US3048801A

US3048801A - Non-reciprocal phase shifter and attenuator

Info

Publication number: US3048801A
Application number: US818919A
Authority: US
Inventors: Robert L Fogel; Herbert T Suyematsu
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1959-06-08
Filing date: 1959-06-08
Publication date: 1962-08-07
Anticipated expiration: 1979-08-07
Also published as: GB873672A; DE1107303B

Description

United States Patent Qfifice 3,048,831 Patented Aug. 7, 1962 3,048,801 NON-RECIPROCAL PHASE SHIFTER AND ATTENUATOR Robert L. Fogel, Torrance, and Herbert T. Suyematsu,

Los Angeles, Calif., assignors to Hughes Aircraft Company, Culver City, Calif, a corporation of Delaware Filed June 8, 1959, Ser. No. 818,919 3 Claims. (Cl. 333-31) The present invention relates to non-reciprocal wave transmission and, more particularly, to a coaxial slowwave structure, partially disk-loaded, having selectively magnetized ferrite elements.

A number of non-reciprocal wave trans-mission devices have been developed for microwave frequencies. The more recent, and most effective of these, utilize magnetized ferrite elements suitably mounted in a waveguiding structure to provide the required non-reciprocal characteristics. The principle of operation upon which these devices are based is the action of the magnetized ferrite element to pass energy in one direction and attenuate or absorb the energy in the other direction. For this operation to attain, it is necessary that the ferrite element be mounted Within the waveguiding structure at a position Where is a net component of circularly polarized radio frequency magnetic field of the mode that is propagating.

In most of the known non-reciprocal ferrite devices, the circularly polarized radio frequency magnetic field exists in the mode of the energy that propagates in the absence of the ferrite element. The ferrite is placed in a position so that the resulting mode has its maximum component of circularly polarized magnetic field in the region of the magnetized ferrite. Thus, for a rectangular waveguide propagating energy in the dominant mode, the ferrite element is mounted within the waveguide to include a position located half way between the longitudinal center line and one side of the broad walls. While this structure is the more common, it is possible under certain conditions to obtain non-reciprocal wave transmission by placing the ferrite element in a position where the radio frequency magnetic field is linearly polarized. Upon magnetization of a ferrite so positioned, a component of circularly polarized magnetic field is created within the ferrite because of the gyromagnetic action. The non-reciprocal efiects resulting from this type of operation are second order, however, and less efiicient than the previously described operation.

Non-reciprocal devices as described in the foregoing paragraphs are suitable for operation at the higher microwave frequencies; however, when such devices are designed for operation at the lower microwave frequencies and at the ultra-high frequency region, they are extremely large. Since coaxial transmission lines are smaller, they are more suitable for the propagation of energy at such referenced lower frequencies and a new approach is required because the radio frequency magnetic field of the dominant TEM mode is linearly polarized. Second order non-reciprocal effects may be readily achieved but for the more eflicient first order non-reciprocal effects, one of several other approaches appear to be more feasible.

One of the referenced aproaches for achieving first order non-reciprocal effects in coaxial transmission line involves the propagation of the first higher order T=E mode which has a circularly polarized component of radio frequency magnetic field. In order to propagate this TE mode within a given size coaxial line, the frequency required is much higher than that of the TEM mode for the same line, or for a given frequency the size of the line required to support the TE mode is much larger. The differences are such that the advantages of the coaxial line over the hollow waveguiding structures are lost. Also energy progagated in this TE mode tends to be converted into the dominant TEM mode by mechanical and electrical asymmetries in the line, such as bends, junctions, etc.

Another approach for obtaining first order non-reciprocal efiects in coaxial transmission line is to provide the line with an inhomogeneous cross section so that the TEM mode cannot be progagated. The dominant mode for this modified type of coaxial line is a hybrid mode containing both the TE and TM modes, although in a few special cases, the dominant mode can be solely the TE or the TM mode. This principle has been utilized with coaxial lines in which the line has been half filled with a material having a high value of dielectric constant. The boundary conditions imposed by this geometry require the existence of longitudinal components of the radio frequency magnetic field which in combination with the circular components of the radio frequency magnetic field provides the required circular polarization at the air-dielectric interface. With rods of ferrite material disposed along the interface and in presence of a static magnetic field non-reciprocal wave transmission is achieved. Several geometries for the dielectric loading of the coaxial line have been explored together with the suitable positioning of the ferrite elements for non-reciprocal Wave transmission.

In'this latter type of coaxial structure, i.e., a structure in which the inhomogeneity is non-circularly symmetric, the relation between the magnitude of the circularly polarized radio frequency magnetic field and the characteristics of the dielectric loading material is not rigorously known because the exact solutions to the wave equation have not yet been obtained. Experimental and approximate theoretical results show that the magnitude of the non-reciprocal effects increases as the value of the dielectric constant for the loading material increases.

To more clearly understand the foregoing results in coaxial transmission line having an inhomogeneous cross section, the difference between the free-space velocities of propagation of the separate media will be assumed to cause a distortion of the principal mode. If the coaxial line is completely filled with either of the different media, a TEM mode will propagate with the free-space velocity of that medium and there will he no longitudinal component of the radio frequency magnetic field. If the cross section of the line is made inhomogeneous, the energy will try to propagate in each medium with the freespace velocity of propagation of that medium. Since the energy must propagate with the same velocity over the entire cross section, however, the velocity of propagation of the resulting distorted mode (in the general case, a hybrid TE plus TM) will have a value between the limiting values of the free-space velocities of propagation of the individual medium and the hybrid mode, in the case under consideration, will have a component of circularly polarized radio frequency magnetic field. It is, therefore, readily apparent that as the ratio of velocities of propagation of the two media is increased, the amount of distortion of the original mode (and, thus in the present instance, the magnitude of the component of the circularly polarized radio frequency magnetic field) is also increased.

While non-reciprocal wave transmission is attainable in accordance with the foregoing, it is to be recognized that in dielectric materials, the dielectric loss increases as the value of the dielectric constant increases. As a result of such relationship, the dielectric loss involved in using materials having a dielectric constant on the order of 15 or higher begins to degrade the performance of the non-reciprocal device in which they are used. It then follows that the efficiency of the non-reciprocal effects is limited in dielectric loaded devices by the ratio 3 of velocities of propagation which, in turn, is limited by the dielectric loss of the dielectric materials.

It is, therefore, an object of the present invention to provide an etficient, compact and simple waveguiding structure for non-reciprocal wave transmission having an inhomogeneous cross section without dielectric materials. In brief, the non-reciprocal wave transmission device vof the present invention is a modified slow-wave-loaded 'waveguiding structure with suitably mounted elements of agnetized ferrite. The slow-wave-loading is partial and is accomplished with conductive elements mounted in spaced-apart relation within the structure to extend across one-half thereof to provide an inhomogeneous cross section and thereby a circularly polarized component of the radio frequency magnetic field at the boundary of the loading elements. The action of the ferrite as mounted at the referenced boundary then provides non-reciprocal operation of the device.

Other objects and advantages of the invention will be readily apparent in the following description and claims considered together with the accompanying drawings, in which:

FIG. 1 is a perspective view, partly in section, of a modified slow-wave coaxial line having a non-reciprocal characteristic in accordance with the present invention;

FIG. 2 is a characteristic curve showing the operation of the device of FIG. 1; and

FIG. 3 is a modification of the device of FIG. 1.

Referring to the drawings in detail, FIG. 1 in particular, a section of coaxial line 11 comprises an inner conductor 12 and an outer conductor 13. A plurality of semi-circular conductive disks 14 are similarly and radially mounted in parallel and spaced-apart relation on the inner conductor 12 to extend transversely with respect thereto. Such disks 14 are electrically connected to the inner conductor 12 and insulated, as by suitable spacing, from the outer conductor 13. In this configuration, there is provided a modified slow-wave structure wherein energy propagates in a hybrid mode which is, in the general form, a combination of the TB and TM modes.

The reason for the propagation of the referenced hybrid mode has been set forth in the previous discussion and may be readily understood by considering that the velocity of propagation in the disk-loaded half-portion of the line is of one value and that of the unloaded half-portion is of another value. Since energy propagates with but one value of the velocity of propagation in such structure, the actual velocity of propagation is one having a value that is intermediate to the two referenced values. Thus,

the inhomogeneity introduced by the partial disk-loading of the inner conductor requires that the resulting mode have a longitudinal component of the radio frequency magnetic field which is combination with the circular component provides a circularly polarized radio frequency magnetic field at or near the boundary between the loaded and unloaded regions of the line.

The theory of operation of the slow wave coaxial line having complete disks mounted on the inner or center conductor is well-known in the art and is generally applicable to the partially-loaded structure described in the preceding paragraphs with minor modifications. Such partially-loaded coaxial line is reciprocal in operation in that waves propagated in either direction are effected in the same manner.

Now to provide a non-reciprocal characteristic to the partially loaded coaxial line two substantially thin elongated strips 16 and 17 of ferrite material are mounted on the diametrical edges of the disks 14 to extend parallel to the inner conductor 12 with one on either side of such conductor. Thus, the strips 16 and 17 are disposed at the boundary between the loaded and unloaded regions of the coaxial line structure where the circularly polarized radio frequency magnetic field of the hybrid mode of propagated energy is present. An adjustable static magnetic field, H, as indicated by arrows 21, is established 4 in a conventional manner to extend diametrically through coaxial line 11 parallel to the broad faces of the strips 16 and 17.

With the ferrite strips 16 and 17 suitably magnetized, the electron spins within the ferrite and the sense of rotation of the circularly polarized radio frequency magnetic field are properly related for only one direction of energy propagation and so provide a non-reciprocal differential phase shift and attenuation.

A 3 length of modified slow-wave coaxial line, as described in the foregoing, having a outside diameter and propagating energy at 3000 mc./s. operates as indicated in FIG. 2. Such FIG. 2 is a plot of the differential phase shift in degrees obtained over a substantially low increment of applied static magnetic field of O to 400 gauss and a resulting curve 26 indicates a range of diiferential phase shift between zero and approximately 150 degrees. Similar operation of a half-slow-wave caxial line 1.3" in length has provided a differential phase shift of +5 degrees with a loss of 0.3 db. with the foregoing structures a VSWR of 1.1 or less over a 12% band width has been obtained by providing matching transformer structure (not shown) at each end of the line.

The results described in the foregoing paragraph show the usefulness of the present invention as a differential phase shifter, which has many applications in the microwave art. In such device the ratio of the velocities of propagation for the loaded and unloaded regions is of the order of 10 to 1. From this it follows that to obtain the same ratio in a dielectrically-loaded waveguiding structure, a material having a dielectric constant of the order of would be required. The losses inherent in a material having such high dielectric constant would be prohibitive and are entirely avoided by the structure of the present invention. Also, by increasing the static magnetic field to a value producing gyro resonance in the ferrite strips 16 and 17, the device is operable as an isolator with substantially high values of attenuation for one direction of propagation and substantially none in the opposite direction.

Referring now to FIG. 3 there is illustrated a second non-reciprocal coaxial wave transmission line 31. In this embodiment a plurality of conductive half disks 32 are radially mounted in spaced-apart and parallel relation in contact with the outer conductor 33 and extend toward, but not in contact with the inner conductor 34. The resulting structure also constitutes a slow-wave-loaded coaxial line and operates in substantially the same manner as set forth for the structure of FIG. 1. Thus, elongated ferrite strips 36 and 37 mounted on either side of and parallel to the inner conductor 34 on the diametric edges of the half disks 32 provide a non-reciprocal transmission characteristic when suitably magnetized. To provide the required magnetization, a conventional adjustable static magnetic field structure (not shown) is mounted externally of the line and establishes a magnetic field, H, through the ferrite strips 36 and 37 parallel to the broad faces thereof as indicated by arrows 39.

The operation of this latter embodiment of the invention is the same as set forth for the structure of FIG. 1. Thus, for substantially low values of applied static magnetic field, difi'erential phase shift of the energy, propagating in one direction through the line, is obtained. Also, by increasing the value of the static magnetic field for gyro resonance in the ferrite, non-reciprocal attenuation results.

With respect to the foregoing non-reciprocal wave transmission device, it is to be realized that the 50% loading factor provided by the semi-circular disks is not limiting. Other geometric configurations providing an inhomogeneous cross section are within the scope of the invention set forth. Also, it is to be realized that the waveguiding structure is not limited to circular coaxial lines as the same principles are applicable with respect to other structures in which the dominant mode of propagation is the TEM mode such as rectangular coaxial line, strip line, and slab line.

Structure has, therefore, been described for providing non-reciprocal wave transmission with minimum losses and physical dimensions. While the salient features of such structure have been described in detail with respect to particular embodiments, it will be readily apparent that numerous modifications and changes may be made Within the spirit and scope of the invention and it is, therefore, not desired to limit the invention to the exact details shown except insofar as they may be set forth in the following claims.

What is claimed is:

1. A non-reciprocal wave transmission device comprising: a coaxial type waveguide supporting radio frequency energy in the TEM mode, said Waveguide having an inner conductor and an outer conductor; slow-wave means disposed within said waveguide, said means including a plurality of spaced-apart conductive half-discs mounted on one of said conductors and spaced from the other conductor, said half-discs being similarly mounted in parallel relation to provide two regions in said waveguide having difierent velocities of propagation; a pair of elongated ferrite strips mounted on said half-discs parallel to said inner conductor along the boundary between said regions with one strip on either side of said inner conductor; said waveguide being operative in a transverse static magnetic field paralleling the broad surfaces of said ferrite strips.

2. A device as claimed in claim 1, wherein said discs are connected to said inner conductor.

3. A device as claimed in claim 1, wherein said discs are connected to said outer conductor.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Seidel: Journal of Applied Physics, February 1957, pages 218-226.

Morgenthaler et 211.: Proceedings of the IRE, November 1957, pages 1S511552.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Robert L. Fogel et ale 5 in the above numbered patfied that error appear tters Patent should read as It is hereby certi n and that the said Le ent requiring correctio corrected below.

"where" insert there d hybrid line 53 Column 1, line 25, after "caxial" read line 39, for "hybrid" rea column 3, for "is" read in column 4, line 18?, for coaxial line 19,, for "90+5" read 90-i 5 line 20 for "with" second occurrence, read With Signed and sealed this 22nd day of Januar (SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of Patents