Patent Abstract:
A radio frequency (RF) thermal isolator and method of manufacture for same. According to one embodiment, the RF thermal isolator includes a first transmission line; a second transmission line of nominally the same dimensions as the first transmission line and axially aligned with the first transmission line, wherein the ends of the transmission lines are separated by a gap having a width that is a very small fraction of the center operating wavelength of the transmission lines; and an electrically conductive sleeve electrically attached to the end of the first transmission line and surrounding the end of the second transmission line and separated from the second transmission line by a gap having a width that is a very small fraction of the center operating wavelength of the transmission lines; wherein the sleeve extends along the second transmission line from the end of the first transmission line for a distance of nominally ¼ of the center operating wavelength of the transmission lines.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to thermal isolation, and more particularly to thermal isolation in radio frequency (RF) transmission lines coupled to cooled systems. 
     2. Related Art 
     Any radio frequency (RF) conductor, such as a cable or waveguide, that includes a metallic component conducts heat. When such an RF conductor is used for connection to a cooled system, heat is transmitted to the cooled system through the RF conductor. The result is a loss of cooling in the cooled system, an increase in the power needed to maintain the desired temperature in the cooled system, or both. 
     One example of a cooled system is a transceiver placed in a dewar cryogenically cooled by liquid nitrogen to approximately 77 degrees Kelvin. By employing high temperature superconductivity (HTS) technology, such systems can achieve reductions in weight, size and RF loss. One potential application for such an HTS transceiver is in a cellular telephone base station, where there is a demand for a low-noise high-performance front end. Another potential application for an HTS transceiver is on board a communications satellite, where there are similar requirements. 
     One approach to achieving thermal isolation is to simply cut a gap in the transmission line. While this approach provides excellent thermal isolation, it unfortunately also produces large ohmic signal loss. 
     Another approach is to use very thin transmission lines to reduce heat flow through the transmission lines. While this approach provides moderate thermal isolation, it also produces moderate signal loss. Further, such transmission lines are unreliable due to their fragility. 
     SUMMARY OF THE INVENTION 
     The present invention is a radio frequency (RF) thermal isolator and method of manufacture for same. According to one embodiment, the RF thermal isolator includes a first transmission line; a second transmission line of nominally the same dimensions as the first transmission line and axially aligned with the first transmission line, wherein the ends of the transmission lines are separated by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines; and an electrically conductive sleeve electrically attached to the end of the first transmission line and surrounding the end of the second transmission line and separated from the second transmission line by a gap having a width that is a very small fraction of the center operating wavelengths at the operating frequency of the transmission lines; wherein the sleeve extends along the second transmission line from the end of the first transmission line for a distance of nominally ¼ of the center operating wavelength at the operating frequency of the transmission lines. 
     In one aspect the gaps have a width that is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of the transmission lines. 
     In one embodiment, each of the transmission lines is a waveguide. In another embodiment, each of the transmission lines is a coaxial cable having an inner conductor and an outer conductor. A center conductor extends axially from the inner conductor of the first transmission line into a cavity in the center conductor of the second transmission line, wherein the center conductor extends beyond the end of the first transmission line for a length that is nominally ¼ of the center operating wavelength at the operating of transmission lines. The cavity extends into the center conductor of the second transmission line for a distance of nominally ½ of the center operating wavelength of the transmission lines. 
     In one aspect the RF thermal isolator includes a mechanical coupler attached between the transmission lines. 
     In one aspect the transmission lines and sleeve are fabricated from a conductive metal. 
     In one aspect the transmission lines and sleeve are fabricated from a composite material coated with a metallic layer. 
     In one aspect the inner conductors of the coaxial cables are hollow, and the cavities within the RF thermal isolator are vented to each other and to the exterior of the RF thermal isolator. 
     The method of manufacture includes electrically attaching an electrically conductive sleeve upon the outer surface of a first transmission line, wherein the sleeve extends beyond an end of the first transmission line for a distance of nominally ¼ of the center operating wavelength at the operating frequency of the first transmission line, and disposing an end of a second transmission line of nominally the same dimensions as the first transmission line within the sleeve such that the second transmission line is axially aligned with the first transmission line and the ends of the transmission lines are separated by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines; wherein the sleeve surrounds the end of the second transmission line and is separated from the second transmission line by a gap having a width that is a very small fraction of the center operating wavelength at the operating frequency of the transmission lines. 
     According to one embodiment, each of the transmission lines is a waveguide. 
     According to another embodiment, each of the transmission lines is a coaxial cable having an inner conductor and an outer conductor, and the method includes forming a cavity in the center conductor of the second transmission line, the cavity having a length of nominally ½ of the center operating wavelength at the operating frequency of the transmission lines; and mounting a center conductor upon the inner conductor of the first transmission line such that the center conductor extends axially from the inner conductor of the first transmission line into the cavity in the center conductor of the second transmission line, wherein the center conductor extends beyond the end of the first transmission line for a length that is nominally ¼ of the center operating wavelength at the operating frequency of the transmission lines. 
     In one aspect the method includes mounting a mechanical coupler between the transmission lines. 
     In one aspect the method includes mounting a mechanical coupler between the sleeve and the second transmission line. 
     In one aspect the method includes mounting a retainer upon the second transmission line; and mounting a mechanical coupler between the sleeve and the retainer. 
     In one aspect the transmission lines and sleeve are fabricated from a conductive metal. 
     In one aspect the transmission lines and sleeve are fabricated from a composite material coated with a metallic layer. 
     In one aspect the inner conductor of the coaxial cables is hollow, and the cavities within the coaxial cables and the sleeve are vented to each other and to the exterior of the RF thermal isolator. 
     In one aspect the gaps have a width that is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of the transmission lines. 
     According to one embodiment, the present invention includes the product made by the process of the methods described above. 
     One advantage of the present invention is that it provides excellent thermal isolation with minimal signal loss. 
     Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a cross-sectional view of a waveguide RF thermal isolator according to a preferred embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of a coaxial RF thermal isolator according to a preferred embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of a coaxial RF thermal isolator according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in terms of the above example. This is for convenience only and is not intended to limit the application of the present invention. In fact, after reading the following description, it will be apparent to one skilled in the relevant art how to implement the present invention in alternative embodiments. 
     The present invention is an RF thermal isolator that provides a very high thermal resistance with no appreciable RF signal loss. The isolator can be used in any transmission line, including waveguides and coaxial cables. The isolator is effective at all RF frequencies, ranging from high frequency up to and including millimeter wave frequencies. 
     The isolator has a very wide bandwidth, sufficient for cellular and satellite applications. For an ultrawide bandwidth application, a plurality of isolator outer chokes are arranged in series, each configured for different frequencies within the bandwidth. By placing several RF thermal isolators in series, one can increase the thermal isolation. 
     FIG. 1 is a cross-sectional view of a waveguide RF thermal isolator  100  according to a preferred embodiment of the present invention. RF thermal isolator  100  includes standard waveguides  102  and  106  and an RF choke  104 . In a preferred embodiment, RF choke  104  is a sleeve fabricated from the same materials as waveguides  102  and  106 . These materials can include conductive metals, such as copper and gold-plated stainless steel, composite materials coated with a metallic layer, and other materials. In one embodiment, RF choke  104  is electrically attached to an end of waveguide  102 . In another embodiment, RF choke  104  is formed by flaring an end of waveguide  102 . 
     In either embodiment, the length of RF choke  104  is L 1 . In a preferred embodiment, L 1  is nominally ¼ of the center operating wavelength at the operating frequency of waveguides  102  and  106 . 
     An end of waveguide  106  extends within RF choke  104 . The ends of waveguides  102  and  106  are separated by a gap g 1 . In a preferred embodiment, g 1  is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of waveguides  102  and  106 . 
     RF choke  104  is separated from the outer surface of waveguide  106  by a gap g 2 . In a preferred embodiment, g 2  is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of waveguides  102  and  106 . 
     In other embodiments, g 1  and g 2  are of different dimensions, selected according to the desired impedance by methods well-known in the art. In general g 1  and g 2  are a very small fraction of the center operating wavelength at the operating frequency of waveguides  102  and  106 . 
     RF thermal isolator  100  presents an RF short circuit path to the signal traversing waveguides  102  and  106 , thereby minimizing RF loss. However, RF thermal isolator  100  presents a thermal open circuit, thereby minimizing heat transmission between waveguides  102  and  106 . 
     In a preferred embodiment, waveguides  102  and  106  and RF choke  104  are held in place by a mechanical couple (not shown). In a preferred embodiment, the mechanical coupler is a tube made from a nonconductive material such as G10 fiberglass, a laminate made of fiberglass laid in epoxy resin. In another embodiment, the mechanical coupler is implemented as one or more fasteners, such as set screws, extending radially inward from RF choke  104  to seat against the outer surface of waveguide  106 . 
     In one embodiment, RF thermal isolator  100  is employed within a spacecraft system designed to operate within a vacuum. Therefore, the cavity within waveguides  102  and  106  is vented to the exterior of the waveguides. 
     FIG. 2 is a cross-sectional view of a coaxial RF thermal isolator  200  according to a preferred embodiment of the present invention. RF thermal isolator  200  includes standard coaxial cables  202  and  206 , an inner conductor extension a sleeve  216 , and  204 . 
     Coaxial cable  202  includes an outer conductor  208  and an inner conductor  210 . Coaxial cable  206  includes an outer conductor  212  and an inner conductor  214 . 
     In one embodiment, sleeve  204  is electrically attached to an end of coaxial cable  202  at its outer conductor  208 . In another embodiment, sleeve  204  is formed by flaring an end of outer conductor  208 . In a preferred embodiment, RF choke  204  is fabricated from the same materials as coaxial cables  202  and  206 . These materials include conductive metals, such as copper and gold-plated stainless steel, composite materials coated with a metallic layer, and other materials. 
     The length of sleeve  204  is L 1 . In a preferred embodiment, L 1  is nominally ¼ of the center operating wavelength at the operating frequency of coaxial cables  202  and  206 . 
     An end of coaxial cable  206  extends within sleeve forming an outer RF choke  204 . Outer conductor  208  of coaxial cable  202  is separated from outer conductor  212  of coaxial cable  206  by a gap g 1 . In a preferred embodiment, g 1  is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of waveguides  202  and  206 . 
     Sleeve  204  is separated from outer conductor  212  of coaxial cable  206  by a gap g 2 . In a preferred embodiment, g 2  is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of coaxial cables  202  and  206 . 
     Inner conductor  210  of coaxial cable  202  is separated from inner conductor  214  of coaxial cable  206  by a gap g 3 . In a preferred embodiment, g 3  is nominally {fraction (1/100)} of the center operating wavelength at the operating frequency of coaxial cables  202  and  206 . 
     In other embodiments, g 1 , g 2  and g 3  are of different dimensions, selected according to the desired impedance by methods well-known in the art. In general g 1 , g 2  and g 3  are a very small fraction of the center operating wavelength at the operating frequency of coaxial cables  202  and  206 . 
     Inner conductor  214  of coaxial cable  206  includes a cavity  218 . Inner conductor extension  216  is electrically attached to inner conductor  210  of coaxial cable  202 . Inner conductor extension  216  extends within cavity  218  for a distance L 2  forming an inner RF choke. Cavity  218  extends beyond inner conductor extension  216  for a distance L 3 . Therefore, cavity  218  has a total depth of L 2 +L 3 −g 3 . In a preferred embodiment, L 1 , L 2  and L 3  are each nominally ¼ of the center operating wavelength at the operating frequency of coaxial cables  202  and  206 . 
     Outer conductors  212  and  208  each have an inner diameter d 1  and an outer diameter d 2 . Inner conductor extension has a diameter d 3 . Inner conductors  210  and  214  have an outer diameter d 4 . 
     In one embodiment, the center operating wavelength at the operating frequency of coaxial cables  202  and  206  is 2.96 inches. Therefore, L 1 =L 2 =L 3 =0.74 inches. Also, g 1 =g 2 =g 3 =0.030 inches, d 1 =0.22 inches, d 2 =0.25 inches, d 3 =0.020 inches, and d 4 =0.087 inches. 
     In a preferred embodiment, coaxial cables  202  and  206  and outer RF choke  204  are held in place by a mechanical couple (not shown). In a preferred embodiment, the mechanical coupler is a tube made from a nonconductive material such as G10 fiberglass, a laminate made of fiberglass laid in epoxy resin. In another embodiment, the mechanical coupler is implemented as one or more fasteners, such as set screws, extending radially inward from outer RF choke  204  to seat against the outer surface of outer conductor  212 . 
     In a preferred embodiment, inner conductors  210  and  214  are hollow to provide venting in a vacuum system, such as a dewar. Inner conductor extension  216  is coupled to inner conductor  210  by a vented plug (not shown) formed within inner conductor  210 . Cavity  218  is formed by placing a vented plug within inner conductor  214  at a distance L 2 +L 3 −g 3  from its opening. 
     RF thermal isolator  200  presents an RF short circuit path to the signal traversing coaxial cables  202  and  206 , thereby minimizing RF loss however, RF thermal isolator  200  presents a thermal open circuit, thereby minimizing heat transmission between coaxial cables  202  and  206 . 
     FIG. 3 is a cross-sectional view of a coaxial RF thermal isolator  300  according to a preferred embodiment of the present invention. RF thermal isolator  300  includes standard coaxial cables  302  and  306 . Coaxial cable  302  includes an outer conductor  308  and an inner conductor  310 . Coaxial cable  306  includes an outer conductor  312  and an inner conductor  314 . 
     An outer RF choke  304  is electrically attached to outer conductor  308 . A retainer  320  is attached to outer conductor  312 . A mechanical coupler  322  is attached to RF choke  304  and retainer  320 . 
     In one embodiment, RF thermal isolator  300  is employed within a vacuum. Therefore, the cavities within coaxial cables  302  and  306  are vented with respects to each other and to the exterior of the coaxial cables. Thus an axial passage  330  is formed within inner conductor  316  and its mounting plug  324  so that the interior of inner conductor  310  and cavity  318  are in fluid communication. Similarly, an axial passage  332  is formed within plug  326  at the end of cavity  318  so that the interior of inner conductor  314  and cavity  318  are in fluid communication. Cavity  318 , the cavity between inner conductor  310  and outer conductor  308 , and the cavity between inner conductor  314  and outer conductor  312  are in fluid communication. This cavity is in fluid communication with the cavity between outer RF choke  304  and outer conductor  312 . The space formed by these cavities is vented to the exterior by a small vent hole  328  in mechanical coupler  322 . 
     CONCLUSION 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be placed therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by any of the above-described example embodiments, but should be defined only in accordance with the following claims and their equivalents.

Technology Classification (CPC): 7