ELECTRICAL TRANSMISSION LINE

Abstract: An electrical transmission line of the twisted pair or coa­ xial cable type is provided. One line has a center conductor, a porous plastic dielectric disposed around the outer periphery of the conductor, the dielectric having specific gravity of 0.5 or less, and having a plastic dielectric sheath disposed around the outer periphery of the porous dielectric, the outer sheath having a melting temperature which is 60% or less of the melting temperature of the porous plastic dielectric. The porous plastic dielectric may be in tape form helically wrap­ ped around the conductor, preferably at a wrap angle of 20 degrees or less. The preferred porous dielectric is expanded polytetrafluoroethylene.

Description: The present invention relates to an electrical transmission line, in particular comprising a twisted pair cable or a coaxial cable and to a multiconductor flat cable transmission line comprising a plurality of individual transmission lines. This type of transmission line ofter employs a porous polytetrafluorehtylene resin as its dielectric. Examples of a transmission line which employs a porous polytetrafluorethylene resin as its dielectric include that in the "Method of Manufacturing Electric Wires Coated with Tetrafluorehtylene Polymer" disclosed in the specification of Japanese Patent Publication No. 21,809/1982. That invention has an arrangement in which, in order to prevent any increase in specific gravity of a porous polytetrafluorethylene resin layer, an unsintered polytetryfluorehtylene resin tape which has previously been rolled and compressed so that its specific gravity is in the range of 1.85 to 2.2 is wound around the outer periphery of a conductor and is then sintered at a temperature of 327° to 400°C. That method, however, still does not completely prevent an increase in the specific gravity of the resin layer. When a conductor having such a resin tape wound thereon is employed as a transmission line, the signal propagation delay time is around 4.0 ns./m, and it is not possible to increase the transmission speed substantially. From DE-A-2920 031, which discloses a transmission line of the type defined in the preamble of claim 1, it is also known a porous plastic dielectric material disposed around the center conductor, wherein the porous plastic material is polytetrafluorethylene (PTFE). FR-A-2133974 discloses a cable in which the dielectric material surrounding the center conductor is PTFE. The present inventors have found from investigations that the specific gravity of a porous dielectric made of a plastic resin starts to increase when heated at a temperature exeeding 60 % of the melting temperature of the resin, and there is virtually no increase in specific gravity of the porous dielectric when heated at a temperature of 60 % or less of the melting temperature of the resin. According to the present invention there is provided an electrical transmission line having the features of claim 1. Preferred forms of the invention provide a transmission characteristic which makes it possible to attain a cable having a signal propagation delay time of about 3.5 ns/m. The invention will now be particularly described by way of example, with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional perspective view of a single-conductor transmission line in accordance with one embodiment of the present invention; Fig. 2 is a cross-sectional perspective view of a coaxial transmission line in accordance with another embodiment; and Fig. 3 is an end elevational view of a plurality of the cables of Fig. 2 fuse bonded together at their respective interfaces to form a multi-conductor flat cable. An electrical transmission line of the twisted pair or coaxial cable type is provided. One line has a center conductor, a porous plastic dielectric disposed around the outer periphery of the conductor, the dielectric having specific gravity of 0.5 or less, and having a plastic dielectric sheath disposed around the outer periphery of the porous dielectric, the outer sheath having a melting temperature which is 60% or less of the melting temperature of the porous plastic dielectric. The porous plastic dielectric is in tape form helically wound around the conductor at a pitch angle of 20 degrees or less. The porous plastic dielectric is a stretched and sintered porous polytetrafluoroethylene resin. This is employed in the form of tape and the tape is wound helically on the conductor at an angle of 20 degrees or less with respect to the longitudinal axis of the conductor. There is, advantageously, substantially no increase in specific gravity of the tape in subsequent processing. On the outer periphery of the porous plastic dielectric of specific gravity of 0.5 or less, there is provided an outer dielectric made of a thermoplastic resin the melting temperature of which is 60% or less of the melting temperature of the porous dielectric. The outer dielectric may be provided by melt extrusion so as to form a sheath over the porous dielectric. It is therefore possible to obtain a transmission line which involves substantially no increase in specific gravity of the plastic porous dielectric and which has a stable propagation delay characteristic. Fig. 1 is a perspective view of an end portion of a single-wire transmission line 1 in accordance with one embodiment of the present invention, the transmission line 1 also being able to be employed as one element of a twisted pair cable or in a multiconductor ribbon cable. This single-wire transmission line 1 includes a conductor 2, and a porous plastic dielectric 3 provided on the outer periphery of the conductor 2 and made of a stretched and sintered porous polytetrafluoroethylene resin having a specific gravity of 0.5 or less, preferably 0.3. On the outer periphery of the porous dielectric 3 is provided, by melt extrusion, a thermoplastic dielectric sheath 4 having a circular cross-section and made of a vinyl chloride resin having a melting temperature (in particular of 170 o C) which is 60% or less of the melting point, i.e. 327 o C, of polytetrafluoroethylene resin constituting the porous plastic dielectric 3. In this embodiment, the porous plastic dielectric 3 is covered with the sheath 4 without substantially increasing the specific gravity, that is, 0.3, of the porous dielectric 3, and the porous dielectric 3 which constitutes the inner layer is protected by the sheath 4 made of the thermoplastic dielectric. As a result, it is possible to obtain a stable high-speed transmission line. It is noted that when a stretched and sintered porous polytetrafluoroethylene resin is employed as the porous dielectric 3, a porous polytetrafluoroethylene tape which has been subjected to extrusion, stretching and sintering may be wound on the outer periphery of the conductor 2, or a porous polytetrafluoroethylene resin layer may be formed on the outer periphery of the conductor 2 by other methods. Further, the conductor 2 may be either a single wire or a twisted wire. Fig. 2 is a perspective view of a coaxial transmission line 5 in accordance with another embodiment of the present invention. In the case of this coaxial transmission line 5, a porous plastic dielectric is provided on the outer periphery of conductor 6, the porous dielectric being formed by helically winding stretched and sintered porous polytetrafluoroethylene tapes 7 and 8 in two layers in opposite directions on the outer periphery of the conductor 6 at a pitch angle of 20 degrees or less with respect to the longitudinal axis of the conductor 6. An outer metallic conductor 9 is provided on the outer periphery of the porous dielectric, and the transmission line 5 is provided with a thermoplastic dielectric sheath 10 made of a vinyl chloride resin and having a rectangular cross-section on its outermost periphery. With the structure in accordance with this embodiment, it was possible to attain an average propagation delay time of 3.53 ns./m in the case where the specific gravity of the porous plastic dielectric 8 was 0.2 to 0.3 and in which an aluminum/polyester layup tape was employed as the outer conductor 9. The coaxial transmission line 5 in accordance with this embodiment can be employed as a single element as shown in Fig. 2 and it is also possible to employ the coaxial transmission line 5 in a flat cable, a rattan-blind-like coaxial flat cable, or the like, by disposing a plurality of coaxial transmission lines 5 parallel to each other and connecting, for example, the respective rectangular sheath surfaces of the transmission lines 5 to each other by fusion bonding as shown in Fig. 3. Under certain conditions, it is possible to obtain a transmission line in which the propagation delay time is about 3.4 ns./m. As a consequence, the transmission line can attain a very high signal propagation speed, which is about 98% of the velocity of light. It is noted that the present invention is not necessarily limited to the above embodiments, and various changes and modifications may be imparted thereto within the scope of the present invention.

Claims: An electrical transmission line (1, 5) comprising: (a) a center conductor (2, 6), (b) a porous, plastic dielectric material (3, 8) disposed around the outer periphery of said conductor (2, 6), wherein said porous plastic dielectric (3, 8) is porous, sintered polytetrafluorethylene (PTFE), (c) a plastic dielectric outer sheath (4, 10) disposed around the outer periphery of the porous dielectric (3, 8) characterized in that (d) the porous dielectric (3, 8) is stretched PTFE having specific gravity of 0.5 or less, (e) the outer sheath (4, 19) has a melting temperature which is 60 % or less of the melting temperature of the porous plastic dielectric (3, 8), and (f) wherein said porous dielectric (3, 8) is in tape form and is helically wound around said conductor (2, 6) at a pitch angle of 20 degrees or less with respect to the longitudinal axis of said conductor (2, 6). A transmission line according to claim 1 wherein said outer sheath (4, 10) has a rectangular cross-section. A multiconductor flat cable transmission line (12) comprising a plurality of individual transmission lines (5) according to claim 1, each having a rectangular cross-section, bonded together at their respective interfaces.

Inventors: HIRANO, TATSUO, WADA, TOSHIO
Applicants: JUNKOSHA CO. LTD.
Application year: 1986
Main classification: H01B7
IPC codes: H01B7, H01B11