US 3248473 A
Description (OCR text may contain errors)
April 26, 1966 G. BUHMANN 3,248,473
LOW-CAPACITANCE TYPE OF HIGH-FREQUENCY CABLE Filed Sept. 6, 1965 Celia/05e 7f3/mig- 4 INVENTOR SUA/THE@ @Uf/MANN BY Q7/5% ATTORNEY United States Patent O 3,248,473 LOW-CAPACITANCE TYPE OF HIGH- FREQUENCY CABLE' Gunther Buhmann, Ludwigsburg, Germany, assignor to International Standard Electric Corporation, New York, N.Y., a corporation ot Delaware Filed Sept. 6, 1963, Ser. No. 307,055 Claims priority, application Germany, Sept. 19, 1962,
St 19,733 Claims. (Cl. 174-29) This invention relates to coaxial cables and more particularly to insulation'for coaxial cables. Present lowcapacitance high-frequency cables are characterized by fairly large spacings between the inner and outer conductors. Coaxial cables of this type utilize air-space insulating and inner conductors consisting of, very thin wire. Because of the need for mechanical strength, the diameter of the thin inner conductor cannot be reduced. If a very low capacitance is desired between the inner and outer conductor, the diameter of the outer conductor can be increased, but this will cause an increase in both the thickness and the stiffness of the cable. In the cases where it is absolutely necessary to avoid this, the electrical values .can only be maintained by reducing the dielectric constant. This is applicable all the more in cases where, for some reason or another, it is necessary to somewhat enlarge the diameter of the inner conductor, eg. in order to provide it with a greater tensile strength, or where it might possibly be necessary to reduce the diameter of the outer conductor, e.g. for reasons of saving space, or where it may be necessary to carry out both measures at the same time.
Since the inner conductor must be guided by securing it concentrically with respect to the outer conductor, supporting constructions are required for the inner conductor. Such supporting constructions of the type known per se, have a deteriorating effect upon the dielectric constant. Therefore, it is not possible to achieve a dielectric constant of unity in the space between the inner and outer conductor. It will always exceed one. In conventional types of low-capacitance coaxial cables supporting constructions are used which consist of an insulating material whose dielectric constant already considerably approaches the value of l. However, if it is necessary, as mentioned hereinbefore, to increase the internal diameter or to reduce the external diameter -for some reason, or another, or if it is necessary to do both at the same time, then the supporting structure for the inner conductor must be provided with a corresponding greater amount of insulating material, and the value of the dielectric con- 'stant will then exceed the ideal value l by a wide margin. This is also true e.g. in the case of low-capacitance highfrequency cables having a characteristic impedance of 150 ohms when considering the coaxial type corresponding to the German Industrial Standard DIN 47268.
Cables of this type are required to have a maximum capacitance of 27 picofarads per meter (pf./m.). Ac cording to the German Industrial Standard DIN 47268 these cables lare grouped in accordance with various diameters. In the 6.6 mm. diameter group the internal diameter of the outer conductor is supposed to range from 6.1 to 6.6 mm., and in the mm. diameter group the internal diameter of the outer conductor is supposed to range from 9.5 to 10.0 mm.
In the case of a given dielectric and a previously fixed internal diameter of the outer conductor, the thickness of the inner conductor is determined by the equation:
loge d lloge D r-g l ICC where:
C=capacitance required evzpermittivity of free space=8.85 l012 farads per v meter er=dielectric constant relative to air D=internal diameter of outer conductor dzexternal diameter of inner conductor Accordingly, with respect to the 6.6 mm. diameter group there will result an inner conductor diameter of 0.3 mm., and with respect to the 10 mm. diameter group there will then result an inner conductor diameter of 0.45 mm. In this case the dielectric constant (er) for polyethylene foam material was assumed to be from 1.45 to 1.50.
From these figures it will be recognized that, in dimen- Sioning the inner conductor, the customarily used copper ternal diameter of the cable. Since the cable diameter will remain unchanged, the diameter group will remain the same. On the other hand, it is possible to reduce the external diameter of the cable in spite of the unchanged diameter of the inner conductor. This will provide the advantage of a more flexible and inexpensive cable. If necessary, it is also possible to combine the modifications of dimensions.
-The above-mentioned andother features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing showing a cross-section of a cable in accordance with my invention, in which:
The cable is constructed as follows:
Approximate diameter when assembled, mm. Inner conductor: Copper, solid, long 0.5 Insulation:
Plastic rope or cord with a pitch of 6-12 mm.
and a diameter of about 0.8 to 1.5 mm. 2-3 Plastic foil tube of polyethylene vfoam material 8 Outer conductor: Copper mesh 9 Protective sheath: Polyvinylchloride 1l Inner conductor 1 is surrounded by a plastic rope consisting of foam material 2 and `a tube 3 of insulating material. Tubing 3, which provides mechanical support, may be eliminated and a solid plastic rope used. The rope helically surrounds the inner conductor. Sheathing 4 of plastic foil surrounds and contains the plastic rope. This plastic foil may consist of cellulose triacetate. The remaining space `between the inner and outer conductor is filled with insulating foam material 5. On top of this is arranged therouter conductor 6, which may have the shape of a copper mesh or of a copper tape, and which is provided on its outside of a protective sheathing 7, e.g. of polyvinylchloride (PVC).
Accordingly, in the proximity of the inner conductor,
this cable construction comprises a supporting arrange- If it is possible to i The rope may be designed as a solid length of polyethylene having a diameter between 0.8 and 1.2 mm.; the patch of this rope being 6-8 mm. With the aid of this rope construction and in combination with the hose or tube of foam material, a dielectric constant of 1.35 mm. is obtained; The rope may be made of polyethylene foam material. This will further reduce the dielectric constant. In order to obtain the necessary tensile strength for foam-material rope, a carrier or support is embedded in it. A thin thread of polyamide having a diameter of about 0.1 mm. may be used for this support. It has been found that this thin thread does not have adverse affects on the electrical characteristics of the cable. With respect to a rope of foam material at a pitch of 9 to 12 mm., a diameter from about 1.0 to 1.5 mm. has proved to be satisfactory. When employing construction of insulation between the inner and outer conductor, including the foam-material tube insulation there is obtainable a dielectric constant ranging from 1.25 to 1.30.
The advantage of such a construction of the insulation betwen the inner and the outer conductor over the conventional types of cables with a characteristic impedance of 150 ohms results from the extremely low dielectric constant achieved. The observance of certain dielectric values, such as a 150 ohms characteristics impedance, and a capacitance of 27 pf/m. permits, with respect to this type of cable construction, a reduction of the external div ameter or an increase of the thickness of the inner conductor; or both measures may be carried out at the same time. It may `be regarded as 4a further advantage of such a cable that it can be manufactured in two manufacturing operations. As regards the conventional types of low-capacitance high-frequency coaxial cables not only is the dielectric constant higher, but, for manufacturing the insulation between the conductors, it is still necessary to provide three and more steps or courses of manufacture.
For the purpose of overcoming the aforementioned deiciencies of conventional types of cables, and for obtaining the already discussed advantages the present-invention provides a low-capacitance and light-weight air-spaceinsulated high-frequency coaxial cable which is supposed to be particularly .featured by the fact that the space between the inner and the outer conductor consists of two coaxial layers arranged in one another, and differing from one another with respect to their dielectric constants, of which the internal layer is constituted by a rope helically enclosing the inner conductor, and comprising a foil of insulating material covering the rope space, and of which the external layer is constituted by a plastics foam material.
If, in accordance with the above-mentioned coaxial insulation, there is manufactured a cable of the l mm. diameter group in accordance with the German Industrial Standard DIN 47268, incorporating an inner conductor having a diameter of 0.45 mm., then there will only be required an internal diameter of the outer conductor of 6.5 mm. instead of 10 mm., with such a cable falling within the next lower diameter group having a dielectric constant of 1.3. However, when providing in the abovementioned insulation construction, a diameter of 0.3 mm. of the inner conductor, which thus would fall under the .former 6.6 mm. diameter group then it will be possible to achieve the electrical values already at an internal diameter of the outer conductor amounting to 4.3
mm. From this it follows that low-capacitance highf frequency cables with a characteristics impedance of ohms can now be manufactured with substantially smaller diameters and less material.
Of course, the inventive construction of the insulation between the inner and the outer conductor, also has advantages for other types of coaxial cable insulations, such as in the case of cables having a characteristic impedance of 60 or 75 ohms. This applies particularly to cases where the cable has a somewhat thicker insulation between the inner and the outer conductor, which may be either equal or greater than 3 mm. Hence, this advantage is particularly noticeable especially in the case of greater diameters and lower characteristic impedances, e.g. in the case of low-capacitance cables. i
While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.
1. A coaxial cable having an extremely small dielectric constant, comprising:
an inner conductor;
an outer conductor;
two layers of foam plastic insulating vmaterial arranged coaxially in one another, both said layers lying between said inner and outer conductors; land a foil sheath of insulating material; wherein the outermost of said layers is formed as a hose; the innermost of said layers is formed as a thin rope, and is helically wound about said inner conductor,
said rope having negligible tensile strength and containing a thread-type insert of uniform tensile strength; and said rope carries said foil sheath, which sheath cylindrically covers said rope and said inner conductor and supports said outermost layer which is arranged thereon.
2. A coaxial cable, according to claim 1, wherein: said foam plastic insulating material comprises polyethylene.
3. A coxial cable, according to claim 2, wherein: said thread-type insert of uniform tensile strength consists of a thin thread of polyamide.
4. A coaxial cable according to claim 1 wherein said foil sheath consists of cellulose triacetate.
5. A coaxial cable according to claim 4 further cornprising an outer protective sheath of polyvinylchloride.
References Cited by the Examiner UNITED STATES PATENTS 2,197,616 3/1938 Lehne et al 174-29 2,436,421 5/1943 Cork 174-29 2,556,224 11/1945 Scott 174-28 2,814,666 4/1953 Maddox 174-27 FOREIGN PATENTS 529,649 11/ 1940 Great Britain.
OTHER REFERENCES Camillo et al.: Foamed Polyethylene Coaxial Cables, in Wire and Wire Products, pp. 649653, June 1958.
ROBERT K. SCHAEFER, Acting Primary Examiner. LARAMIE E. ASKIN, IOHN F. BURNS, Examiners.