two excitations result in equal excitations with phases that result in counter-rotating circulator polarizations within the junction. The requirement for circulation in terms of these excitations is that their reflection coefficients corresponding to the eigenvalues for the scattering matrix be displaced in phase by 120.degree..

The significance of gap 27 can be understood when it is recalled that in an ordinary H-plane resonant junction, the electric fields are everywhere parallel to the axis of symmetry. The region formed by gap 27, however, has a dielectric constant and permeability product that is different from that of the region occupied by the gyromagnetic material of cylinder 25 so that the phase constants of the two regions differ. This creates an electric field in the plane of the interface between the two regions. Thus, the counter-rotating excitations launch waves as dielectrically supported modes in cylinder 25, travelling up cylinder 25 to be reflected at boundary 23 and to couple back into the junction at gap 27. Thus, the phase of the counter-rotating modes are determined by the length of cylinder 25 and the degree of its magnetic polarization.

In the absence of the pin 28 the ferrite cylinder 25 does not support an in-phase mode with transverse electric fields. With pin 28, however, such a mode is supported. Being confined to the region of the pin, its resonances are determined by the length of pin 28. As noted above, the counter-rotating modes have only transverse electric fields at the axis of symmetry, and, therefore, are essentially not affected by pin 28.

The relationships can be seen from FIG. 2 which shows typical reflection coefficients 31, 32 and 33 in phase degrees of the three modes in the above-described prior art structure as they vary with frequency. Thus, the counter-rotating modes as represented by curves 31 and 32 are resonant characteristics. A moment's reflection will indicate that the center of the linear region designated by point 30 corresponds to that of zero phase shift at resonance at a frequency F.sub.1, when cylinder 25 is unmagnetized. The low frequency end corresponds to a range of +180.degree. and the high frequency end corresponds to a resonant phase shift of -180.degree.. Curves 31 and 32 are phase separated above and below point 30.degree. by 60.degree. by controlling the Faraday rotation parameters of cylinder 25, including its length, composition and magnetization. Pin 28 is then employed to position the in-phase mode curve as represented by curve 33, also a resonant characteristic, so that its most linear portion falls within the band of intended operation in a given junction with a phase 120.degree. away from the phase of the nearest rotating mode of curve 31. Circulation is then possible over the range designated A in which the curves generally parallel each other as indicated.

Note that this range A is bounded on the high and low frequency ends by regions in which the phase characteristics become non-linear as they follow the typical pattern of phase shift in a resonant circuit. In these high and low regions the phase spacing between the characteristics as required for circulation no longer exists and is responsible for the band limiting effects in the prior art structures.

With this background in mind, the principles of the present invention may be understood from FIG. 3. In all cases in which the structures, materials or principles of operation are the same as those described above, the detailed description thereof need not be repeated.

Referring then to FIG. 3, a waveguide junction is shown only in a cross section taken through the junction region but corresponding in every way to the kind of junction formed by guides 20, 21 and 22 of FIG. 1 and for which the top and bottom conductive boundaries are designated 40 and 41, respectively. Gyromagnetic cylinders 42 and 43 are included on the axis of symmetry of the junction with an end of each adjacent respectively to the top and bottom conductive boundaries 40 and 41. In the embodiment illustrated both cylinders 42 and 43 have equal diameters. In accordance with the invention, however, cylinder 42 has an axial length l.sub.1 equal .lambda..sub.1 /4, where .lambda..sub.1 is the electrical wavelength at the frequency F.sub.1 mentioned in connection with FIG. 2 and cylinder 43 has an axial length l.sub.2 equal .lambda..sub.2 /4, where .lambda..sub.2 is the electrical wavelength at the frequency F.sub.2, shown on FIG. 2 above F.sub.1. Since the cylinders act as quarter wave shorted stubs for the counter-rotating modes, they are resonant, respectively, at the frequencies F.sub.1 and F.sub.2.

A conductive pin 44, similar to the one described in connection with FIG. 1, is included in an axial hole within cylinder 42 to tune the in-phase mode in the manner of the prior art to a dielectric mode resonance in body 42. Considering cylinder 42 and pin 44 as a coaxial shorted stub it will be convenient to think of this resonance as one which occurs when the electrical length of pin 44 is one-quarter wavelength at the in-phase mode frequency. In accordance with the present invention this resonance should occur at a frequency such as F.sub.3 on FIG. 2 which lies between F.sub.1 and F.sub.2. Thus, the previously described characteristic 33 is descriptive of the reflection coefficient produced by the in-phase resonator including pin 44. While the in-phase mode resonance will be treated hereinafter in terms of the conductive pin, it should be kept in mind that the in-phase mode may be tuned without a conductive pin by proper control of the diameter of cylinders 42 or 43, exclusively, or by any resonant structure which couples primarily or differentially the in-phase mode in the band of interest. An example of this principle will be specifically illustrated in connection with FIG. 7 hereinafter.

Assuming then that the previously described characteristics 31 and 32 are also descriptive of body 42, reflection coefficients of body 43 may be defined by characteristics 34 and 35 of FIG. 2. The frequency F.sub.2 must be located so that the +180.degree. phase shift point of curves 34 and 35 overlaps, that is, occurs at a lower frequency, than the -180.degree. phase shift portion of curves 31 and 32. In addition, frequency F.sub.2 must be sufficiently far from F.sub.1 that the two resonances do not "pull" each other and degenerate into a single resonant mode. When these criteria are met, the reflection coefficients sum to form a continuum between linear portions of the respective characteristics as represented by the dotted characteristics 36 and 37. Circulator action is, therefore, achieved over the range designated B extending over the full linear portion of curve 33.

While cylinders 42 and 43 are illustrated as having equal diameters and different physical lengths, it should be noted that their electrical length is the operative parameter and may be effected by changing diameters and/or the dielectric constants of the materials from which they are formed either with or without different physical lengths. It has been found that the presence or absence of the pin-containing holes does not appreciably affect their electrical lengths.

In FIG. 4 the region of circulation is further extended by introducing a second conductive pin 53 into the second cylinder 54 which provides a second resonance for the in-phase mode forming a continuum with the resonance of the first pin 44 in cylinder 52. Note that cylinder 54 is short and fat and cylinder 52 is long and thin. In both cases it will be assumed that these dimensions are proportioned to produce resonances more or less like those described in FIG. 3.

Conductive pins 53 and 44 are both similar to pin 44 of FIG. 3. Pin 44 has a length that is a quarter wavelength for the in-phase mode at a frequency F.sub.3. Additionally pin 53 has a length that is a quarter wavelength for this mode at a higher frequency F.sub.4 to produce a new resonance as shown by the broken line characteristic 39 on FIG. 2. As shown on FIG. 2, the frequency F.sub.3 will in general fall between F.sub.1 and F.sub.2 and the frequency F.sub.4 will fall above the frequency F.sub.2. Furthermore, the frequency F.sub.3 and F.sub.4 are so spaced from each other that their respective resonance characteristics 33 and 39 merge as shown by the dotted characteristic 38. Circulator action is now obtained over the band C extending from the low frequency end of the linear portion of characteristic 33 to the high frequency end of the linear portion of characteristic 35. Since the mode involved in the pin resonance is the lowest order mode for a dielectric waveguide with a central conductor, the pin length required for the resonance for the in-phase mode is generally less than the ferrite length required for the resonances in the rotating modes. Thus, the pins normally do not project from the ferrite bodies.

FIG. 5 illustrates an alternative embodiment of the invention in which the relative positions of the components are, in effect, reversed. Specifically, a pair of gyromagnetic cylinders 55 and 56 having different lengths and being separated by a thin conductive septum 57, are suspended between conductive boundaries 40 and 41 by spacers of low dielectric constant material 58 and 59. Conductive pins 60 and 61 are included within cylinders 55 and 56, both pins extending from conductive septum 57. The dimensions of cylinders 55 and 56 and of pins 60 and 61 are chosen as described above. Thus, the counter-rotating modes are generated at the gaps produced by dielectric spacers 58 and 59, propagate in opposite directions to be respectively reflected by septum 57 interposed at different distances from the dielectric gaps to produce the round trip phases as described above. Pins 60 and 61 act as separate quarter wavelength shorted studs for the in-phase mode having lengths similarly measured from the terminating septum 57.

In each of the foregoing embodiments the resonant structure for either the in-phase mode or for the counter-rotating modes took the form of a quarter wavelength shorted stub at the appropriate wave frequency. It is possible, however, to obtain these resonances with an equivalent half wavelength open stub and such may be an advantage in a particular embodiment at extremely high frequencies where fractional wavelengths become increasingly small. FIG. 6 shows one such illustrative embodiment. Thus, gyromagnetic cylinders 71 and 72 are supported and separated by low dielectric constant spacers 73 and 75 between conductive boundaries 40 and 41 and by spacer 74 between the cylinders. Cylinders 71 and 72 are resonant for the rotating mode at the required frequency when they are one-half wavelength long. Similarly, pins 76 and 77, included within the bodies, are resonant for the in-phase mode when they are one-half wavelength long.

At frequencies in the millimeter wave range even a one-half wavelength pin causes problems because of its small dimension, the necessity of the hole in the gyromagnetic cylinder, and the difficulty of making good electrical contacts with the conductive waveguide housing or septum.

FIG. 7, therefore, illustrates one further alternative for tuning the in-phase mode. Conductive boundaries 80 and 81 are provided with conductive enlargements forming cylindrical cavities 82 and 83 extending concentrically with the axis of symmetry of the junction. The gyromagnetic bodies 84 and 85 are seated in cavities 82 and 83 and typically, but not necessarily, fill said cavities. The cavity diameters must be large enough to permit transmission of the lowest order TM mode when the cavities are treated as sections of gyromagnetically loaded circular waveguide. In this embodiment, all three modes couple to resonances determined by the axial dimensions of the junction. The resonances for the in-phase mode are determined by the lengths of the cavities 82 and 83, since the RF field modes involved cannot be supported in the unshielded portions of the gyromagnetic bodies. However, the rotating modes couple to RF field modes on the unshielded portion of the gyromagnetic bodies and these modes blend smoothly into RF field modes in the shielded cavity portion. Thus, for example, the in-phase mode sees resonances when the cavities 82 an 83 are one-quarter wavelength long while the rotating modes see resonances when the gyromagnetic bodies 84 and 85 are three-quarter wavelength long. The dimensions of the cavities and gyromagnetic bodies are proportioned to produce the separate spaced resonances as described hereinbefore. The dimensions of the resonators and the gyromagnetic bodies are then proportioned to produce the separate, spaced resonances as described hereinbefore.

In accordance with a further feature of the invention, the separate frequencies of resonance may be more widely spaced from each other to further increase the band of circulation by the use of one additional resonant circuit common to each of the previously described resonances. Referring to FIG. 8, the junction region includes a large diameter, conductive disk or platform 91 adjacent conductive boundary 40. Adjacent to disk 91 is a first cylinder of gyromagnetic material 92 including a conductive pin 93 connected to disk 91. A second cylinder of gyromagnetic material 94 is located adjacent conductive boundary 41 and includes a second conductive pin 95. In general, the function of conductive disk 91 is to produce a low Q resonance that merges with and smoothes out the separate relatively high Q resonances of gyromagnetic cylinders 92 and 94, and of conductive pins 93 and 95. The nature of this smoothing and the proportions of all components required therefor may most easily be explained by reference to the reflection coefficient versus frequency characteristics of each of the components.

Referring to FIG. 9, it may be assumed, therefore, that the resonances in the low frequency portions of curves 101 and 102 centered about F.sub.1 represent the counter-rotating resonant characteristics for cylinder 94 while the high frequency portions of curves 101 and 102 represent the corresponding resonances centered about F.sub.2 represent the counter-rotating resonant characteristics for cylinder 92. The low and high frequency resonances in curve 103 at F.sub.3 and F.sub.4, respectively, may be assumed to represent the resonances associated with pins 95 and 93. In accordance with the invention, the separations between F.sub.1 and F.sub.2 and between F.sub.3 and F.sub.4 are substantially equal and significantly greater than the corresponding separations described with reference to FIG. 2. More particularly, they are spaced so that there is little if any overlap between the two resonances in any one of the curves 101, 102, or 103, and each resonance retains its characteristic S shape spanning the reflection coefficient phase range of 360.degree.. A typical spacing between F.sub.1 and F.sub.2 and between F.sub.3 and F.sub.4 is in the order of 20 percent of the midband operating frequency F.sub.5. Disk 91 is proportioned to provide a resonant transformer between the gyromagnetic cylinders and the connecting waveguides. The resonant frequency of the transformer is typically at or near the center operating frequency F.sub.5. The disk, 91, typically, but not necessarily, has dimensions of one-quarter wavelength at F.sub.5 as measured from the outer edge of the gyromagnetic cylinder 92 to the outer edge of the disk. The broken curves 104, 105 and 106, included here for illustrative purposes, have been generated from curves 101, 102 and 103 by a simple impedance transformation through such a quarter wave transformer. This transformation may be made using the well-known transmission line equations for input impedance or it may be carried out graphically on a Smith Chart. The procedure is to convert the reflection coefficients of FIG. 9 to impedances, transform the impedances through the quarter wave line, and reconvert the impedances to reflection coefficients. The residual ripple on curves 104, 105 and 106 is determined by both the shape of the curves 101, 102 and 103 and the impedance and length of the transformer. When the transformer impedance and length is optimized for minimum ripple, the phase separation between the transformed reflection coefficients is substantially 120.degree. from F.sub.6 to F.sub.4 or over the band D of FIG. 9.

In all cases in which quarter wavelength sections are specified it is understood that any odd number of quarter wavelengths is obviously intended. Likewise when half wavelength sections are specified, it is understood that any even multiple of quarter wavelengths is intended.

Claims

What is claimed is:

1. A circulator operating over a broadband of electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded region, first and second axially aligned bodies of magnetically polarized gyromagnetic material disposed upon the axis of symmetry of said common region, means including said bodies for producing a first pair of spaced resonances within said band for each of a pair of counter-rotating electric fields induced in said common region, and means for producing a second pair of spaced resonances within said band for an electric field along said axis of symmetry, one of said second pair of resonances falling at a frequency between said first pair of resonances and the spacing between said resonances being such that said resonances produce a broad frequency range in which the phases between said electric fields are maintained at substantially 120.degree..

2. A circulator operating over a broadband of electromagnetic wave energy comprising a conductively bounded structure having a plurality of branches symmetrically extending away from a conductively bounded region, first and second axially aligned cylinders of magnetically polarized gyromagnetic material disposed upon the axis of symmetry of said common region, the sum of the axial lengths of said cylinders being such relative to the distance between conductive boundaries measured along said axis to leave a dielectric gap adjacent to at least one end of each of said cylinders, said axial lengths being significantly different from each other and so proportioned that each body is resonant for counter-rotating modes at a respectively different frequency falling within said broadband.

3. The circulator of claim 2 wherein said dielectric gap is located between opposing ends of said cylinders.

4. The circulator of claim 2 wherein said conductive boundaries are enlarged in the area of said axis of symmetry to form cavities and wherein said cylinders are seated in said cavities.

5. The circulator of claim 2 including a thin conductive core within at least one of said gyromagnetic cylinders having such a length as to produce a resonance for a third mode different from said rotating modes at a third frequency falling between the different frequencies of said rotating modes.

6. The circulator according to claim 5 wherein the electrical lengths of said cylinders are one-quarter wavelength of the frequency for which they are resonant and wherein an end of each cylinder is terminated in a conductive discontinuity.

7. The circulator according to claim 5 wherein the electrical lengths of said cylinders are one-half wavelength of the frequency for which they are resonant and wherein the ends of each cylinder are terminated in a dielectric discontinuity.

8. The circulator of claim 5 including a thin conductive core within at least one of said gyromagnetic cylinders having such a length as to produce a resonance for a third mode different from said rotating modes at a third frequency falling between the different frequencies of said rotating modes, a second thin conductive core within the other of said gyromagnetic cylinders having such a length as to produce a resonance for said third mode at a frequency different from said third frequency, and a conductive body disposed on said axis and acting as a low impedance transformer to smooth together the characteristics of said resonances between said frequencies.

9. The circulator of claim 5 including a thin conductive core within the other of said gyromagnetic cylinders having such a length as to produce a resonance for said third mode at a frequency different from said third frequency.

10. The circulator according to claim 9 wherein the spacings between said frequencies of resonance are such as to produce a broad range in which the reflection coefficients of said respective modes are different by 120.degree. at substantially every frequency within said range.