Air-insulated Coaxial High-frequency Cable

Abstract

A tubular inner conductor has a first diameter and is provided with annular corrugations adjacent ones of which are spaced from each other by a distance smaller than 0.3 times the first diameter. A coaxial tubular outer conductor surrounds the inner conductor and defines therewith a clearance. The outer conductor is provided with helical corrugations and has a larger second diameter. Spacers are located in the clearance and engage the corrugations of the inner and outer conductors. Adjacent ones of these spacers are spaced axially of the conductors by a distance which is equal to between 0.6 and 0.75 times the sum of the first and second diameters.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to a coaxial cable, and more particularly to an air insulated coaxial high-frequency cable.

Coaxial high-frequency cable of the air insulated type is largely governed in its power-handling capacity and efficiency by low resistance and leakage losses. The proportion of the leakage losses is particularly important, because these leakage losses -- resulting from the dielectric losses of the insulating material -- rise proportionally to the frequency. The resistance losses, caused by the high frequency resistance of the conductors, increase only with the root of the frequency. The resistance loss decreases as the diameter of the cable increases, that is the larger the cable diameter the lower the resistance loss. The leakage loss, however, is independent of the cable diameter and is determined only by the dissipation factor and the relative dielectric constant of the insulating material. In terms of the construction of coaxial high-frequency cables this means that the selection of suitable insulating materials is the more important, the greater the cable diameter and the greater the operating frequency at which the cable is to be used. In addition to these requirements the insulating material which is employed should be capable of being readily manufactured and processed, and should have high elasticity. The insulating material is, of course, used for the spacers which space the inner from the outer conductor, and the latter must be so configurated that the high mechanical stresses which occur in the cable during the manufacture, installation and operation -- for instance twisting and/or relative shifting of the conductors -- are negated as much as possible.

A further important consideration is the longitudinal uniformity when such cables are used for the transmission of television signals, and in the decimeter and centimeter wave range.

One construction known from the prior art utilizes spacers which are of an advantageous configuration insofar as their manufacture is concerned. Such spacer uses a profiled strand of synthetic plastic material which is wrapped helically about the inner conductor. However, this construction also has an essential disadvantage, namely the fact that the main strand of insulating material which is wrapped about the inner conductor and is provided with spacing portions, is located over its entire length in the region of highest field strength and cannot be properly anchored so that it tends to shift on the inner conductor, especially in large-diameter cables.

A further proposal according to the prior art uses individual injection molded spacers having spokes or projections which are spaced circumferentially about the inner conductor and are connected by annular portions of T-shaped cross-section, whereas the cross-section of the spokes themselves resembles a cross. This construction avoids the disadvantages of the first-mentioned prior-art construction, but has its own problems in terms of its support on the inner conductor. Moreover, it -- as all the other prior-art constructions -- has the disadvantage that while it engages the inner conductor it does not properly engage the outer conductor in a sense that could prevent the latter from shifting relative to the inner conductor. Moreover, the second prior-art proposal using the individual spacers does nothing to prevent them from twisting or turning and thereby displacement in direction of the longitudinal axis of the cable. This is an effect which occurs when during the manufacture of the cable forces act upon the spacers in a rotational sense, for instance due to the operation which causes the helical profiling of the outer conductor.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to overcome the aforementioned problems in air insulated coaxial high-frequency cables.

More particularly it is an object of the present invention to provide a novel cable of this type wherein the requirements made of longitudinal uniformity and accuracy of the centering of the conductors relative to one another, are met, in order to provide the desired high-frequency power-handling capacity and efficiency, but wherein the frequency range is not disadvantageously influenced.

In keeping with these objects, and with others which will become apparent hereafter, one feature of the invention resides in an air insulated coaxial high-frequency cable which, briefly stated, comprises a tubular inner conductor having a first diameter and provided with annular corrugations adjacent ones of which are spaced from each other by a distance smaller than 0.3 times the first diameter. A coaxial tubular outer conductor surrounds the inner conductor and defines therewith a clearance. The outer conductor is provided with helical corrugations and has a larger second diameter. Spacers are located in the clearance and engage the corrugations of the inner and outer conductors, and adjacent ones of these spacers are spaced axially of the conductors by a distance equal to between 0.6 and 0.75 times the sum of the first and second diameters.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a fragmentary axial section through a cable according to the present invention;

FIG. 2 is a cross-section through a spacer used in the cable of FIG. 1;

FIG. 3 is a section taken on line III--III of FIG. 2; and

FIG. 4 is a section taken on line IV--IV of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Discussing the drawing now in detail and referring firstly to FIG. 1 it will be seen that the cable illustrated there has an inner conductor 1 which is tubular and provided with annular corrugations. Its outer diameter is identified with character d. The inner conductor 1 is located coaxially within the tubular outer conductor 3 whose diameter is designated with reference character D. The conductors are held in precise coaxial relationship by a plurality of axially spaced spacers 2.

Reference to FIG. 1 shows a side-elevational view of one of the spacers 2 which are located in the clearance between the conductors 1 and 3. The spacer 2 has three spacer portions 4a, 4b and 4c which are offset circumferentially through 120.degree. relative to one another. The inner end of each of the portions 4a, 4b and 4c is provided with a part-circular contact plate or portion 6 formed in the plane of symmetry of the respective portions 4a, 4b or 4c with a ridge 7. This ridge 7 extends into the depression between two ridges of the corrugations on the inner conductor, whereas the portion 6 bridges the depression and is supported on the adjacent ridges bounding the same. The outer end of each of the portions 4a, 4b and 4c is provided with a pair of ridges, those of the portion 4a being identified with reference numeral 8a, those of the portion 4b with reference numeral 8b, and those of the portion 4c with reference numeral 8c. These ribs or ridges 8a, 8b and 8c extend axially of the spacer 2 and, hence, axially of the cable when the spacer is installed. They serve to anchor the spacer with respect to the outer conductor 3 in that they become plastically deformed on engagement with the profiling of the latter.

Each of the spacers 2 further is provided with arcuate circumferentially extending springy portions. The portion 5a connects the portions 4a and 4b, and similarly the portion 5b connects the portions 4b and 4c. It will be seen that the portions 5a and 5b do not define a circumferentially complete annulus, but rather have extensions 10a and 10b, respectively, which project beyond the portions 4a and 4c towards one another but leave a gap as shown in FIG. 2. These projections 10a and 10b thus constitute additional support for engagement with the inner surface of the outer conductor 3. A marking 9 is provided, located in the plane of symmetry of the respective spacer 2 and serving to facilitate the proper positioning of the spacers on the inner conductor.

The portions 4a, 4b and 4c are of H-shaped or double-T-shaped cross-section, as shown in the sectional view of FIG. 4, and the portions 5a and 5b are of analogous cross-section, as shown in FIG. 3.

The present invention has many advantages which are of considerable importance in this particular field. By having the center-to-center spacing of the annular corrugations of the inner conductor 1 be less than 0.3 times the diameter d, I am able to obtain particularly advantageous bending characteristics for the construction. It has been observed that when an inner conductor having a larger spacing is bent, as is inevitable during the manufacture and certainly during the installation of such cables, it stands to become permanently deformed at the bend, that is a sharp bend tends to form rather than a mere curve. This is avoided with the present invention.

Moreover, the proper anchoring of the spacers 2 in the corrugations of the inner conductor 1, which is now possible with the construction according to the present invention, makes it impossible for the spacer 2 to shift axially of the inner conductor 1 due to turning movement.

In addition, the mechanical connection between inner and outer conductor via the spacers 2 will always be retained under all operating conditions, aside from which the particular configuration of the cross-section of the spacers 2 is such as to have a beneficial influence in terms of their mechanical performance and in terms of their lack of interference with the electrical characteristics of the cable. This is particularly important when such cables are to be installed in high television towers.

By having the center-to-center spacing of axially adjacent ones of spacers 2 in the range of between 0.6 and 0.75 times the sum of the diameters d and D, that is of the electrically effective diameters of the inner and outer conductors, I am able to make optimum utilization of the operating frequency of the cable and to use the cable -- despite its longitudinally periodical structure -- up to the border frequency which is determined by the wave-type conversion. It has been observed that larger center-to-center spacing of the spacers 2 results in a reduction of the useful operating frequency range, and smaller spacing results in a shift of the border frequency which is electrically not utilizable and is obtained at the expense of having to use more spacers and therefore more plastic material which increases the cost of the cable. This border frequency, incidentally, is so selected in accordance with the present invention, in keeping with the requirements of the specific geometric characteristics of the cable, that it will be above the border frequency which is determined by the wave-type conversion. This achieves the additional advantage that the lowpass characteristic of the longitudinally periodic structure is made ineffective.

By having the portions 4a, 4b and 4c provided with a radially diverging double-T profile, and by providing the ribs 8a, 8b and 8c, the ribs can engage the profiling of the outer conductor 3 under plastic deformation. Using a double-T profile instead of a single-T profile or the cross-shaped profile known from the prior art, the larger surface area of the portions 4a, 4b and 4c results in a reduction of the heat resistance between inner and outer conductor. This in turn increases the transmissible R.F. power and at the same time results in a stiffer construction of the portions 4a, 4b and 4c. These advantages are further enhanced by the fact that the portions 6 with the ribs 7 are provided because this particular construction permits the portions 4a, 4b and 4c to be readily produced by injection molding with the use of a single two-part mold. The ribs 7 extend into the depressions between consecutive ridges of the annular corrugations of the inner conductor 1, surrounding the inner conductor 1 over approximately 30-50 percent of arc. The dimensioning of the portions 6 such that in axial direction of the cable their dimension corresponds to 1.0 - 1.8 times the spacing between consecutive corrugations, provides for a further beneficial influence upon the bending characteristics of the cable as well as the longitudinal uniformity thereof. By the engagement of the ribs 7 between consecutive corrugations, and by the embracing of the inner conductor over the aforementioned portion of arc by the ribs 7 and the portion 6, the respective spacer 2 will be reliably anchored to the inner conductor 1 even under maximum stresses upon the cable, and it increases the mechanical stresses which can be transmitted to the inner conductor 1 even in the region of maximum heating. This anchoring is completed by the engagement of the ribs or ridges 8a, 8b and 8c with the corrugations of the outer conductor 3. The fact that these ribs 8a, 8b and 8c extend in axial direction of the cable makes it possible for them to extend into the profiling of the outer conductor, irrespective of the tolerance position of the conductors and spacers, and the ribs then become plastically deformed so that an anchoring effect is obtained which prevents turning and/or shifting of the spacers 2 relative to and between the inner and outer conductors, and which assures that the inner and outer conductors will always be properly centered and the length uniformity of the cable maintained, that is the conductors cannot longitudinally shift relative to one another.

By having the portions 5a and 5b also configurated of double-T-shaped profile, a minimum amount of material is required to produce them while a maximum amount of strength is obtained for these portions. The projections 10a and 10b extend by approximately 5.degree.-10.degree. passed the portions 4a and 4c, respectively, and this further enhances the mechanical stability of the spacers 2, in conjunction with the double-T profile of the portions 4a, 4b and 4c and of the portions 5a and 5b.

In addition, this configuration assures that that portion of the spacers 2 which is in contact with the uneven profile of the outer conductor will be wider, which is advantageous in terms of the mechanical stresses which can be absorbed by the spacers as well as in terms of the electrical characteristics of the cable and of the prevention of relative axial shifting of the conductors.

By providing the marking 9 I obtain a further advantage. The position of this marking makes it possible to arrange the spacers 2 on the inner conductor 1 in the same in which they were originally located in the injection molding form. This negates the deviation between the electrically effective mass symmetry plane and the geometrical symmetry plane, which results from the deviation in the two portions of the mold cavity provided in the two mold sections, that is the deviation as to the dimensioning of these cavities. However, even if the intended use is not made of the marking 9, that is if the spacers 2 are not arranged in accordance with the information conveyed by these markings, then the presence of the marking in the cross-sectional plane of symmetry of the respective spacer does not disadvantageously influence the electrical characteristics of the cable.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in an air insulated coaxial high-frequency cable, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. An air insulated coaxial high-frequency cable, comprising a tubular inner conductor having a first diameter and provided with annular corrugations adjacent ones of which are spaced from each other by a distance smaller than 0.3 times said first diameter; a coaxial tubular outer conductor surrounding said inner conductor and defining therewith a clearance, said outer conductor being provided with helical corrugations and having a larger second diameter; and spacers located in said clearance and engaging the corrugations of said inner and outer conductors, adjacent ones of said spacers being spaced coaxially of said conductors by a distance equal to between 0.6 and 0.75 times the sum of said first and second diameters.

2. A cable as defined in claim 1, wherein said spacers are of synthetic plastic material.

3. A cable as defined in claim 1, wherein said spacers each comprise a plurality of spacing portions extending radially across said clearance and being of substantially H-shaped cross-section, each of said spacing portions having a radially outer periphery provided with a pair of axially extending ribs which engage respective corrugations of said outer conductor, and a radially inner periphery including a part-circular support portion which engages said inner conductor and embraces the same over substantially 30.degree.-50.degree. of arc, said support portion extending in axial direction by a distance corresponding to between 1.0 and 1.8 times the center-to-center spacing between consecutive ones of said annular corrugations.

4. A cable as defined in claim 1, wherein said spacers each comprise a plurality of circumferentially offset spacing portions, and circumferentially extending springy portions connecting respective ones of said spacing portions, said springy portions being of substantially H-shaped cross-section and forming a circumferentially incomplete annulus.

5. A cable as defined in claim 1; and further comprising a marking provided in the cross-sectional plane of symmetry of the respective spacer for facilitating positioning of the latter with reference to said inner conductor.