Patent Application
20070205008
Coaxial cables for high frequency signal transmission may be designed for specific operating impedances by adjusting the spacing between the inner conductor and the surrounding outer conductor. To design a coaxial cable for high impedance characteristic, the distance between the inner conductor and the outer conductor is increased and or a dielectric with a higher specific gravity is used. However, application of dielectric materials with higher specific gravities increases the materials cost, weight and signal loss characteristics of the cable. To minimize the overall diameter of a high impedance cable, where high signal power capacity is not a design parameter, the diameter of the inner conductor may be minimized down to that of a fine wire.
A coaxial cable with a fine wire inner conductor, surrounded by a foam dielectric that is covered by the outer conductor presents several manufacturing challenges. A fine wire inner conductor is very fragile. This makes it difficult to smoothly guide the inner conductor with the required precision through a traditional continuous coaxial cable manufacturing process.
Prior high impedance fine wire inner conductor coaxial cables have been observed with an unacceptably high number of longitudinal voids in the dielectric foam, proximate the fine wire inner conductor. These voids introduce variances to the dielectric value of the area between the inner and outer conductor, create a moisture/corrosion path within the cable and also allow the position of the inner conductor within the foam dielectric to vary. Together, these factors introduce a significant error between the designed and the measured characteristic impedance of the finished cable that may vary length to length of the cable.
A prior art coaxial cable with void(s) 5 around the fine wire inner conductor 10, for example as shown in
Competition within the coaxial cable industry has focused attention upon electrical characteristic uniformity, defect reduction and overall improved manufacturing quality control.
Therefore, it is an object of the invention to provide a coaxial cable and method of manufacture that overcomes deficiencies in such prior art.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.
The inventor has recognized the reason voids appear in prior high impedance fine wire inner conductor coaxial cables.
The foam dielectric area of a high impedance cable will be larger than in an otherwise similar low impedance cable. During the foam dielectric expansion step, the foam dielectric relies upon the thermal mass of the inner conductor to assist with the curing of the dielectric foam towards the center of the cable rather than just towards a cooling quench flowing around the exterior. Even if a traditional thin adhesive coating of an unexpanded plastic is present around the inner conductor, if insufficient inner conductor thermal mass is present to receive heat transfer from the dielectric foam, i.e. cool the core of the foam dielectric as it is expanded, the foam dielectric will pull away from the inner conductor, creating voids around the inner conductor.
The inventor's research has verified that applying a thick outer layer of adhesive resin around the fine wire inner conductor increases the thermal mass and improves the inner conductor mechanical characteristics during further manufacturing steps. The increased thermal mass and improved mechanical characteristics of the coated fine wire inner conductor results in a fine wire inner conductor coaxial cable with significant improvements in impedance characteristic uniformity and ease of use.
As shown in
The adhesive resin coating 20 is surrounded by a foam dielectric 15 which is surrounded by the outer conductor 30. In the exemplary embodiment, the foam dielectric 15 and adhesive resin coating 20 are polyolefin resins selected to have compatible molecular properties. The adhesive resin coating 20 also is selected to provide suitable adhesion to the inner conductor 10 as well as acceptable signal loss characteristics.
The fine wire inner conductor 10 of the first embodiment may have a steel core for improved tensile strength. Copper or other high conductivity metal electroplating may be applied to the steel core to protect it from corrosion and improve conductivity. An outer layer of tin may also be applied to simplify soldered connections to the inner conductor.
The outer conductor 30 may be a solid aluminum or copper material with or without corrugations, as desired. Alternatively, foil and or braided outer conductor(s) 30 may also be applied. If desired, a plastic outer protective sheath may be added.
During a continuous manufacturing process according to the invention, as shown in
A second extruder 45 applies a foam dielectric resin layer to the coated inner conductor 25 that expands into the foam dielectric 15 upon exiting the second extruder 45. Expansion is aided by passage through a quench area 50, as shown in
The foam dielectric 15 coated inner conductor 25 may be cured for a desired period or passed directly to the outer conductor 30 application process (not shown). The desired outer conductor 30 may be applied, for example by seam welding a solid metal outer conductor 30, coaxial with the inner conductor 10, around the foam dielectric 15. Methods for applying outer conductor 30 to a foam dielectric 15 coated inner conductor 25 are well known in the art and as such are not described in further detail here.
To minimize material requirements, the adhesive resin coating 20 thickness may be adjusted until an acceptable level of void(s) 5 is obtained in the finished coaxial cable.
The invention has been demonstrated with respect to a first exemplary embodiment. One skilled in the art will appreciate that the cable design and manufacturing process herein is applicable to coaxial cables having a foam dielectric thickness corresponding to a characteristic impedance greater than 85 ohms and solid inner conductors of up to 0.1 inch in conductor diameter. For lower impedance and or thicker inner conductor cables, the thermal mass of the inner conductor 10, uncoated, should be sufficient to avoid the appearance of the void(s) 5 described herein, during curing of the foam dielectric 15 as long as the inner conductor 10 is not delivered to the second extruder 45 for foam dielectric 15 coating at an excessive temperature.
Although the manufacturing process is described as a continuous process, the process may be divided into several discrete sections with work in progress from each section stored before feeding the next section, without departing from the invention as claimed.
Where in the foregoing description reference has been made to ratios, integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth.
While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant's general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.
