Low loss thermal block RF cable and method for forming RF cable

• US 6,207,901 B1
• Filed: 04/01/1999
• Issued: 03/27/2001
• Est. Priority Date: 04/01/1999
• Status: Expired due to Fees

• Current Assignee: Northrop Grumman Corporation
• Original Assignee: TRW Inc. (Northrop Grumman Corporation)
• Inventors: Smith, Andrew D., Allen, Barry R.
• Primary Examiner(s): Kincaid, Kristine
• Assistant Examiner(s): Mayo, III, William H.
• Application Number:
  US09/285,032
• Time in Patent Office: 726 Days
• Field of Search: 174/117.F, 174/117.FF, 174/36, 174/21.C, 174/28, 174/29, 174/102.R, 174/102.SP, 333/12
• US Class Current: 174/102.00R
• CPC Class Codes: H01P 1/30 for compensation of, or protection against, temperature or moisture effects ; for improving power handling capability H01P1/04, H01P1/08 take precedence
• CPC Class Codes: H01P 3/06 Coaxial lines

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.

Claims

1. An RF cable for transmitting RF waves over a band of wavelengths which encompasses a wavelength λ

, the RF cable comprising;

a) a coaxial inner conductor including;

i) a first inner conductor section;

ii) a second inner conductor section laterally spaced from the first inner conductor section;

iii) a third inner conductor section including opposed end portions, one of said end portions being transversely spaced from, and coextending over a length of about λ

/4 with, the first inner conductor section, and the other of said end portions transversely spaced from, and coextending over a length of about λ

/4 with, the second inner conductor section, thereby forming a discontinuous thermal flow path along the inner conductor; and

iv) a dielectric material between the end portions of the third inner conductor section and each of the first and second inner conductor sections;

wherein the first, second and third inner conductor sections are composed of an electrically conductive material; and

b) a coaxial outer shield surrounding the inner conductor, the outer shield including;

i) a first outer shield section;

ii) a second outer shield section laterally spaced from the first outer shield section;

iii) a third outer shield section including opposed end portions, one of said end portions of the third outer shield section being spaced from, and coextending over a length of about λ

/4 with, the first outer shield section, and the other of said end portions being spaced from, and coextending over a length of about λ

/4 with, the second outer shield section, thereby forming a discontinuous thermal flow path along the outer shield; and

iv) a dielectric material between the end portions of the third outer shield section and each of the first and second outer shield sections;

wherein the first, second and third outer shield sections are composed of an electrically conductive material.

2. The RF cable of claim 1, having an insertion loss of about −

0.2 dB at an RF wave frequency of about 5 GHz to about 15 GHz.

3. The RF cable of claim 1, wherein the inner conductor and the outer shield each have an input end and an output end, the RF cable having a thermal load of about 10 mW at an input end temperature of about 300K and an output end temperature of about 77K.

4. The RF cable of claim 1, wherein the third inner conductor section has a length of about λ

, and the third outer shield section has a length of about λ

/2.

5. The RF cable of claim 1, wherein the first, second and third inner conductor sections and the first, second and third outer shield sections each have a thickness equal to at least about 3-4 skin thicknesses of the thermally conductive material.

6. The RF cable of claim 1, further comprising means for maintaining the inner conductor and the outer shield in a substantially fixed configuration.

7. The RF cable of claim 1, comprising an input end, an output end, and a connector disposed at each of the input end and the output end.

8. The RF cable of claim 1, wherein the first inner conductor section includes exposed portions at opposed lateral ends of the RF cable for electrical connection to the RF cable.

9. An RF cable for transmitting RF waves over a band of wavelengths which encompasses a wavelength λ

, the RF cable having an input end, an output end and a longitudinal axis, the RF cable comprising;

a) a coaxial inner conductor including;

i) a metallic first inner conductor section;

ii) a metallic second inner conductor section axially spaced from the first inner conductor section; and

iii) a metallic third inner conductor section having a length of about λ and

including opposed end portions, one of said end portions being radially spaced from, and coextending over an axial length of about λ

/4, with the first inner conductor section, and the other of said end portions being radially spaced from, and coextending over an axial length of about λ

/4 with, the second inner conductor section, thereby forming a discontinuous axial thermal flow path along the inner conductor; and

iv) a dielectric material between said end portions of the third inner conductor section and each of said first inner conductor section and said second inner conductor section; and

b) a coaxial outer shield surrounding the inner conductor in a concentric configuration, the outer shield including;

i) a metallic first outer shield section;

ii) a metallic second outer shield section axially spaced from the first outer shield section;

iii) a metallic third outer shield section having a length of about λ

/2 and including opposed end portions, one of said end portions of the third outer shield section being radially spaced from, and coextending over an axial length of about λ

/4 with, the first outer shield section, and the other of said end portions of said third outer shield section being radially spaced from, and coextending over an axial length of about λ

/4 with, the second outer shield section, thereby forming a discontinuous axial thermal flow path along the outer shield;

iv) a dielectric material between the end portions of the third outer shield section and each of the first outer shield section and the second outer shield section, respectively; and

c) means for maintaining the inner conductor and the outer shield in a substantially fixed configuration;

wherein (i) the inner conductor and the outer shield each have an input end and an output end, the RF cable having a thermal load of about 10 mW at an input end temperature of about 300K and an output end temperature of about 77K; and

(ii) the RF cable having an insertion loss of about −

0.2 dB at an RF wave frequency of about 5 GHz to about 15 GHz.

10. The RF cable of claim 9, wherein the first, second and third inner conductor sections and the first, second and third outer shield sections each have a thickness equal to at least about 3-4 skin thicknesses of the metallic material.

11. The RF cable of claim 9, wherein the first inner conductor section includes exposed electrical connection portions at opposed ends of the RF cable.

12. An RF cable for transmitting RF waves over a band of wavelengths which encompasses a wavelength λ

, the RF cable having a longitudinal axis and comprising;

a) a coaxial inner conductor including;

i) an electrically conductive first inner conductor section having a diameter; and

ii) an electrically conductive second inner conductor section having a first portion having about the diameter of the first inner conductor section and a second portion having a smaller diameter than the first portion, the second portion being radially spaced from and coextending over a length of about λ

/4 with, the first inner conductor section, thereby forming a discontinuous thermal flow path along the inner conductor; and

b) a coaxial outer shield surrounding the inner conductor, the outer shield including;

i) an electrically conductive first outer shield section having a diameter;

ii) an electrically conductive second outer shield section axially spaced from the first outer shield section and having about the same diameter as the first outer shield section; and

iii) an electrically conductive third outer shield section including opposed end portions and an intermediate portion, the end portions each having a larger diameter than the intermediate portion and the intermediate portion having about the same diameter as the first and second outer shield sections, one end portion being radially spaced from, and coextending over a length of about λ

/4 with, the first outer shield section, and the other end portion being radially spaced from, and coextending over a length of about λ

/4 with, the second outer shield section, thereby forming a discontinuous thermal flow path along the outer shield.

13. A method of forming an RF cable for transmitting RF waves over a range of wavelengths which encompasses a wavelength λ

, the method comprising the steps of;

a) providing a substrate having a top edge, a bottom edge opposed side edges, and a face, the substrate being comprised of an electric insulator;

b) forming a strip pattern of an electrically conductive material on the face of the substrate, the strip pattern including;

i) a first strip;

ii) a pair of second strips spaced from the first strip in a transverse direction which extends from the bottom edge toward the top edge of the substrate, the second strips being substantially aligned with each other in a longitudinal direction;

iii) a pair of third strips spaced from the second strips in the transverse direction, the second strips being substantially aligned with each other in the longitudinal direction; and

iv) a fourth strip spaced from the third strips in the transverse direction;

wherein the first, second, third and fourth strips are substantially parallel to each other; and

c) winding the substrate in the transverse direction to form the RF cable having a spiral configuration and defining a longitudinal axis, the RF cable comprising;

i) a coaxial inner conductor including;

1) the first strip having a spiral configuration and including opposed end portions;

2) the second strips radially spaced from the first strip, each second strip having a spiral configuration, the second strips each including an end portion having, the end portions of the second strips each coextending with one of the end portions of the first strip over a length of about λ

/4, thereby forming a discontinuous thermal flow path along the inner conductor;

ii) a coaxial outer shield surrounding the inner conductor in a concentric configuration, the outer shield including;

1) the third strips radially spaced from the second strips, each third strip having a spiral configuration, the third strips each including an end portion;

2) the fourth strip radially spaced from the third strips, the fourth strip including opposed end portions, the end portions of the fourth strip each coextending with an end portion of one of the third strips over a length of about λ

/4, thereby forming a discontinuous thermal flow path along the outer shield.

14. The method of claim 13, wherein the inner conductor includes exposed portions at opposed lateral ends of the RF cable for electrical connection to the RF cable.