Resonance absorption-type microstrip line isolator

Abstract: A resonance absorption-type microstrip line isolator comprising a ground conductor (5), a magnetic member (1) pro­ vided on the ground conductor (5), and a central conductor (4a) provided on the magnetic member (1), portions of the magnetic member (1) on both sides of the central conductor (4a) being magnetized oppositely. The magnetic member may be replaced by a composite member constituted by at least two magnetic members (1) and at least one nonmagnetic dielectric member (6). Further, to achieve a miniaturization of the iso­ lator, the central conductor (4a) may be in a meandering shape.

Description: The present invention relates to a small and inexpensive isolator usable in the ranges of VHF, UHF and microwaves. Isolators are widely used as indispensable parts for microwave apparatuses in wide ranges of microwave applications for the purposes of protecting transistors at high power, interstage matching, removing unnecessary radiations, etc. Recently, because of dramatic miniaturization of other microwave elements, the isolators have come to occupy considerably large space relative to other elements in overall microwave apparatuses. For instance, there are some microwave apparatuses, several tens % of whose space is occupied by isolators. Further, considerable percentages of the costs of the overall microwave apparatuses are attributed to the isolators. Accordingly, demands are increasing for the miniaturization and cost reduction of the isolators. In general, various types of isolators are already known as shown in FIG. 1 (See, for instance, Konishi et al., "Recent Microwave Circuit Technology Using Ferrite," Denshi Tsushin Gakkai (Electronic Communications Association) pp. 70-104, 1969). The same reference numerals are assigned to the corresponding parts in all of Figs. 1 (a)-(f). Specifically, Fig. 1 (a) shows an isolator utilizing a Faraday effect in a circular waveguide 3a. Fig. 1 (b) shows an isolator having a rectangular waveguide 3 in which the displacement of an electric field is utilized. Fig. 1 (c) shows an isolator having a ferrite slab 1 whose edge guide mode is utilized. Fig. 1 (d) shows an isolator comprising a usual junction circulator 11, one terminal of which is connected with a dummy load 2a. Fig. 1 (e) shows an isolator comprising ferrite members 1 at positions of a circularly polarized wave in a rectangular waveguide 3 for absorbing it by resonance. Fig. 1 (f) shows an isolator comprising a microstrip line for generating a circularly polarized wave for resonance absorption. In the first four isolators shown in Figs. 1 (a)-(d), an absorption element 2 or a dummy load 2a is provided for absorbing the energy of a microwave propagating backwardly . On the other hand, in the case of the isolators of resonance absorption type shown in Figs. 1 (e) and (f), microwave ferrite members 1 themselves act as microwave absorbers. Incidentally, in all of Figs. 1 (a)-(f), 1 represents a soft ferrite member suitable for a microwave, 2 a microwave absorber, 2a a dummy load, 3 a rectangular waveguide, 3a a circular waveguide, 4 a central conductor of a microstrip line, 5 a ground conductor of a microstrip line, 6 a dielectric member, and H ext an external magnetic field. To achieve the miniaturization of an isolator, the smaller the number of parts, the more advantageous. In this sense, the resonance absorption-type isolator, which does not need a microwave absorber separately, appears to be more suitable. However, such type of an isolator is not widely used at present. The reason therefore is not clear, but it may be considered that a means for exciting a circularly polarized wave for resonance absorption is complicated, meaning that the number of parts are not necessarily reduced. Another reason is that since it positively employs a non-linear phenomenon like resonance, the harmonic generation of high-frequency wave undesirable to the microwave apparatuses is inevitable. Further, a nonreciprocal phase shifter is known from the article "Microstripline Ferrite Devices Using Surface Field Effects for Microwave Integrated Circuits", by Eric E. Riches et al, IEEE Transactions on Magnetics, Vol. MAG-6, No. 3 (1970), pages 670 - 673. This nonreciprocal phase shifter comprises a ferrite substrate, a microstripline disposed on the ferrite substrate and a C-shape electromagnet, wherein both ends of said electromagnet are in contact with the ferrite substrate so that the magnetic flux flows in parallel with the ferrite substrate. OBJECT AND SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to overcome the problems of the above conventional resonance absorption-type isolators, thereby providing a small, inexpensive isolator. This object is solved by a resonant absorption-type microstrip line isolator according to claim 1. To achieve this object, there is provided according to the present invention an isolator comprising a ground conductor; a magnetic member provided on the ground conductor; and a central conductor provided on the magnetic member, portions of the magnetic member on both sides of the central conductor being magnetized oppositely. The magnetic member may be replaced by a composite member constituted by at least two magnetic members and at least one nonmagnetic dielectric member. Further, to achieve the miniaturization of the isolator, the central conductor may be in a meandering shape. BRIEF DESCRIPTION OF THE INVENTION Figs. 1 (a)-(f) are schematic views showing various conventional isolators; Fig. 2 (a) is a schematic perspective view showing the distribution of an electromagnetic field of a microstrip line; Fig. 2 (b) is a schematic plan view showing the distribution of an electromagnetic field of a microstrip line; Fig. 3 is a cross-sectional view showing the isolator according to one embodiment of the present invention; Fig. 4 is a cross-sectional view showing the isolator according to another embodiment of the present invention; Fig. 5 is a cross-sectional view showing the isolator according to a further embodiment of the present invention; Fig. 6 is a cross-sectional view showing the isolator according to a still further embodiment of the present invention; and Fig. 7 is a cross-sectional view showing the isolator according to a still further embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention will be explained in detail referring to the attached drawings. Fig. 2 shows the distribution of an electromagnetic field of a microstrip line with a dielectric member for explaining the basic principle of the present invention. In general, a microwave propagating in the microstrip line is in a TEM mode, and in the vicinity of the central conductor 4, both of lines of electric force 7 and lines of magnetic force 8 are perpendicular to the direction of microwave propagation. However, since the lines of magnetic force 8 are closed ones, they are in the shape of loop around a point at which an electric field is maximum, as shown in Fig. 2 (a). This means that circularly polarized wave components of a microwave magnetic field are considerably distributed around the central conductor 4 of the microstrip line. However, unlike in the rectangular waveguide 3 show in in Fig. 1 (e), the region of circularly polarized wave is not localized drastically. Thus, when a microwave propagates from above in Fig. 2 (b), there exist circularly polarized wave components of the clockwise direction on the left side and those of the counterclockwise direction on the right side, when viewed from above. Fig. 3 shows the principle of the resonance absorption-type microstrip line isolator according to one embodiment of the present invention, which is invented based on the electromagnetic field distribution of the microstrip line shown in Fig. 2. In Fig. 3, the isolator comprises a microwave ferrite member as a magnetic member 1 in place of the dielectric member 6 in Fig. 2, and ferrite portions on both sides of the central conductor 4 are magnetized in the opposite polarities by a pair of permanent magnets 9. By this structure, when a magnetic field H ext is applied to a resonance point in ferromagnetic resonance, an energy of a microwave propagating as if it springs up from the plane of paper showing Fig. 3 is absorbed by the microwave ferrite member 1. On the other hand, in the case of a microwave propagating as if it sinks vertically into the paper plane, almost no microwave is absorbed. Thus, this structure has a function as an isolator. In Fig. 3, since the microwave ferrite member 1 extends to areas where the lines of magnetic force in the microwave are not in the shape of circularly polarized waves, a loss of microwave passing through the isolator in the forward direction, namely an insertion loss is increased modestly. Fig. 4 shows another embodiment of the present invention which can alleviate the above problem. In this isolator, a portion of the magnetic member just under the central conductor 4, where there are substantially no circularly polarized wave components, is replaced by a nonmagnetic dielectric member 6. Outside portions of the magnetic member 1 are also replaced by another nonmagnetic dielectric member 6, but this replacement is not always necessary. Thus, by using a composite member constituted by at least two magnetic members 1 and at least one nonmagnetic dielectric member 6, the insertion loss of the resonance absorption-type isolator can be greatly reduced. Incidentally, the composite member may be constituted by vertically overlapping a magnetic member and a nonmagnetic dielectric member, unlike the lateral arrangement of magnetic members and a dielectric member as shown in Fig. 4, without changing the principle of the present invention shown in Fig. 3. To make sure that a necessary level of a loss in the backward direction is achieved, the isolators as shown in Figs. 3 and 4 need relatively large sizes. This is because the energy distribution of the microstrip line is concentrated almost immediately below the central conductor 4, meaning that strong coupling of the microwave ferrite member 1 and the electromagnetic energy of a microwave propagating therethrough cannot be achieved. To achieve strong coupling, the microstrip line should be made longer. However, this makes difficult the miniaturization of the isolator. Fig. 5 shows a further embodiment of the present invention for solving the above problem, in which a central conductor 4a is in a meandering shape to achieve a large effective length of the central conductor 4a. In Fig. 5, the meandering central conductor 4a is bent at two points, but it should be noted that it may be bent any times. Incidentally, in Fig. 5, four magnetic members 1 and five nonmagnetic dielectric members 6 are combined. As the number of bending of the central conductor 4a increases, the numbers of the magnetic members 1 and the nonmagnetic dielectric members 6 increase correspondingly. A bending pitch of the central conductor 4a is equal to an alternating pitch of the magnetic members 1 and the nonmagnetic dielectric members 6, while always satisfying the requirement that the central conductor 4a extends only on the nonmagnetic dielectric members 6. In Fig. 5, the bending portions of the central conductor 4a extend partially from the composite member, but it is possible to provide nonmagnetic dielectric members thereunder, if necessary, for impedance matching. Also, magnetized members may be placed outside the composite member. Fig. 6 shows a still further embodiment of the present invention, in which the microwave ferrite members 1 as shown in Figs. 3 and 4 are magnetized. Since the permanent magnets 9 are placed adjacent to the central conductor 4, they should not be metal magnets because if so an electromagnetic field mode is deteriorated. Accordingly, ferrite magnets are used for the permanent magnets 9 in this embodiment. Also, instead of using permanent magnets 9 under the ground conductor 5 as in Figs. 3 and 4, a soft magnetic material is used for the ground conductor 5a in this embodiment. By this structure, the isolator can be thin, and the deterioration of its characteristics can be prevented because images of the permanent magnets 9 appear under the ground conductor 5a by electric imaging. Since the soft magnetic material generally does not have such a high electric conductivity, the ground conductor 5a is desirably plated with gold, silver or copper. In addition, a thin conductor can be inserted between the ground conductor 5a and the composite member to achieve the same effect. The permanent magnets 9 have opposite magnetic poles to those closer to the central conductor 4, and these opposite magnetic poles act to weaken a magnetic field H ext . In order to avoid this, a soft magnetic yoke 10 is mounted to top ends of the permanent magnets 9 in this embodiment. By this structure, the magnetic poles of the permanent magnets 9 disappear apparently. Fig. 7 shows a still further embodiment of the present invention, in which a meandering central conductor 4a is placed on a composite member consisting of a plurality of magnetic members 1 and a plurality of nonmagnetic dielectric members 6 arranged alternately. In this case, microwave ferrite magnetic members 1 are alternately magnetized by a ferrite magnet 9a having a plurality of magnetic poles. In this embodiment too, the pitch of the magnetic poles of the permanent magnet 9a is the same as that of the composite member and the bending pitch of the central conductor 4a. Also, the ground conductor 5a may be similarly made of a soft magnetic material. With the structure shown in Fig. 7, a microstrip line isolator, in which resonance absorption takes place at 5 GHz, is provided, and when it has a size of about 5 mm x about 5 mm, its insertion loss is 3 dB and its backward loss is 10 dB. Thus, by the principle of the present invention, an extremely small isolator can be achieved. With respect to the magnetic materials usable for the magnetic member, microwave soft ferrite is explained, but it should be noted that a garnet-type magnetic material composed mainly of Y₂O₃ and Fe₂O₃ (YIG) can also be used.

Claims: A resonance absorption-type microstrip line isolator comprising a ground conductor (5; 5a), a magnetic member (1) provided on said ground conductor and a central conductor (4; 4a) provided on said magnetic member (1), characterized in that at least one permanent magnet (9; 9a) is disposed above said magnetic member (1) such that portions of said magnetic member on both sides of said central conductor (4; 4a) are vertically magnetized in opposite directions, so that a microwave propagating in one direction is absorbed by resonance and a microwave propagating in an opposite direction is not absorbed. The isolator according to claim 1, wherein a pair of permanent magnets (9) is disposed on both sides of said central conductor (4) above said magnetic member (1), the opposite magnetic poles of said permanent magnets facing said magnetic member. The isolator according to claim 1 or 2, wherein said magnetic member (1) is a microwave ferrite member. The isolator according to any of claims 1 to 3, wherein said magnetic member (1) is made of a garnet-type magnetic material composed mainly of Y₂O₃ and Fe₂O₃. The isolator according to any of claims 1 to 4, wherein at least two magnetic members (1) and at least one nonmagnetic dielectric member (6) are arranged alternately, said central conductor (4; 4a) being provided on said nonmagnetic dielectric member and at least one permanent magnet (9) being disposed above said magnetic members and having a plurality of alternately opposite magnetic poles arranged such that said magnetic members are vertically magnetized alternately in opposite directions. The isolator according to claim 5, wherein said central conductor (4a) is in a meandering shape. The isolator according to claim 5 or 6, wherein said ground conductor (5a) is made of a soft magnetic material. The isolator according to any of claims 1 to 7, wherein a pair of permanent magnets (9) is disposed on both sides of said central conductor (4; 4a) below said ground conductor (5; 5a), said permanent magnets below said ground conductor having magnetic poles directed oppositely such that portions of said magnetic member (1) on both sides of said central conductor are vertically magnetized in opposite directions.

Inventors: TAKEDA SHIGERU, TSUBOI TAKASHI, TAKEDA, SHIGERU, TSUBOI, TAKASHI
Applicants: HITACHI FERRITE LTD, HITACHI METALS LTD, HITACHI FERRITE LTD., HITACHI METALS, LTD.
Application year: 1989
Main classification: H01P1
IPC codes: H01P1