US3010086A - Microwave isolator - Google Patents

Description

Nov. 21, 1961 H. sElDEl.

MICROWAVE lsoLAToR Filed NOV. 17, 1958 A7' TORNEI iinired states @arent dldd Patented Nov. 2l, .196i

3,illi,86 MHCRWAVE ESOLATGR Harold Seidel, llanwood, Ni., assignor to Bell Telephone Laboratories, incorporated, New York, NX., a corporation of New Yaris Y' Filed Nov. i7, lSS, Ser. No. 774,547 8 Claims. (Cl. 3332-24) This invention relates to electromagnetic wave transmission systems and more particularly to transmission structures having nonreciprocal attenuating properties for use in such systems.

The use of materials having gyromagnetic properties to obtain both reciprocal and nonreciprocal effects in microwave transmission circuits is Widely known and has found numerous and varied applications in propagation structures of both the waveguide and the transmission line types. A rsum of early work done in this field is contained in an article entitled The Behavior and Application of Ferrites in the Microwave Region by A. G. Fox, S. E. Miller and M. T. Weiss, Bell System Technical Journal, January 1955, pages -103. The Proceedings of the Institute of Radio Engineers, Volume 44, No. 1G, October 1956, is devoted in major part to a more recent survey of the uses and characteristics of ferrites.

Included among the new transmission components that have found widespread use in the microwave art is the so-called isolaton The isolator may be defined as a circuit element which is transparent to electromagnetic waves propagating therethrough in one direction, designated the forward direction, whereas electromagnetic waves propagating in the opposite, or reverse, direction are attenuated by the isolator to the extent required by the system.

In the above-mentioned article by Fox, Miller and Weiss three distinct classes of isolators are described. l'hese include the Faraday rotation isolators, the fielddisplacernent isolators and the resonance isolators, The advantages and limitations of the several types disclosed are described in an article by C. Bowness entitled Microwave Ferrites and Their Application, published in the July-August 1958 issue of the Microwave Journal, Volume l, page 13. ln brief, all three classes are found to be temperature and frequency sensitive and, in addition, the thin resistive varies or lossy strips used in conjunction with the Faraday rotation isolators and the field displacement isolators, more particularly the latter, tend to unduly limit their power handling capabilities.

It is therefore the object of this invention to introduce nonreciprocal attenuation over a broader band of temperature and frequency variations and at higher power levels.

Recognizing that in the presence of gyromagnetic materials the waveguide modes differ radically in almost every essential detail from the propagating modes normally `associated with the conventional unloaded waveguide, it is proposed to utilize these anomalous modes to produce new and useful results. In particular, it is proposed to utilize that certain class of higher order modes for which the energy tends to concentrate about an interface or boundary of the gyrornagnetic medium. This concentration about the boundary produces extremely large radio frequency magnetic field densities in a relatively small portion of the gyromagnetic materials. Within this region of the material there is induced, by this class of modes, highly turbulent electron spin systems having extreme variations in the alignment of the magnetization vectors associated with such spin systems. In this nonuniformly induced state of alignment of the magnetic spins there is a greater tendency for the material to absorb radio frequency energy, resulting in what may be referred to as a self-loss, nonresonant attenuator. lt

is self-loss in the sense that power absorption takes place in the gyromagnetic medium itself rather than in some external lossy material, and it is nonresonantl in that it operates at -a direct current magnetic biasing field intensity far below that required to induce the usual resonance conditions. This mode of operation is in marked contrast to the arrangement of the spin systems as they exist in the usual resonance attenuators wherein substantially all the magnetization vectors are aligned parallel to the direct current biasing field.

While it is recognized that imperfections in any practical transmission system will tend to induce higher order modes of the type herein considered, the prior art has arduously sought to minimize this tendency by appropriately shaping and proportioning the gyromagnetic materials. Attenuation of microwave energy has been achieved by either using external lossy materials in association with the gyromagnetic material or by resonantly biasing the gyromagnetic material itself. By contrast, it is the purpose of this invention to produce maximum disruption of the normal propagating modes by inserting discontinuities in the wave path in the region of the gyromagnetic material and thereby to convert substantially all of the wave energy to higher order mode energy. These higher order modes being bound very tightly as surface waves to the interface `of the gyromagnetic material are then highly attenuated due to the very inefficient use of said material as a transmission medium. A crude analogy of the operation of an attenuator in accordance with the present invention would be to compare the loss induced in this type of attenuator to that induced in the conventional conduction system when large currents are caused to flow through conductors having extremely small cross-sectional dimensions.

It is ytherefore a more specific object of this invention to introduce attenuation in electromagnetic wave sys tems by inducing a high degree of nonuniformity in the electron spin systems of gyromagnetic materials.

It is a further object of this invention to induce such nonuniformity by concentrating, into a restricted region, the radio frequency magnetic fields of the higher order turbulent modes associated with such materials.

lt is another object of this invention that such attenuation be nonreciprocal.

In accordance with the broad principles of the invention, the intensity and distribution of the higher order turbulent modes characteristic of the transmission modes in gyromagnetic media are greatly enhanced by means of scattering elements longitudinally distributed along the gyromagnetic medium.v The transmission path and the igyromagnetic material are so shaped and oriented with respect to each other as to minimize the effect of the scattering elements for one direction of propagation and to enhance it for propagation in the reverse direction.

ln a preferred embodiment of the invention, the two elements of a two-element transmission path are separated over a longitudinally extending region by a magnetically polarized gyromagnetic medium. One of the elements is smaller than the other of said elements, producing a relatively high field density in the region of the smaller element and adjacent to one of the interfaces of the gyromagnetic material. Scattering means are longitudinally distributed along the interface adjacent to the smaller element. For one direction of propagation along the path,

the higher order modes introduced by the scattering means are concentrated about the adjacent interface of the gyromagnetic material and are attenuated. For propagation in the opposite direction, the higher order modes tend to concentrate about the far interface of the gyromagnetic material, but because of the distance from the scattering elements the coupling is small and the resulting attenuation correspondingly small.

An isolater constructed in accordance with the invention differs fundamentally from the prior art isolators in that its nonreciprocal operation does not depend upon some secondary characteristic of the transmission path created through the direction of propagation of the wave energy, but rather it is an inherent characteristic of the action of the turbulent mode on the gyromagnetic material itself, and depends Adirectly upon the direction of propagation of the wave energy. rThus, by properly shaping and proportioning the two elements and the gyromagnetic material, the attenuation can be controlled as a function of the direction of wave propagation to obtain an isolator having a high reverse-to-forward loss ratio.

It is a feature of the invention that it operates over a broad frequency band and is substantially insensitive to changes in the operating temperature or frequency. It is a further feature of the device that it may be scaled down to very small physical dimensions in a given frequency range, there being no cut-ofi characteristic associated with the device, i.e., there is no requirement that the transverse dimensions of the structure be comparable to a wavelength of the frequencies of interest.

These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings, in which:

FIG. l is a perspective View of the invention showing the scattering means in relationship to the gyromagnetic material;

FIG. 2 shows, by way of illustration, the iield distribution of a dielectric loaded waveguide in a cut-off state;

FIG. 3 shows, by way of illustration, the iield distribution in an anisotropic wave transmission path;

FIG. 4 shows, by way of illustration, the electric eld distribution in the gyromagnetic material;

FIG. 5 shows one possible -type of uniformly distributed scattering means; and

FIG. 6 is a perspective view of the second embodiment of the invention showing alternate scattering means, shaping of the gyromagnetic material, and lthe use of heat sinks embedded in the gyromagnetic material.

In FIG. l there is shown an attenuator inaccordance with the invention comprising a conductive channel, whose cross-sectional dimensions can be small compared to a wavelength, as will be explained in greater detail hereinafter. Suitably supported within channel 10, by beads or other means not shown, is a thin conductive member il. Member 1i extends longitudinally within channel 10 and together with the wide and narrow walls thereof constitute a two-wire Wave supporting structure lil-#11. For convenience, hereinafter, all the two-wire systems described will be referred to as strip transmission lines or simply lines. Y

Located to one side of member 11 and extending the full height of channel and the full width between member l1 and channel 16 is an element 12 of material capable of exhibiting gyromagnetic properties over a range of operating frequencies of interest. The term gyromagnetic material is employed here in its accepted sense as designating the class of magnetically polarizable materials having unpaired spin systems involving portions of the atoms thereof that are capable of being aligned by an external magnetic polarizing iield and which exhibit a significant precessional motion at a frequency within the range contemplated by the invention under the combined influence of said polarizing field and an orthogonally directed varying magnetic iield component. This precessional motion is characterized as having an angular momentum, a gyroscopic moment and a magnetic moment. Typical of such materials are ionized gases, paramagnetic materials and ferromagnetic materials, the latter including the spinels such as magnesium aluminum ferrite and aluminum zinc ferrite, and the garnet-like materials such as yttrium iron garnet.

Element l2, is biased by a steady magnetic eld at right angles to the direction of propagation of wave energy along the line. As illustrated in FIG. l, 'this ield may be supplied by a single solenoid structure comprising a magnetic core i3 having pole pieces N and S bearing against the top and bottom wide walls, respectively, of channel 1). Turns of wire 14 on core i3 are connected through switch and rheostat i6 to a source of magnetizing current 17. The biasing eld, however, may be supplied by an electric solenoid with a magnetic core of other suitable physical design, by a solenoid without a core, by a permanent mag net structure, or element ll may be permanently magnetized if desired.

The amplitude of the biasing field is a function of the signal bandwidth. In particular, .the relationship between the biasing ield and the signal frequency is given by where wh is the upper limit of the frequency band; w1 is the lower limit of the frequency band; 'y is equal to 2.8 106 megacycles per oersted; M is the saturation magnetization, and

H is the effective internal biasing field.

The essence of the present invention resides in the discontinuities 1S distributed along the surface of member 1i nearest the gyromagnetic material l2.

In applicants paper entitled Character of Waveguide Modes in Gyromagnetic Media published in the Bell System Technical Journal, Volume 36, March 1957, on pages 409-426, the transmission of wave energy in isotropic and in anisotropic waveguides is considered. While this article deals specifically with waveguide structures, the nature of the energy distribution of the higher order modes Within the gyromagnetic material is equally applicable to the two-conductor transmission system of FIG. 1. In this article it is shown that in the presence of gyromagnetic materials the waveguide modes diler radically in almost every essential detail from the propagating modes normally associated with the conventional unloaded or isotropic waveguide.

When it is stated that the higher order modes of a guide of restricted size are in a cut-oit state, it means that there is a longitudinally decaying field associated with the cut-olf modes, but that the transverse crosssection of the guide has associated with it waves of an harmonic nature, at least over restricted regions. Such a field distribution is shown in FIG. 2 in which there is shown a waveguide 20 partially filled with dielectric material 21. Here, at least over the dielectric filled section, there are observed transverse harmonic energy distributions. These are represented by vertical variations 22 and horizontal variations 23. The longitudinally decaying iield normally associated with the cut-off state is shown by means of curve Z4.

'In an anisotropic wave transmission path, the wave energy distribution is inverted. That is, cut-olf may be induced with respect to one or both of the transverse dimensions of the path, while retaining the harmonic character of the wave in the longitudinal distribution. In contradistinction to the conventional guide, the energy associated with these higher order modes propagates within the path. This situation is illustrated in FIG. 3 in which there is shown a waveguide 30, partially filled with some gyromagnetic material 31. The vertical harmonic variation is given by curve 33, and the longitudinal harmonic variation, representing a propagating wave, is given by curve 32. The cut-off horizontal transverse distribution is represented by the exponentially decaying wave 34.

There remains, however, one degree of comparison or similarity between the modes of the two differently loaded guides. This similarity resides in the fact that cut-off, whenever induced by the higher order mode distribution, becomes progressively sharper the greater the order of the modes present. This relates equally to the horizontal transverse distribution 34 of FIG. 3 irrespective of the fact that it is a transverse rather than a longitudinal distribution. rl`hus, with increasing order of distribution, the energy is more tightly boundpto the interface of the gyromagnetic material. Since this energy is propagating, the transmission of these higher order modes becomes increasingly inefficient as a consequence of the highly restricted portion of the guide through which the energy is being channeled.

The problem of stimulating the higher order modes is greatly simplified if the gyromagnetic material is initially located in a region of the wave path having a high density, nonuniform eld distribution. These conditions are fairly well met in a two-element transmission path of the type shown in FIG. 1. As may be seen by referring to FIG. 4, the electric field lines il in such a structure are densely concentrated at the boundary of the inner conductor l1, diverge within the gyromagnetic material 12 and terminate on the walls 10. Thus, within the gyromagnetic material 12, the electric field lines have those preferred characteristics which tend to facilitate the generation of the higher order modes of the class heretofore described.

If the energy interchange process between the line and the gyromagnetic material is thought of in terms of a perturbation phenomenon, the device may be considered as consisting of a pair of coupled transmission paths. The first path, comprising the strip transmission line, is a low loss path having a velocity of propagation substantially that of free space, whereas the second path, comprising the incoherently aligned electron spin systems of the gyromagnetic material, is a high loss path having a velocity of propagation an order of magnitude or more slower than the strip transmission line.

Because of the large difference in phase velocities there is, to a first order, no interaction between the two transmission systems. 'lhis situation can be effectively altered by interrupting the longitudinal symmetry of the device in the regions of high current densities. For the twoelement embodiment shown in FIG. 1, high current densities occur over the region in which conductor l1 and material 12 are nearest each other. As shown, the symmetry is interrupted by placing irregularities along the surface of element l1 immediately adjacent to the gyromagnetic material. 'Ihese irregularities or interruptions of random size and distribution create the higher order space harmonics, which, as indicated above, tend to concentrate their wave energy at a boundary of the gyromagnetic material. in the presence of these higher order space harmonics a sympathetic interaction is established between the two transmission system. Energy is scattered at the interface into higher and more complex modes, a process which facilitates the repeated extraction of energy from the two line system at one phase velocity, and transfers it to the gyromagnetic transmission system at another phase velocity. The energy transferred from the line to the gyromagnetic material now finds itself in a far lossier medium and this energy is quickly dissipated.

In a broad-band system, the irregularities are randomly distributed along conductor 1l as in FIG. 1 and have cross-sectional areas which are also randomly varied both as to size and shape. The random nature of both the distribution and the configuration of the scattering elements is intended to avoid specific space harmonic selection which might create excessive frequency sensitivity. Hence, the discontinuities or interruptions of whatever nature are made aperiodic. On the other hand, in a narrowband system, the attenuation per unit length of ferrite may be increased by careful selection of the higher order modes generated. In the narrow-band situation, the scattering elements, as shown in FIG. 5, are uniformly distributed to provide periodic interruptions ofthe incident Wave. Thus, in FIG. 5, indentations of equal size and shape are longitudinally distributed along element lll. The indentations are physically separated from each other by the longitudinal intervals 5l. The periodic spacing tends to favor the space harmonics associated with a relatively narrow band of frequencies, thus making the attenuator more frequency sensitive than the random arrangement of FIG. 1.

The higher order modes have been characterized as surface waves bound to some interface of the gyrornagnetic material. It may be shown from analysis that, over a given frequencyrange and in the presence of a given polarizing field Hdc, the energy in the gyromagnetic material is primarily bound to the far surface 19, adjacent to the wall of channel lil, for wave propagation in the forward direction, while it is primarily bound to the near interface 20, adjacent conductor l1, for propagation in the reverse direction. The energy coupled between the strip transmission line mode and the modes in the gyromagnetic material at either interface is related to the integrated product of the relative fields associated with each of these modes. Since the overlap of the mode fields is very large in the reverse direction because of the proximity of interface 263` to the conductor 11, a significant coupling of energy occurs, which energy is eventually dissipated in the gyromagnetic material. Conversely, the overlap of the fields is very small for propagation in the forward direction because of the relatively large distance between surface 19 and the conductor l1. Consequently, the energy exchange and the resulting attenuation are far less for propagation in the forward direction than for propagation in the reverse direction. As a consequence, the operation of an attenuator so constructed is nonreciprocal.

FIG. 6 shows an alternate embodiment of the invention in which the mode conversion mechanism has been altered and in which the isolation ratio is improved and other minor changes made. Thus, for example, because the attenuation is not particularly affected by the width s of conductor 1l in FIG. l, it has been replaced in the embodiment of FIG. 6 by a smooth cylindrical wire 6l. In the embodiment of FIG. l, the non-uniformity in the system which produces mode conversion resided in changes in the boundary conditions along conductor 1-1 in the region of the gyromagnetic material. In the embodiment of FIG. 6, mode conversion is induced by causing changes in the dielectric 'constant of the wave path in the region of the gyrornagnetic material. This is accomplished by replacing the single element of gyromagnetic material with numerous smaller segments 62 of a gyromagnetic material having a high dielectric constant such as, for example, one of the ferrites, which are longitudinally distributed along the wave path. The effect of breaking up the gyromagnetic material in the manner shown is to allow the surrounding dielectric material (in this embodiment, air) to fill the regions 63 between segments 62;. Because of the large difference in the dielectric constants between ferrite and air, substantial local changes in the effective dielectric constant of the wave path are established in the immediate vicinity of the ferrite material. The overall effect is to create, by means of such dicontinuities in the wave path, a plurality of dielectric scattering elements along the inner surface of elements 621. Obviously, the choice of dielectric material used must be considered in relationship to the type of gyromagnetic material used in order to produce the desired local changes in the dielectric constant of the Wave path over the region of interest.

The purpose of tapering the segments 62 is to reduce the forward loss of the isolator by making it more difficult for the higher order modes to tend tol be established on the far interface. While the forward losses in this type of isolator tend to be small, they are nevertheless nite. lt would be desirable to be able to reduce them still further if possible. This can be done by increasing the difference in the phase velocity between the low loss strip transmission line and the high loss path which, for the forward direction is that portion of the ferrite near the far interface. Since the phase velocities of the high order modes vary directly as the height of the gyromagnetic material in the vicinity of the interface and inversely as the mode order, decreasing the height for a given mode, decreases the already slower mode phase velocity in the gyromagnetic material still further. Thus, as the disparity in the relative phase velocities in the two paths increases, the attenuation per unit length decreases. The near interface, however, being the full height of the channel, is unaffected, and coupling to this surface, in the reverse direction, is not reduced.

Because the wave energy concentrated within the gyromagnetic material is so intimately associated with its boundary, conductive heat sinks may be imbedded within the material to improve the heat dissipating properties of the -attenuator and increase its power handling capabilities. Such an arrangement is also included in FG. 6 wherein a series of conductive posts 64 extend from conductive Iwall 65 of channel 60 to about halt' way into each of the segments 62. Energy attenuated within the material generates heat which is conducted by the posts 64 to conductive surface 65 and dissipated to the surrounding environment. This has the eect of reducing the size of the isolator for a given power level or increasing the permissible power level for a given size isolator. It is obvious that such an arrangement can be used with other embodiments of this type of isolator.

It had been mentioned earlier that an isolator of the type described herein could be made extremely small compared to a wavelength. This results from the fact that the strip transmission line, being a two-conductor system, does not have a cut-cti frequency in the sense that a waveguide transmission path does. Hence, the dimensions of the strip transmission line (or low-loss path in this type isolator) can be made extremely small. Similarly, the higher order modes in the gyromagnetic material (which comprises the high-loss path) are also substantially independent of the free-space wavelength. It is rather the distribution of these modes in the gyromagnetic material that provides the nonreciproeal losses, and these tend to maintain a constant ratio that is substantially independent of guide size. However, there is a limitation which was mentioned with regard to the structure of yFlG. 6 and which must be considered in designing small attenuators. As mentioned earlier, reducing the height of an interface increases the relative diierence in' phase velocity between the wave energy propagating along the strip transmission line and the higher order modes which tend to establish themselves at the surfaces of the gyromagnetic material. The phase velocity of the wave energy propagating along the strip transmission line remains substantially constant as its size decreases. However, the phase velocity of the higher order modes decreases with decreasing height. This additional difference in phase velocities tends to reduce the coupling from the strip transmission line mode to the higher order modes. Thus, while the reversetoforward loss ratio remains substantially constant, the attenuation per unit length decreases as the size of the isolator decreases. The design of the isolator is, therefore, a matter of compromise between cross-sectional size and length, with any particular design depending upon the particular application at hand.

in all cases it is understood that the above-described arrangements are illustrative of a small number of thc many possible specihc embodiments which can represent applications ol the principles of the invention. Numerous and varied other arrangements can readily #be devised in accordance with these principle-s by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. An isolator for electromagnetic wave energy com* prising a two-element transmission path supportive of said wave energy in the TEM mode of wave propagation over a range of operating frequencies, one of said elements having a surface area substantially smaller than the surface area of; the other of said elements, an element of magnetically polarizable material exhibiting gyromagnetic eltects over said frequency range and supportive of a class of higher order modes of wave propagation different than said TEM mode disposed between'at least a portion of said unequal surface areas, means for mag-l netically biasing said material in a direction transverse to the direction of wave propagation along said path and means for coupling substantially all of said wave energy `from said TEM mode of wave propagation to said higher order modes comprising a plurality o sub stantially lossless electrical discontinuities longitudinally distributed along the interface of said material adjacent to said smaller surface.

2. The combination according t0 claim 1, wherein said discontinuities comprise a plurality of successive changes in the boundary conditions along said smaller surface.

3. In an electrical system a two-conductor transmission path supportive of electromagnetic wave energy comprising a conductively bounded enclosure having a longitudinally extending conductive member disposed therein, an element of magnetically polarizable material exhibiting gyromagnetic eiiects over the operating frequency range of said path partially lilling said enclosure asymmctrically located within said path between said member and a portion of the inner surface of said enclosure, means for magnetizing said element in a direction transverse to the direction of wave propagation and means for inducing a system of higher order modes concentrated along one of the boundary surfaces of said element comprising a plurality of irregularities longitudinally distributed along said conductive member over an interval coextensive with said element, said inducing means and said material being in coupling relationship to dissipate within said material substantially all of said wave energy.

4. An isolator comprising a section of two conductor transmission line supportive of a given mode of wave propagation having a solely transverse electric and a solely transverse lmagnetic. eld distribution, the first of said conductors having a sur-face area substantially greater than the surface'area of the other of said conductors, means for applying wave energy to said line in said given mode having a first velocity of propagation, and a plurality of magnetically polarized elements of gyromagnetic material longitudinally distributed along a region of said line disposed between a portion `of the sur-faces of said two onductors, said elements being supportive of a class of higher order modes of wave propagation having propagation velocities substantially different than said iirst velocity of propagation, said plurality of elements comprising means for coupling substantially all of said wave energy from said given mode of wave propagation to said higher order modes of wave propagation to dissipate within said elements the energy associated with said higher order modes for only one direction of propagation.

5. The combination according to claim 4 wherein each of said elements of gyromagnetic material has a transverse dimension in that portion of said region adjacent to said first conductor that is substantially smaller than the transverse dimension of each of said elements in that portion of the region adjacent to said other conductor.

6. The combination according to claim 4 wherein said elements have substantially the same size and shape and are uniformly distributed along said path.

7. The combination according to claim 4 wherein said elements are of unequal size and shape and are aperiodically distributed along said path.

8. The combination according to claim 4 wherein at 9 least one conductive post extends into each of said ele- 2,849,683 ments, said posts connecting to the rst of said conductors. 2,849,684 2,922,125 References Cited in the le of this patent UNITED STATES PATENTS 5 2,777,906 shockley Jan. 15, 1957 13,223 2,834,947 Weisbaum May13,1958 6 1 10 Miller Aug. 26, 1958 Miller Aug. 26, 1958 Suhl Jan. 19, 1960 FOREIGN PATENTS Australia Aug. 6, 1958 France Sept. 15, 1958

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