1. Field of the Invention
The present invention relates to waveguide circulators, and more particularly to improved power handling capabilities for ferrite waveguide circulators through the use of thermally-conductive dielectric attachments.
2. Description of the Related Art
Ferrite circulators have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications. A waveguide circulator may be implemented in a variety of applications, including but not limited to transmit/receive (T/R) modules, isolators for high power sources, and switch matrices. One important application for such waveguide circulators is in space, especially in satellites where extreme reliability is essential and where size and weight considerations are very important. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120° and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, a gyromagnetic effect is created that can be used for circulating the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of circulation between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. Radio frequency (RF) energy can be routed with low insertion loss from one waveguide arm to either of the two output arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction.
Generally, these three-port waveguide switching circulators are impedance matched to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conventional three-port waveguide switching circulators typically have one or more stages of quarter-wave dielectric transformer structures for purposes of impedance matching the ferrite element to the waveguide interface. The dielectric transformers are typically used to match the lower impedance of the ferrite element to the higher impedance of the air-filled waveguide so as to produce low loss. Thin adhesive bondlines are used to attach the transformers to the ferrite element and the waveguide structure, so they also provide a thermally conductive path from the ferrite element to the waveguide structure for transferring heat out of the ferrite element.
Previous patents have described approaches for achieving broad bandwidth through the addition of impedance matching elements. Broadband circulators have high isolation and return loss and low insertion loss over a wide frequency band, which is desirable so that the circulator is not the limiting component in the frequency bandwidth of a system. Broad bandwidth also allows a single design to be reused in different applications, thereby providing a cost savings. These previous approaches for achieving broad bandwidth generally involve the addition of quarter-wave dielectric transformers or steps in the height or width of the waveguide structure to thus achieve impedance matching the ferrite element to the waveguide port. For example, previous approaches have disclosed achieving impedance matching by providing a step or transition in the waveguide pathway. This technique eliminates the standard dielectric transformers, thereby eliminating a thermal path for conducting heat out of the ferrite element. This technique also relies on the presence of a significant gap or spacing between adjacent ferrite elements, increasing the size and weight of the structure. These methods all require impedance matching elements in addition to the ferrite element in order to achieve acceptable performance. Other approaches include changing the shape of the ferrite resonant structure to achieve broadband performance. However, these ferrite structures are restricted to fixed-bias applications with a single direction of circulation.
Referring now to
Resonant section 130 exists where the legs of device 101 converge inside the three apertures 135. As would be evident to those possessing an ordinary skill in the pertinent arts, the dimensions of resonant section 130 determine the operating frequency for circulation in accordance with conventional design and theory. The sections 140 of the ferrite element in the area outside of the magnetizing winding apertures 135 may act as return paths for the bias fields in the resonant section 130 and as impedance transformers out of the resonant section. Faces 150 of the ferrite element are located at the outer edges of the three legs.
Referring now to
The conventional components described above may be disposed within the conductive waveguide structure 100, which is generally air-filled. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conductive waveguide structure 100 may include waveguide input/output ports 105. Ports 105 may provide interfaces, such as for signal input and output, for example. Empirical matching elements 104 may be disposed on the surface of conductive waveguide structure 100 to affect the performance. Matching elements 104 may be capacitive/inductive dielectric or metallic buttons that are used to empirically improve the impedance match over the desired operating frequency band.
Referring now to
The purpose of a ferrite circulator is to circulate RF power from one port to another while absorbing a minimal amount of the circulating power. All of the dielectric and ferrite materials in circulators absorb some power, but the majority of the power absorbed by a ferrite circulator is contained in the ferrite element due to the relatively high volume of the ferrite element 101 and the high electrical and magnetic loss tangents of the ferrite material. In conventional single-junction waveguide circulators, such as illustrated in
Accordingly, a need exits for a ferrite circulator that incorporates thermally conductive dielectric attachments in order to maximize the area of contact with the ferrite for improved heat transfer beyond the present art, thereby allowing ferrite circulators to operate at higher average microwave power levels.
The present invention improves upon the geometry of conventional ferrite circulators in order to increase the average power handling and decrease the temperature rise in the ferrite and associated adhesive bondlines. Embodiments of the present invention utilize thermally conductive dielectric attachments on the sides of the ferrite element. These attachments significantly improve the thermal conductivity of the path from the ferrite element to the waveguide structure. If the attachments are good thermal conductors—such as, for example, boron nitride, aluminum nitride, or beryllium oxide—they can be relatively thin (for example, less than about 0.02″ thick for operations at about 20 GHz) to minimize the dielectric loading impact on RF performance while still improving the thermal performance of the circulator. Embodiments of the present invention decrease the maximum temperature of the ferrite element and associated bondlines, thus improving the performance and survivability of ferrite circulators in high power applications. Because of the increasing power handling capabilities in embodiments of the present invention, the ferrite circulators are suitable for a broader range of applications, making them a viable alternative to other switch technologies in high average power applications.
In one aspect of the invention, a ferrite waveguide circulator is provided. The circulator includes a waveguide structure having an internal cavity, the waveguide structure including a plurality of ports extending from the internal cavity. The circulator also includes at least one ferrite element disposed in the internal cavity, said ferrite element including at least one leg having at least two side surfaces and one face surface. At least one thermally-conductive dielectric attachment is affixed to at least one of the side surfaces of the ferrite element.
In another aspect of the invention, a ferrite waveguide circulator is provided having a waveguide structure having an internal cavity, the waveguide structure including a plurality of ports extending from the internal cavity. The circulator also includes at least one ferrite element disposed in the internal cavity, said ferrite element including at least one leg having at least two side surfaces and one face surface. At least one thermally-conductive dielectric attachment is affixed to at least one of said face surfaces of the ferrite element.
In a further aspect of the invention, a system for circulating microwaves in a waveguide is provided. The system includes a waveguide structure having an internal cavity forming an input port and one or more output ports; a ferrite element that substantially exclusively couples microwaves from said input port to one of said output ports, wherein the substantially exclusive coupling is responsive to an activation of at least one magnetizable winding associated with said ferrite element; and at least one thermally conductive dielectric attachment affixed to the ferrite element so as to conduct thermal energy away from said ferrite element.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by the instrumentalities and combinations particularly pointed out hereinafter.
The accompanying drawings are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification. The accompanying drawings illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the figures:
Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.
The embodiments of the present invention increase the average power handling over conventional ferrite circulators by decreasing the temperature rise in the ferrite and associated adhesive bonds. Due to the electrical and magnetic losses inherent in ferrite materials, the ferrite elements in circulators absorb a portion (generally around 2%) of the microwave power that passes through the devices. As ferrite is a relatively poor thermal conductor, the absorbed power results in high internal temperatures in the ferrite and the adhesives used to attach the ferrite to the traditional quarter-wave dielectric transformers and dielectric spacers. The high temperatures in the ferrite result in a degradation in performance, as the material properties of the ferrite change with temperature. The high temperatures in the adhesives can cause failures in the bondline resulting in outgassing or weakening of the bond.
Thermal conductance is inversely proportional to length and proportional to area. Thus, the thermal conductance of the path from the ferrite element to the heat sinking waveguide structure can be improved by maximizing the surface area contact and minimizing the length of travel of the absorbed power out from the ferrite element to the attached dielectrics. Both of these requirements are met through the use of thermally-conductive dielectric attachments as utilized in this new invention. The basic thermal design problem for circulators is to get the absorbed heat out of the ferrite element though dielectric attachments to the waveguide structure, which is an excellent thermal conductor. The traditional dielectric attachments are the dielectric spacers and quarter-wave dielectric transformers. For impedance matching purposes, the area of the dielectric spacer is generally minimized and kept within the resonant section of the ferrite element. Thus, its area of contact with the ferrite element is limited to a small Y-shaped cross-section in the center of the part. The traditional quarter-wave dielectric transformers are a quarter-wavelength long with a width on the order of ⅓ that of the ferrite element, again resulting in a small area of contact. Since the height and length of the ferrite element are usually 1.5 to 2 times longer than the width of the ferrite element, the traditional dielectric attachments have the thermally undesirable combination of a small area of contact at a long distance from the heat source.
Generally, embodiments of the present invention utilize thermally-conductive dielectric attachments on the sides of the ferrite element, which maximize the area of contact and minimize the path length from the ferrite element out to the thermally-conductive dielectric attachments. Because these thermally-conductive dielectric attachments are good thermal conductors, such as boron nitride, aluminum nitride, or beryllium oxide, they can be relatively thin (less than about 0.02″ thick for operations at about 20 GHz) to minimize the dielectric loading effects without impacting the thermal performance. Generally, the thermally-conductive dielectric attachments may be made from any otherwise suitable material having a thermal conductivity of at least 0.01 W/(in.2·° C.). So, a primary advantage of the new invention is to decrease the maximum temperature of the ferrite element and associated adhesive bondlines in order to improve the performance and survivability of ferrite circulators in high power applications. For example, a switch of the present invention, operating near 20 GHz, was found to handle 1.8 times as much power as a traditional switch for the same temperature rise in the ferrite element. Looking at this another way, the temperature rise in the ferrite for the present invention was only 56% of the temperature rise in the traditional switch for equal power levels. There are many RF switching applications where alternate switch technologies, such as pin diode or mechanical switches, are used because of their power handling capabilities. This invention broadens the applications for ferrite switches, making them a viable alternative to other switch technologies in high average power applications.
Referring to
Referring now to
The conductive waveguide structure may be air-filled. Conductive waveguide structure 200 may also include waveguide input/output ports 205, 206, and 207. Waveguide ports 205, 206, and 207 may provide interfaces for signal input and output. The empirical matching elements 204 may be disposed on the surface of conductive waveguide structure 200 to affect the performance characteristics. Matching elements may be capacitive/inductive dielectric or metallic buttons used to empirically improve the impedance match over the desired operating frequency band.
Still referring to
As the RF signal propagates through the waveguide structure 200, some power is absorbed in the various switch elements, and the majority of this absorbed power is contained in the ferrite element 201 due to its relatively high volume and high electrical and magnetic loss tangents. The waveguide structure 200 acts as a heat sink for the ferrite element 201. In conventional circulators, the thermal paths between these two parts are limited by the intersecting area between the ferrite element and the dielectric spacers and quarter-wave dielectric transformers. In the embodiment illustrated in
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While exemplary embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous insubstantial variations, changes, and substitutions will now be apparent to those skilled in the art without departing from the scope of the invention disclosed herein by the Applicant. Accordingly, it is intended that the invention be limited only by the spirit and scope of the claims, as they will be allowed.
This application claims priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/752,339, filed on Dec. 20, 2005, which is incorporated herein by reference. Not Applicable Not Applicable
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