Abstract:
An RF cable contains an coaxial inner conductor and a coaxial outer shield surrounding the inner conductor in a concentric arrangement. Quarter-wave series sections in the inner conductor and the outer shield severs a direct thermal path along the RF cable, providing low thermal loading for a cryogenic-to-ambient temperature interconnection. The resonant structure of the RF cable permits propagation alternating current and blocks direct current. A method of forming the RF cable comprises depositing metal on a substrate composed of a polymer film having very low thermal conductivity, and winding the metallized substrate into a tubular configuration. The inner conductor may extend laterally beyond the outer shield to provide points of electrical contact.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to the field of electromagnetic wave transmission and, more particularly, to a transmission cable for radio frequency (RF) waves. 
     BACKGROUND ART 
     In many RF electronic circuit configurations, there is a need to supercool the electronic circuits for improved performance. For example, a thermally cooled amplifier has a lower noise figure than an amplifier operated at ambient temperature. Emerging cryogenic microwave receiver systems that provide enhanced speed and sensitivity include cryogenic cooled components such as cooled mixers and superconductive components for handling signals. These systems place difficult demands on signal connections. The connections to these systems include one end typically at ambient temperature, and an opposite end at a cryogenic temperature. It is highly advantageous to reduce heat conduction along the RF coaxial signal connections to maintain the receiver components at the cryogenic temperature without placing excessive demands on the receiver system refrigeration unit, which commonly has limited cooling capabilities. Input and output via the connections is difficult because the connections need to present minimal thermal load while simultaneously minimizing transmission loss to the input and output signals. The efficiency and power dissipation in the refrigeration units is determined by the refrigeration power supply. The lower the heat load imposed by RF connections, the lower the temperature the refrigeration unit can cool the amplifier, producing a lower overall amplifier noise figure. Consequently, it is important to reduce the heat leakage along RF connections to the cryogenic system. 
     The problem of providing an input/output RF connection is fundamentally challenging because all materials having high electrical conductivity also have high thermal conductivity. No existing coaxial RF connection solves this problem. 
     In addition, connections for such cryogenic systems should have low insertion loss, which is a measure of transmission efficiency. Low insertion loss relates to reduced power loss during transmission. 
     Thus, there is a need for an improved RF connection that has (i) very low thermal conductivity, and (ii) low insertion loss over a range of frequencies. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved RF cable that has (i) very low thermal conductivity, and (ii) low insertion loss over a wide band of frequencies. The RF cable can transmit RF waves such as microwaves at modest currents between points at widely varying temperatures, such as between ambient and cryogenic temperatures. The RF cable transmits RF waves over a band which encompasses more than an octave in the frequency spectrum. The RF waves are typically microwaves, but can be other RF waves as well. 
     The RF cable comprises a coaxial inner conductor and a coaxial outer shield surrounding the inner conductor in a concentric configuration. The inner conductor can include a first inner conductor section, a second inner conductor section axially spaced from the first inner conductor section, and a third inner conductor section. The third inner conductor section has a length of about λ and includes opposed end portions each having a length of about nλ/4, where n is typically equal to one. One end portion coextends with the first inner conductor section at a break, and the other end portion coextends with the second inner conductor section at another break. The breaks are quarter-wave series sections. The inner conductor sections form a discontinuous axial thermal flow path along the inner conductor. The inner conductor sections are comprised of a highly electrically conductive material to achieve low electrical losses. A dielectric material can be provided between the end portions of the third inner conductor section and each of the first and second inner conductor sections. 
     The outer shield can include a first outer shield section, a second outer shield section axially spaced from the first outer shield section, and a third outer shield section. The third outer shield section has a length of preferably about λ/2 and includes opposed end portions each having a length of preferably about λ/4. One end portion coextends with the first outer shield section at a break, and the other end portion coextends with the second outer shield section at another break, thereby forming a discontinuous thermal flow path along the outer shield. The first, second and third outer shield sections are comprised of a highly electrically conductive material. A dielectric material can be provided between the end portions of the third outer shield section and each of the first and second outer shield sections. 
     The RF cable includes at least one break in each of the inner conductor and the outer shield. The breaks prevent the direct flow of heat along the inner conductor and the outer shield, and enable resonant transmission and good electrical conductance. 
     The RF cable can include, for example, a single break in each of the inner conductor and the outer shield. In this construction, the coaxial inner conductor comprises a first inner conductor section and a second inner conductor section, coextending over a length of preferably about λ/4. The coaxial outer shield comprises a first outer shield section and a second outer shield section, also coextending over a length of preferably about λ/4. 
     The RF cable can comprise means for maintaining the inner conductor and the outer shield in a substantially fixed configuration. For example, an electrical connector can be provided at the input and output ends. Dielectric material with low thermal conductance can be used to position the concentric conductance. The interior of the RF cable can be maintained at a low selected pressure to provide very low thermal conductance. 
     The RF cable can have a spiral configuration. The spiral configuration can be formed by depositing a highly electrically conductive material, typically a metal, onto a substrate having very low thermal conductivity, such as a dielectric material sheet. The substrate is wound in a spiral configuration, typically around a form having very low thermal conductivity, to form the spiral configuration. Breaks in the inner conductor and the outer shield form a discontinuous axial thermal flow path along the RF cable. The spiral configuration includes exposed end regions of the metal that enable direct electrical contact to the RF cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood from the following description, appended claims and accompanying drawings, where: 
     FIG. 1 is a longitudinal cross-sectional view of a double-break RF cable in accordance with the invention; 
     FIG. 2 is a longitudinal cross-sectional view of a single-break RF cable in accordance with the invention; 
     FIG. 3 illustrates an RF cable in accordance with the invention having a single break in the inner conductor and two breaks in the outer shield; 
     FIG. 4 is an RF schematic illustration of the RF cable of FIG. 3; 
     FIG. 5 shows the calculated insertion loss versus the electromagnetic wave frequency for single and double-break RF cables in accordance with the invention; 
     FIG. 6 is a top plan view of a metallized substrate prior to winding the substrate to form a spiral-shaped RF cable in accordance with the invention; 
     FIG. 7 is a perspective view of the spiral-shaped RF cable; 
     FIG. 8 is an axial cross-section in the direction of line  8 — 8  of FIG. 7; and 
     FIG. 9 is a transverse cross-section in the direction of line  9 — 9  of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an RF cable  20  in accordance with the invention. The RF cable  20  comprises an inner conductor  22  and an outer shield (current return)  24  surrounding the inner conductor  22  in a concentric, coax within a coax arrangement. The RF cable  20  defines a longitudinal axis A—A. 
     The inner conductor  22  comprises a first inner conductor section  26 , a second inner conductor section  28  axially spaced from the first inner conductor section  26 , and a third inner conductor section  30  partially within each of the first and second inner conductor sections in a coaxial configuration. As shown, the first and second inner conductor sections  26 ,  28  can be tubular shaped and of substantially the same diameter. The third inner conductor section  30  is also tubular shaped and has a smaller diameter than the first and second inner conductor sections  26 ,  28 . The inner conductor sections  26 ,  28  are preferably parallel to each other. Breaks  32  prevent direct axial heat flow along the entire length of the inner conductor  22 . 
     The inner conductor sections  26 ,  28 ,  30  are formed of an electrically conductive material to reduce RF losses. The material can be a metal such as copper, aluminum, gold, silver and the like. 
     The inner conductor sections  26 ,  28 ,  30  typically have a thickness equal to at least about 3-4 skin depths to enable sufficient electrical current flow along the inner conductor  22 . The skin depth is related to the electrical conductivity of the material and to the RF frequency. For example, the skin depth of copper at a microwave frequency of about 10 GHz is about 1 micron. 
     A dielectric material  36  can be provided between the first and second inner conductor sections  26 ,  28  and the third inner conductor section  30  at opposed end portions  34  of the third inner conductor section. The dielectric material  36  has low thermal conductivity so that heat flow from the first inner conductor section  26  to the third inner conductor section  30 , and from the third inner conductor section  30  to the second inner conductor section  28  is low. The dielectric material  36  can be, for example, “MYLAR,” a polystyrene polymer. 
     The outer shield  24  can comprise a first outer shield section  42 , a second outer shield section  44  axially spaced from the first outer shield section  42 , and a third outer shield section  46  partially surrounding each of the first and second outer shield sections  42 ,  44  in a coaxial configuration. The first and second outer shield sections  42 ,  44  are typically tubular shaped and of substantially the same diameter. The third outer shield section  46  is typically tubular shaped and has a greater diameter than the first and second outer shield sections  42 ,  44 . The outer shield sections  42 ,  44 ,  46  are preferably parallel to each other. Breaks  48  prevent direct axial heat flow along the outer shield  24 . 
     A dielectric material  50  can be provided between the first and second outer shield sections  42 ,  44  and the third outer shield section  46  at opposed ends  49  of the third outer shield section. The dielectric material  50  reduces heat flow from the first outer shield section  42  to the third outer shield section  46 , and from the third outer shield section  46  to the second outer shield section  44 . 
     The interior space  51  of the RF cable  20  can be filled with a dielectric material (not shown). The dielectric material contributes to the low thermal conductivity of the RF cable  20 . Alternately, the interior space  51  can be maintained at a vacuum pressure or filled with a gas such as air at an elevated pressure. 
     The input end  38  and the output end  40  of the RF cable  20  can be closed using respective electrical connectors  52 ,  53  to provide mechanical support and maintain the inner conductor  22  and the outer shield  24  in relative alignment, and to provide a gas seal to maintain the selected pressure within the interior space  51 . For example, the connectors  52 ,  53  can be SMA-type connectors. 
     The RF cable  20  can be used for RF transmission at modest currents. For example, weak signals from an antenna are typically at the microwatt level and at a peak current of about 0.2 mA. The RF cable  20  can be used for transmission to a system including electronic circuits at a low temperature, such as a cryogenically-cooled microwave receiver system (not shown). The input end  38  of the RF cable  20  can be at a temperature of about 300K, and the output end  40  at a cryogenic temperature up to about 80K. The cryogenic refrigeration systems conventionally used in microwave receiver systems have low cooling capacity. Accordingly, it is important to reduce heat conduction into the system. The efficiency and power dissipation of the refrigeration system is determined by the system&#39;s refrigeration power supply. The RF cable  20  reduces RF input thermal power to the refrigeration system, enabling the refrigeration system to cool an associated amplifier to a lower temperature to produce a lower overall amplifier noise figure. The RF cable  20  is particularly suitable for front end receiver and low noise RF applications. 
     The RF cable  20  blocks direct current (d.c.) flow because the breaks  32 ,  48  in the inner conductor  22  and the outer shield  24 , respectively, form an axially discontinuous electric charge flow path. Alternating current (a.c.) can flow along the entire length of the RF cable  20  due to the relative positioning of the inner conductor  22  and the outer shield  24 . More specifically, the inner conductor  22  and the outer shield  24  form sections Q each of a length of about nλ/4, where λ is a wavelength within the range of RF wavelengths transmitted along the RF cable  20 , and n is an odd integer of at least one. The sections Q preferably have a length of about a quarter wave (λ/4), and are referred to herein as “quarter-wave series sections”. The quarter-wave series sections maintain a low insertion loss over a wider RF wave frequency range than longer section lengths such as 3λ/4 and 5λ/4. The third inner conductor section  30  has a length of preferably about λ, and the third outer shield section  46  has a length of preferably about λ/2. The inner conductor  22  and the outer shield  24  can each have an arbitrary total axial length. The RF flow is under resonant conditions due to the presence of the quarter-wave series sections Q. The RF cable  20  characteristic impedence can be matched with the characteristic impedence of the RF input transmission line to the RF cable  20 . Accordingly, the RF cable  20  has good electrical conductance, despite the presence of the breaks  32 ,  48 . 
     The RF cable  20  has very low thermal conductivity. Particularly, the RF cable  20  has an estimated thermal load of only about 10 mW from a direct multi-watt coaxial RF connection, at an input end  38  temperature of about 300K and an output end  40  temperature of about 80K. This advantage is achieved by the breaks  32 ,  48  and the low thermal conductivity of the dielectric material  36 ,  50 . 
     As shown in FIG. 2, an alternative RF cable  60  in accordance with the invention comprises a coaxial inner conductor  62  and a coaxial outer shield  64 , with only a single break  66  in the inner conductor  62  and only a single break  68  in the outer shield  64 . The inner conductor  62  comprises a first inner conductor section  70  and a second inner conductor section  72  partially inside the first inner conductor section  70 . The inner conductor sections coextend over a length Q, which is preferably about λ/4. The second inner conductor section  72  has a length of preferably at least about λ/2. The outer shield  64  comprises a first outer shield section  74  which is partially surrounded by a second outer shield section  76 . The first and second outer shield sections  74 ,  76  coextend over a length Q, which is preferably about λ/4. The inner conductor sections  70 ,  72  and the outer shield sections  74 ,  76  are preferably substantially parallel to each other. 
     A dielectric material  78  having low thermal conductivity can be provided between the first and second inner conductor sections  70 ,  72 , and between the first and second outer shield sections  74 ,  76 , to reduce heat flow. 
     The RF cable  60  has an input end  80  and an output end  82 . Input and output connectors  84 ,  85  can be provided at the input end  80  and the output end  82 , respectively, to maintain a substantially fixed configuration of the inner conductors  62  and the outer shield  64 , and to maintain a selected pressure within the interior space  86  of the RF cable  60 . For example, the selected pressure can be maintained within the inner conductor  62 . The connectors  84 ,  85  can each be, for example, an SMA-type connector. 
     The quarter-wave series sections Q enable the transmission of RF waves under resonant conditions, and also enable good electrical conductance of the RF cable  60 . The breaks  66 ,  68  enable low thermal conductivity of the RF cable  60 . 
     An alternative RF cable  100  in accordance with the invention is shown in FIG.  3 . The RF cable  100  comprises a coaxial inner conductor  102  and a coaxial outer shield  104 . The inner conductor  102  includes a first inner conductor section  106  and a second inner conductor section  108 . The second inner conductor section  108  includes a first portion  110  preferably having about the same diameter as the first inner conductor section  106 , and a second portion  112  having a smaller diameter than the first portion  110 . The second portion  112  is inside of and coextends with the first inner conductor section  106  over a length Q preferably equal to about λ/4, such that the section  114  is a quarter-wave series section. The lengths L 1  and L 2  of the first and second inner conductor sections  106 ,  108 , respectively, are arbitrary. 
     The outer shield  104  includes a first outer shield section  116 , a second outer shield section  118  and a third outer shield section  120 . The first and second outer shield sections  116 ,  118  preferably have about the same diameter. The third outer shield section  120  includes end portions  122  each having a diameter greater than the diameter of the first and second outer shield sections  116 ,  118 , and an intermediate portion  124  having about the same diameter as the first and second outer shield sections  116 ,  118 . The end portions  122  surround and coextend with the respective first and second outer shield sections  116 ,  118 , over a length Q preferably equal to about λ/4, such that the sections  126  are quarter-wave series sections. Thus, the RF cable  100  includes a single break in the inner conductor  102  and two breaks in the outer shield  104 . 
     FIG. 4 is an RF schematic of the RF cable  100  of FIG.  3 . The different regions A-G as referenced in FIG. 3 are depicted. The regions A and G have lengths of L 1  and L 2 , respectively, and the regions B-F each have a length of about λ/4. 
     The insertion loss of the RF cables  20  and  60  is predicted to be very low over a relatively wide band of electromagnetic wave frequencies. The insertion loss is an indication of the transmission efficiency and can be defined as follows: 
     
       
         insertion loss=10 log 10 ( P   out   /P   in )  
       
     
     where insertion loss is given in decibels (dB), P out  is the power at the output end of the RF cable, and P in  is the power at the input end. An insertion loss of zero represents no loss of power. FIG. 5 shows the calculated insertion loss, over the frequency range of 0-20 GHz, of the double-break RF cable  20  and the single-break RF cable  60 , having quarter-wave series sections of a length equal to about λ/4 at 10 GHz. At 10 GHz, the RF cables  20 ,  60  operate at about perfect resonance. The insertion loss is only about −0.2 dB at 10 GHz, and about this very low value over the frequency range of from about 5 GHz to about 15 GHz. Overall, the single-break RF cable  60  and double-break RF cable  20  have comparable insertion loss characteristics. The frequency range over which the insertion loss is near zero generally increases as the number of breaks in the RF cable is increased. 
     Thus, the RF cable according to the present invention provides the advantages of very low thermal conductivity, good electrical conductance, and low insertion loss over a wide frequency band. 
     FIG. 7 illustrates a double-break RF cable  150  according to the invention having a spiral configuration. Referring to FIG. 6, the RF cable  150  can be formed by metallizing selected portions of a substrate  152  composed of a material having a low coefficient of thermal conductivity. Suitable materials for forming the substrate  152  include “MYLAR” and like polymer dielectric materials. The substrate  152  has a top edge  154  and a bottom edge  156 , and comprises regions R 1 , R 2  and R 3 , having respective side edges  158 ,  160 ,  162 , and respective widths W 1 , W 2  and W 3 . The illustrated configuration of the substrate  152  can be formed by cutting the regions C 1  and C 2  from a rectangular shaped substrate. The substrate  152  has an axial center line B—B and a transverse center line C—C. The substrate  152  can have a typical thickness of from about 0.25 mil to about 1 mil. Reducing the substrate  152  thickness reduces thermal conduction along the RF cable  150 . 
     A material having high electrical conductivity to reduce electrical losses is deposited on the surface  164  of the substrate  152  in the form of strips. The material can be a metal such as copper, aluminum, gold, silver and the like. The metal is applied at the regions  166 ,  168 ,  170  and  172  of the substrate  152 . The applied metal preferably has a thickness of at least 3-4 skin thicknesses. 
     The metal can be deposited on the substrate  152  by a conventional thin film deposition process such as chemical vapor deposition. The metal can be patterned using a conventional photoresist mask formed on the substrate  152 . 
     The metal is applied at selected areas of the surface  164  of the substrate  152 . A first metallic strip  166  of a length of preferably about λ is formed near the bottom edge  156  of the substrate  152 . A pair of laterally spaced, second metallic strips  168  are also formed at the region R 1  and transversely spaced from the first metallic strip  166 . The second metallic strips  168  are axially spaced and axially aligned with respect to each other. The second metallic strips  168  each coextend with the first metallic strip  166  along a length Q equal to preferably about λ/4. A pair of laterally spaced, third metallic strips  170  are formed at the region R 2 . A fourth metallic strip  172  of a length of preferably about λ/2 is formed at the region R 3 . The third metallic strips  170  each coextend with the fourth metallic strip  172  over a length Q equal to preferably about λ/4. The metallic strips are preferably parallel to each other on the substrate. 
     The RF cable  150  is formed by winding the metallized substrate  152  in the transverse direction C—C, beginning at the bottom edge  156  of the substrate  152 . The substrate  152  can be wound, for example, around a suitable form such as a glass rod (not shown) comprised of a low thermal conductivity material. The form can be removed after the RF cable  150  is formed or optionally left inside the RF cable  150 . The RF cable  150  has a continuous, spiral configuration. The second metallic strips  168  extend furthest laterally at both ends of the RF cable  150 , thereby providing electrical connection points. 
     FIG. 8 illustrates an axial cross-section of the RF cable  150 . 
     FIG. 9 shows a transverse cross-section of the RF cable  150 . As shown, the metallic strips  166 ,  168 ,  170  and  172  each have a spiral cross-sectional configuration and are concentrically positioned relative to each other in a coax within a coax configuration. The first metallic strip  166  and the second metallic strips  168  are separated from each other by the substrate  152  to form the inner conductor  174 . The third metallic strips  170  are separated from the second metallic strips  168  by the substrate  152 . The fourth metallic strip  172  is separated from the third metallic strips  170  by the substrate  152  to form the outer shield  176 . 
     The predicted thermal conductivity of the RF cable  150  is very low due to the thinness of the metallic strips  166 ,  168 ,  170 ,  172 , and to the thinness and low thermal conductivity of the substrate  152 . 
     Although the present invention is described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. In particular, the number of coaxial coupled sections are not limited. The number of quarter-wave series sections in the inner and outer coaxial conductors can be increased to provide more bandwidth. Therefore, the scope of the appended claims is not limited to the description of the preferred embodiments contained herein.