CIRCULATOR HAVING AN IMAGE MAGNET

Abstract: A circulator (24) for use in RF communication systems. Included in the circulator are a conductive pattern (28) which defines a circulator junction and a magnet (40) disposed above the conductive pattern to provide magnetic bias. A substrate (26) below the conductive pattern preferably carries a ground plane (38) on its lower surface. Below and preferably abutting the ground plane is a magnetic layer (44) situated so as to have an image magnet induced in it, whereby the image magnet provides a substitute for a second, conventional magnet.

Description: CIRCULATOR HAVING AN IMAGE MAGNET Field of the Invention This invention is directed to the field of RF circulators of the type commonly used in RF communication systems and devices. BacJcground of the Invention Circulators are commonly used in microwave systems to interconnect multiple devices so as to inhibit unwanted reflections between the devices. See, for example, the article by Knerr entitled "A Microwave Circulator That's Smaller Than a Quarter," Bell Laboratories Record, March 1973, pp 79 - 84. The physical construction of a conventional RF circulator of the type to which the invention is addressed is shown schematically in Figure 1. The illustrated circulator 10 includes a ferrite substrate 12 whose upper surface 14 (shown greatly enlarged) carries a conductive pattern. That pattern typically includes three conductive strips connected to a central circular pattern which defines a circulator junction. The three strips are usually attached to the junction at points 120 degrees apart around its circumference to provide input/output ports to the junction. The junction itself couples RF energy among the input/output ports. OMPI The bottom surface of the substrate 12 carries a ground plane 16 (shown greatly enlarged). Disposed above the surface 14 is a magnet 18 which is separated from the surface 14 by a spacer 20 to prevent the magnet 18 from interfering with the RF field associated with the circulator junction. Another magnet 22 is situated beneath the ground plane 16 and in vertical alignment with the upper magnet 18. With this arrangement, the magnets 18 and 22 serve to provide a magnetic bias which induces proper operation of the circulator junction. The type of two-magnet circulator shown in Figure 1 is well known and is discussed in the literature. See, for example, the article by Ho entitled "Design Techniques for Low Loss, Miniature Microwave Microstrip Circulator at L-Band," Proceedings of the Electronic Components Conference, April, 1978, pp 277 - 280. An obvious disadvantage of conventional two-magnet circulators is the need for a second magnet and the additional cost associated therewith. Other types of circulators which employ a single magnet are also reported, as in the previously mentioned article by Knerr. For some applications, the type of single-magnet circulator shown in the Knerr article is too costly and too complex. This disadvantage is due in part to the need for a two-piece steel housing which presumably serves as a return path for magnetic lines of flux. In addition, a single-magnet type circulator is not efficient enough for certain applications. Accordingly, it is a general object of the invention to provide an improved RF circulator. It is a more specific object of the invention to provide such a circulator which is less costly than conventional two-magnet circulators, but which provides substantially equivalent performance. OMP It is another object of the invention to provide an RF circulator which achieves high levels of performance without the need for costly and bulky housings or the like which are frequently required to provide an adequate return path for magnetic lines of flux. Brief Description of the Figures Figure 1, referred . to previously, is a schematic side view of a conventional RF circulator. Figure 2 is a schematic side view of an RF circulator according to the invention. Figure 3 is a top view of the conductive pattern which is disposed on the upper surface of the substrate shown in Figure 2. Figure 4 depicts the equivalent magnetic structure of the circulator shown in Figure 2. Summary of the Invention Broadly stated, the circulator described herein includes a conductive pattern which is arranged to form a circulator junction, and a magnet disposed above the conductive pattern. A ferrimagnetic substrate is disposed beneath the conductive pattern and preferably carries the pattern on its upper surface. An electrically conductive ground plane is preferably carried by or on the lower surface of the substrate. Below the ground plane, a magnetic layer, preferably of cold rolled steel, is disposed in relation to the magnet so as to induce an image magnet in the magnetic layer, thereby eliminating the need for a second magnet. With this image magnet, closely confined and concentrated lines of magnetic flux are established through and perpendicular to the plane of the circulator junction. The improved circulator eliminates the need for a second conventional magnet, permits the magnetic layer to act as a support for the circulator (or for more than one circulator mounted thereon) and eliminates costly or bulky steel housings associated with some conventional circulators. Description of the Preferred Embodiment Referring to Figure 2, the improved circulator 24 preferably includes a suitable ferrimagnetic substrate 26 of ferrite or garnet, for example. Immediately above the substrate 26 is a conductive pattern 28 (shown greatly enlarged) which is arranged to form a circulator junction and a plurality of strips connecting the junction to input/output ports. Preferably, the conductive pattern is formed directly on the upper surface of the substrate 26. Referring briefly to Figure 3, the conductive pattern 28, typically formed of silver or copper, is shown as forming a circular circulator junction 30 to which three conductive strips 32, 34, and 36 are connected. The function of these strips is to couple the junction 30 to three input/output ports. When a suitable magnetic bias is directed through the junction 30 and the underlying substrate 26, RF energy coupled to one of the strips is passed via the junction 30 to another strip with little loss and with minimum reflections. The theory of such operation and junction design is discussed in an article by Fay and Co stock entitled "Operation of the Ferrite Junction Circulator," IEEE Transactions on Microwave Theory and Techniques, January, 1965, pp. 15 - 27. It should be understood that the illustrated conductive pattern is exemplary of what may be used and that other suitable patterns may be employed. OMPI Referring again to Figure 2, an electrically conductive ground plane 38 (shown greatly enlarged) is disposed immediately below the substrate 26 and is preferably formed of silver or copper on the bottom surface of the substrate 26. This ground plane provides a termination plane for electric lines of force and the return path for currents as is typical in microstrip circuitry. A magnet 40 is disposed above the conductive pattern 28 and is preferably separated therefrom by an insulating spacer 42 which preferably has a relative permeability of one. The magnet 40 may be a cast Alnico 8 magnet from Indiana General Corp. of Valparaiso, Indiana. To avoid the use of a second magnet below the ground plane 38 and yet achieve the degree of efficiency commonly found in two-magnet circulators, I substitute for the conventional second magnet a magnetic layer 44 and dispose the layer 44 in relation to the magnet 40 so as to induce an image magnet in the magnetic layer 44. The layer 44 is preferably located immediately below and abutting the . ground plane 38 to ensure that an effective image magnet is induced therein by the magnet 40. Referring to Figure 4, it can be seen that the North (N) and South (S) poles of the magnet 40 induce in the layer 44 an image magnet 46 having the illustrated N and S poles. Of .course, the polarity of the magnet 40 may be reversed, and a similar reversal will occur in the polarity of the image magnet. The effect of this image magnet, in cooperation with the magnet 40, is to establish closely confined and concentrated lines 48 of magnetic flux through the circulator junction 30 and perpendicular to the plane of the circulator junction. As shown by the dashed lines 48, the lines of flux do not diverge substantially, but are highly concentrated to induce an appropriate magnetic field in the substrate below the circulator junction for orienting the magnetic domains therein in a fashion to develop proper junction operation. Preferably, the layer 44 is made of cold rolled steel and may extend horizontally beyond what is shown in Figures 2 and 4 to form another magnetic layer for an adjacent circulator. In addition to eliminating the need for a second magnet, the layer 44 provides the only needed support for the circulator and may, indeed, support adjacent circulators (not shown) which may be identical to the illustrated construction. The illustrated embodiment has been found to be a practical and improved alternative to conventional circulators. For example, a circulator constructed according to the present invention for operation in the range of from 6420 to 7130 megahertz has been found * to provide insertion loss and average isolation character¬ istics which are nearly equivalent to corresponding characteristics of two-magnet isolators. Although the invention has been described in terms of a preferred embodiment, it will be obvious to those skilled in the art that various alterations and modifica¬ tions may be made to suit a particular application. Accordingly, it is intended that all such alterations and modifications be considered as within the spirit and scope of the invention as defined by the appended claims. OMPI

Claims: What is claimed is: 1. An RF circulator having a plurality of input/output ports, comprising: a conductive pattern arranged to form a circulator junction for coupling RF energy to and from the input/output parts; a magnet disposed above the conductive pattern; a ferri agnetic substrate disposed below the conductive pattern; an electrically conductive ground plane •disposed below the substrate; and a magnetic layer below the ground plane selected and disposed in relation to the magnet so as to induce an image magnet in the magnetic layer, such that closely confined and concentrated lines of magnetic flux are established through and perpendicular to the plane of the circulator junction. 2. A circulator as set forth in claim 1, wherein said magnetic layer is disposed to act as a support for the circulator. 3. A circulator as set forth in claim 2, wherein said magnetic layer is formed of cold rolled steel. 4. A circulator as set forth in claim 2 further including an insulating spacer disposed between the magnet ' and the conductive pattern. 5. In an RF circulator having a conductive pattern arranged to form a circulator junction, having a magnet disposed above the circulator junction and having a magnetic substrate disposed below the circulator junction, the improvement comprising: a magnetic layer disposed immediately below the substrate and in vertical relationship with the magnet so as to induce an image magnet in the magnetic layer

Inventors: HARGIS ROBERT NORMAN, HARGIS, ROBERT, NORMAN
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1984
Main classification: H01P1
IPC codes: H01P1