Marriage and Werner, Some Non-Reciprocal Coaxial Devices at 2 Gc/s in Part B Supplement Number 21, Vol. 109, The Proceedings of the Institution of Electrical Engineers, June 1961, TKl I4 V. 109 P&B Suppl. 21-22; Title page and pages 147-152..
This invention pertains generally to improved ferrimagnetic microwave isolators employing a twin-line or multiple-line transmission line. As employed herein the term "ferrimagnetic" is intended to connote materials having negligible electrical conductivity and exhibiting either ferrimagnetic or ferromagnetic effects.
Isolators of the past have been made in either waveguide, stripline, micro-strip, or coaxial configuration. Waveguide isolators are characterized by their relatively large size and their narrow bandwidths. Waveguide isolators are also frequency dispersive and require a relatively large amount of ferrimagnetic material. Stripline isolators are generally designed as terminated circulators. Terminated stripline circulators exhibit reactive variations over their relatively narrow bandwidths and they have large reflection coefficients out-of-band. Coaxial isolators are usually of the resonance type and their isolation and insertion loss are functions of their transmission lengths in the ferrimagnetic material. The isolation per unit length is relatively small for resonance isolators of the coaxial type, giving rise to long lengths for practical devices.
It is accordingly a primary object of the present invention to provide a microwave isolator of relatively small size and light weight construction.
An additional object of the present invention is to provide a microwave isolator which exhibits negligible reactive variations over the operating bandwidth and maintains a low reflection coefficient out-of-band.
It is a further object of the present invention to provide a microwave isolator which has a relatively large bandwidth.
Another object of the present invention is to provide an isolator whose isolation is a function of the transmission length in the ferrimagnetic material, but whose isolation per unit length is relatively high.
In accordance with the present invention, the above and other objects are achieved by means of a microwave isolating device employing a twin-line or multiple-line transmission line configuration, as opposed to hollow waveguide, coaxial line, stripline or microstrip. In the exemplary embodiments described herein, the isolator body includes a plurality of lines, all but one of which are at dc and RF ground. Two coaxial connectors disposed at either end of the device have their center conductors connected to that one line which is not at dc and RF ground. The lines rest upon a dielectric chip whose dielectric constant and thickness along with the relative dimensions of the lines determine the characteristic impedance of the transmission system.
In order to achieve isolation a ferrite chip or chips are placed between the lines and a magnetic field is provided in substantially the same plane as is described by the lines and the ferrite chips. With the polarity of the DC magnetic field reversed, the preferred direction for insertion loss and isolation is reversed.
With the above considerations and objects in mind, the invention itself will now be described in connection with preferred embodiments thereof given by way of example and not of limitation, and with reference to the accompanying drawings, in which:
FIG. 1 is a perspective view, partially broken away, showing an isolator of this invention for connection in an external transmission line;
FIG. 2 is a partially exploded view showing another embodiment of this invention;
FIG. 3 is an end elevational view of another embodiment of the invention; and
FIG. 4 is an exploded view showing another embodiment of this invention.
Referring now to the drawing, with particular reference to FIGS. 1 and 2, there is shown one form of an unbalanced twin-transmission line isolator 3 in accordance with the present invention adapted for connection into any transmission line 2. In the exemplary embodiment shown, a pair of elongate conducting members 4 and 5, such as of steel plated with silver over nickel plate, are mounted in parallel spaced-apart relationship by means of their attachment to conductive end members 6, such as of brass. Each end member is provided with a coaxial connector 7 for connecting the isolator 3 into the transmission line 2 where isolation is desired. Member 4 is shorter than member 5 and is electrically connected such as by brazing to the center conductors 8 of the coaxial connectors 7 located in a central region of end members 6. Member 5 is connected directly to end members 6 such as by screws 9 passing through apertures 11 in end members 6 and into threaded bores 12. Thus, member 6 is at the dc and RF ground of the transmission line 2.
A dielectric sheet or plate member 13 such as titanium dioxide 13 coated with a conductive material 14 such as silver in areas conforming to side surfaces of members 4 and 5 is affixed to one side surface of the members 4 and 5 such as by soldering or by silver epoxy. The members 4 and 5 can, however, be simply placed on plate 13. As pointed out above, the dimensions of the dielectric member 13 and the conductive members 4 and 5 and the dielectric constants of these members determine the characteristic impedance of the transmission system.
In order to render this transmission device an isolator, a ferrimagnetic body 15 is suitably mounted such as with epoxy between members 4 and 5. The particular composition of the ferrimagnetic member 15 is not a necessary element of the present invention and any substantially electrically nonconductive materials exhibiting ferrimagnetic or ferromagnetic effects may be employed. Pure or substituted Yitrium iron garnet polycrystalline materials are exemplary for this purpose.
A steady state or DC magnetic field H is applied to the body 15 in one sense or the other in a direction substantially parallel to the plane of the dielectric member 13, and of magnitude that is connected with ferrimagnetic resonance at the applied microwave frequency and for the given geometry. As shown in FIGS. 1 and 2, the magnetic field is established by permanent magnets 16 and 17 positioned respectively on top of conductive members 4 and 5. By selecting conductive members 4 and 5 of a material having a high magnetic permeability, such as steel, these conductive members serve as pole pieces for establishing the field in body 15. The members 4 and 5 could themselves be magnets for establishing the magnetic field, or the field could be established by a solenoid 18 as shown in FIG. 3.
With the DC magnetic field passing through the ferrimagnetic body 15 in a selected direction, microwave energy entering one of the coaxial connectors 7 will appear at the other coaxial connector 7 with little insertion loss while microwave energy flowing in the opposite direction will experience an insertion loss which is related to the length of the ferrimagnetic member 15. This insertion loss is, for practical devices, normally such that the output signal level is from 10 to 30 decibels below that of the input signal level. Where the polarity of the DC magnetic field is reversed from that which was originally assumed above the directions of energy flow for minimum and maximum insertion loss are reversed.
A cover housing 1 encloses the isolator elements located between end plates 6.
The invention has been described above in some detail with particular reference to FIGS. 1-3 where members 4, 5 and 15 are shown specifically as being rectangular in cross section. However, it will be evident to those skilled in the art that the invention is equally applicable to isolators employing round, triangular or other cross sections for these members. Further, it will be understood that the ferrimagnetic member 15 may have tapered or stepped ends for broadband operation. It is further understood that the connectors 7 may take the form of hollow waveguide connectors with central coupling loops or coupling posts being connected to the ends of member 4.
Referring now to FIG. 4, there is shown another embodiment of the present invention in the form of a balanced three transmission line version similar to the embodiment of FIGS. 1-3 but including an additional transmission line connected at dc and RF ground and an additional ferrimagnetic element.
In this embodiment the two electrically conductive members 24 and 25 at dc and RF ground are connected to the end plate 6 and positioned on silver coated portions of the dielectric base member 13' at opposite sides of the plate. The third transmission line is in the form of a metal strip 26, such as plated on the dielectric base member 13', which is connected by tabs 8' to the center conductor of the coaxial line 7 at each end of the device. A ferrimagnetic member 27 is positioned between each of the conductors 24 and 25 and the center transmission line 26.
The magnetic field is established in these ferrimagnetic elements 27 by a magnetic structure including a plate magnet 28 that rests on the top of and spans the gap between the conductive members 24 and 25 and a central wedge member 29 located in the gap between the ferrimagnetic members 27.
The construction of the isolator of this invention is such that the thickness, or transverse dimension, of the ferrimagnetic members is substantially less than the transverse dimension of the electrically conductive members for establishing the proper field pattern for the isolator.
The dielectric sheet or plate serving as the base of the isolator can be modified to effectively tune the isolator and this modification can take the form of additional dielectric block or plate portion that can either be moved around to different locations of the underneath surface of the plate member or secured in a particular location when a precisely desired tuning characteristic is achieved.
While it is believed that the above description and accompanying drawings clearly provide sufficient information for a person skilled in the art to practice the present invention, the following two illustrative examples are provided for further illustrating the invention.
A twin transmission line isolator of the type illustrated in FIGS. 1 and 2 was built with the following dimensions and characteristics:
Element Nos. Material Length Width Height __________________________________________________________________________ 4 and 5 cold rolled steel .4 in. .092 in. .032 in. 13 Titanium Dioxide .4 in. .279 in. .075 in. 15 Polycrystalline .375 in. .092 in. .045 in. YIG; Transtech, Inc. Type G-113 16 and 17 Type F-620 .375 in. .092 in. .125 in. Ferrite Magnet, D. M. Stewart Co. __________________________________________________________________________
A device constructed in accordance with this embodiment has a 0.7 insertion loss, 23 db isolation and 1.15:1 VSWR for the frequency range 2.2 to 2.3 GHz and less than 1.5 db insertion loss from 10 MHz to 4.5 GHz.
An isolator constructed in accordance with the embodiment of FIG. 4 has the following characteristics:
Element Nos. Material Length Width Height __________________________________________________________________________ 24 and 25 cold rolled steel .5 in. .093 in. .187 in. 13' Titanium Dioxide .5 in. .562 in. .025 in. 27 Polycrystalline .450 in. .173 in. .045 in. YIG; Transtech, Inc. Type G-113 28 F-620 Ferrite .500 in. .434 in. .156 in. Magnet, D. M. Stewart Co. 29 F-620 Ferrite .450 in. .040 in. .125 in. Magnet, D. M. Stewart Co. __________________________________________________________________________
An isolator of this embodiment was constructed having the insertion loss, isolation, and VSWR for the 2.2 to 2.3 GHz range as the preceding embodiment and less than one db insertion loss from 10 MHz to 6 GHz.