EP0531312A1 - Antenna structure and method of making same - Google Patents

Abstract

Antenna structure in which the antenna takes the form of a helical strip (12) which is wound around a hollow cylindrical sheath (10), both of which are wound with resin. An integral mounting flange (16) is provided at one end. The structure is produced using a cylindrical core (20) having a flange at one end which is sheathed with a sleeve (10) of fiberglass, for example, on which is then wound a conductor (12 ) such as a copper strip, to form the antenna, the assembly then being coated with an appropriate resin and the core removed. The thickness and the resistance of the fixing flange (16) are obtained, preferably by superposition of multiple hollowed discs (40) centered around the cylindrical core and pressed against the flange before the coating operation.

Description

ANTENNA STRUCTURE AND METHOD OF MAKING SAME

Field of the Invention

This invention relates to antenna apparatus for radiating and/or receiving electromagnetic wave signals, and to methods for making same. In its preferred embodiment it relates particularly to helical antennas suitable for launching into, and use in, space, for example in launched satellite applications.

Background of the Invention

Usual antenna structures, particularly for use at extremely high frequencies, (for example at UHF or L-band frequencies) are known which, because of their desired electrical characteristics, tend inherently to be rather fragile. One such antenna structure is the helical antenna, which consists generally of a helix of a suitable conductor, such as copper; one or more of such antenna structures may be assembled in an array on a reflector, although they are also useful when used individually. Such helical antennas are normally cantilever-mounted at one end, and therefore rely upon their own strength and rigidity to maintain their geometric integrity. While in some ground-based applications this may be feasible, when the antenna structure is to be launched into space on a satellite, such self-supporting structures are subject to deformation or destruction, or at least some damage to their operating characteristics.

Various attempts have been made in the past to provide a supporting structure for such types of antennas, the supporting structure being intended to provide the desired rigidity and strength without interfering with the electrical properties of the antenna itself. For satellite use, such supporting structure should be highly resistant to the shock and vibration of launching, of relatively light weight, transparent to the electromagnetic waves with which it is to operate, and chemically inert. It is also desirable that it be inexpensive and easy to fabricate in exactly the desired configuration, and it preferably includes, integral therewith, a suitable means, such as a flange at one end, for mounting it. While antenna support structures are known which may provide one or several of the above-mentioned desired features, applicants are not aware of any which provides a majority, or all, of these features in one structure to the extent desired.

Accordingly, it is an object of the present invention to provide a new and useful antenna structure, and a method for^' making same.

Another object is to provide such an antenna structure which is highly resistant to shock, which will accommodate a relative large range of temperatures, which is light in weight, and which is chemically stable.

A further object is to provide a method for making such antenna structure which is simple, inexpensive, and does not require any machining steps.

Brief Summary of the Invention

These and other objects of the invention are achieved by the provision of an antenna structure comprising a braided sleeve of substantially inert material substantially transparent to electromagnetic waves, on which the conductive material of the antenna element is placed. This assembly is impregnated with and covered by an inert, hardened resin which is also substantially transparent to electromagnetic waves and highly resistant to mechanical damage or deformation in response to high levels of shock and vibration, while also tolerating a wide range of temperatures.

The antenna structure is preferably made by providing a mandrel the outer surface of which has a shape like that desired of the supporting structure, but of somewhat lesser diameter, and then forming on the exterior of that mandrel a braided sleeve of a suitable material, of which fiberglass is an example; the conductive antenna material, for example copper foil, is then placed on the exterior of the braided cover and the assembly placed in a closely fitting mold into which a liquid resin is delivered to suffuse the braid and helical strip. The liquid resin so supplied impregnates the braid and covers the conductive antenna material and bonds it to the braid in the desired antenna configuration. The resin is then cured, as by heating, the assembly removed from the mold, and the mandrel removed. If the mandrel is of a shape to be held captive in the braided sleeve, it is originally made of a frangible or soluble material, so it can be removed by breaking or dissolution, while if it is not held captive it may be made of a suitable metal, such as aluminum, and removed merely by pulling it out.

In a preferred embodiment, the antenna element is in the form of a helical strip of metal foil, wound around the braided sleeve, and the sleeve terminates in an outwardly- extending mounting flange, produced by providing a similar flange on the original mandrel and braiding the braid material over and around the upper side and periphery of the mandrel flange prior to potting of the assembly. Electrical contact may be made to the helical entennas by removing a small amount of the resinous coating near the flange and securing a conductor to the thus-exposed metal foil. In a preferred embodiment the thickness and hence strength of the mounting flange is enhanced by placing one or more impregnable lamina against a face of the flange prior to application of the resinous material.

In the completed product, the copper strip is embedded in and protected by the potting resin, and the impregnated braid provides a strong, shock-resistant and vibration-resistant supporting structure as desired, which is transparent to electromagnetic waves and which tolerates a wide range of temperature variation; in addition, it is light because of the low densities of the fiber and the resin, and because it is hollow; it is configured exactly as desired for the selected antenna geometry; and it is made by a process which does not require any machining_.. In addition, in the preferred embodiment, the mounting flange is provided without requiring any separate and expensive steps.

Brief Description of Figures

These and other objects and features of the invention will be more readily understood form a consideration of the following detailed description, taken with the accompanying drawings, in which:

Figure 1 is a side elevational view of a completed antenna structure in accordance with a preferred embodiment of the invention, with parts broken away, and which is circularly symmetrical about its longitudinal axis.

Figure 2 is a sectional view taken along lines 2-2 of Fig. 1;

Figure 3 is a fragmentary view, partly in section, showing the annular lamina preferably used to build up the thickness of the final flange;

Figure 4 is a perspective view of such an annular lamina;

Figure 5 is a side view of a braided mandrel suitable for use in making an antenna structure in accordance with the invention;

Figure 6 is a side view of a portion of the braided mandrel, with the helical foil wrapped about it to form the antenna; and

Figure 7 is an enlarged fragmentary side view of the completed antenna structure. Detailed Description of Specific Embodiments

Referring now to the specific example of the antenna structure and method for making it which is set forth below, and without thereby in way limiting the scope of the invention, it is assumed that the objective in this case is to provide an antenna structure such as is shown in Figures 1 and 2, which consists of a resin-impregnated braided sleeve 10 carrying a helical strip 12 of copper which is embedded in the same resinous body 14 as is the braided sleeve. This assembly is provided with an integral resin-impregnated flange 16 at one end thereof by which the antenna structure is to be mounted, for example on an electromagnetic wave reflector.

Referring now to Figure 5, there is shown therein a suitable mandrel 20 for making such an antenna for operation at L-band frequencies, with the braided sleeve 10A formed thereon. In this example the mandrel may be of aluminum, and in the shape shown it is easily withdrawn from the final molded object at the end of the process; in other cases where the mandrel is shaped so as to be captured in the molded assembly it may be made of a soluble or readily frangable nature so that it can be removed it from the final product.

The outside of the mandrel 20 has the dimensions desired for the final product, less the thickness of the material of the final product; that is, it defines an appropriate size and shape for the interior surface of the final cylindrical antenna support structure. In this example, it is assumed that the L-band antenna structure is to have approximately the following dimensions:

Length of cylindrical portion of structure = about 4.356X10^"1 meters;

Length of conical portion at one end = about 1.16x10 -1 meters; Diameter of cylindrical portion = about 5.54xl0^"2 meters;

Taper of conical end = about 25°, whereby its smaller terminal end has a diameter of about 4.78xl0^"2 meters;

Thickness of flange 16 along longitudinal axis of antenna = about 5.1xl0^"3 meters, with a diameter of about 1.3X10^"1 meters.

In manufacture, the mandrel 20 is placed vertically in position along the vertical axis of a conventional braider with its conical end facing upwardly, and is gripped by a controlled vertical lifting device which moves upwardly while the braiding machine operates. Typically, a 72 carrier braider may be employed to produce a conventional two-over, two-under braid, the braiding material being 250-yield fiberglass and the braiding being accomplished in the usual "maypole" fashion, suitably by a commercial braiding machine such as is manufactured by Wardwell Branley Company, Central Falls, R.I. Although the final exact adjustment of the operating parameters is preferably performed experimentally in known manner, a lifting rate of the order of 2.5 meters per second will normally produce the preferred 45° angle braid over the exterior of the mandrel.

In order to provide a flange 16 of the desired thickness, it is preferred, before the braiding operation, to install on and against the flange at least several thin, gasket-shaped annular pads or laminas such as 40 (Figs. 4 and 5), typically of woven fiberglass, which are slid onto the mandrel 20 by way of its tapered end until they seat against the flange. In this example, about 30 such pads each about 3 mils in thickness may be used. This gives the final flange a thickness about twice that of the remainder of the structure, for example about 5.1xl0^"3 meters, and correspondingly increased strength. The copper helix 12 is preferably a foil of about 3 mils thickness and about 1.3xl0^~2 meters width. The pitch of the helix which it forms around the braided mandrel is about 14.5°, giving 9.5 turns ih all, two of these turns being on the conically tapered end section of the braided support structure. The exact configuration of the helical antenna strip is determined by the electrical characteristics desired by the designer, and for the purposes of this invention may take.any of a relatively wide variety of forms in different applications.

The potting or resin-embedding can be of a standard type -commonly referred to as resin transfer molding (RTM), using an epoxy resin. The mold contains a female cavity which fits closely around the exterior surface of the braided mandrel, with a very slight clearance sufficient to permit some flow of the liquid resin when it is under pressure.

As is common in such RTM procedures, the pressurized resin in flowable form is applied through an opening near the bottom of the mold, the top of the mold typically being open to atmosphere, or provided with a degree of vacuum to assist the flow of the resin through the interior of the mold. Pressure differentials of approximately 3.4xl0⁵ Pascals are typical, although these parameters may be varied substantially to achieve optimum conditions for the particular application. What is desired is to assure that after a predetermined, relatively short, interval of time, the braid is uniformly impregnated with the resin, and a thin continuous smooth coating of the resin is formed on the exterior of the assembly in the very small space between the mold and the exterior of the braiding. Typically, the thickness of the exterior layer of the resin is about 3 mils or even less.

Typical curing times are about 2 hours with a mold temperature of about 150°C, these parameters again being variable depending on the exact size and nature of the product being potted and the resins used.

The completed part is then removed from the mold and, after cooling, the central mandrel is removed. In this example, the mandrel can be pulled out from the flange end; in other applications wherein the mandrel has both increased and reduced diameters at both ends or in the interior portions of the product, the trapped mandrel is preferably made of a soluble or frangible material which at this point in the process is dissolved or broken up to remove it from the interior of the braided sleeve and to the desired completed antenna structure.

To mount the assembly, holes such as 50 may be drilled through the flange 16 to accommodate supporting screws or bolts. In some applications, relatively large numbers of such antenna structures are mounted on a common reflector to form what is known as an "antenna farm", useful in space satellites. As mentioned above, electrical contact may be made to the helical antenna by removing a small portion of the resin covering the end of the helix next to the flange, and securing an electrical lead thereto.

It is noted that no machining processes are required in making the antenna structure, and that the individual steps in the fabrication are of types which those skilled in the art can readily provide. The resultant structure is of accurate dimensions and is relatively light in weight, is transparent to electromagnetic wave, is highly resistant to shock and vibration, and tolerates a wide range of temperature changes. The copper helix is firmly supported and held in place, while at the same time being protected by a thin layer of hardened resin.

As mentioned, other electrical designs of antennas will require correspondly different support structures, the shape of the supporting structure in each case being such as to support the antenna in exactly its desired electrical configuration.

Thus, while the invention has been described with particular reference to the specific embodiments in the interest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse from those specifically described without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. The method of making an antenna structure which comprises:

forming a braided sleeve by braiding an electrically-insulating material over a mandrel;

placing a conductive antenna element on the exterior of said braided sleeve in its ultimately desired configuration;

suffusing the assembly of said braided sleeve and said antenna element with an electrically-insulating liquid potting material to impregnate said braided sleeve and to cover said antenna element with said potting material;

thereafter hardening said potting material to form a rigid composite structure with said antenna element firmly supported in said desired configuration and said antenna element covered with said hardened potting material; and

thereafter removing said mandrel from said rigid composite structure.

2. The method of claim 1 wherein said sleeve is of tubular form and said antenna element extends along and about said sleeve in helical form.

3. The method of claim 1 wherein said antenna element is of conductive foil.

4. The method of claim 3 wherein said foil is a copper strip arranged in spaced-apart helical turns along said sleeve.

5. The method of claim 4, wherein the material of said braided sleeve and the material of said potting material are both substantially transparent to electromagnetic waves.

6. The method of claim 1, wherein said suffusing comprises placing said assembly in a closely fitting mold and forcing said potting material in flowable form into said mold until said impregnation of braided sleeve and said covering of said antenna element is complete, wherein said potting material is a thermosetting resin and wherein said hardening comprises heating said resin to cure and harden it.

7. The method of claim 1 wherein said mandrel is flanged at least at one end to provide said sleeve with a mounting f1ange.

8. The method of claim 7, comprising strengthening said flange by placing one or more annular rings of woven electrically-insulating material over said sleeve and against said flange prior to said suffusing with said potting material, whereby said rings are also suffused and covered with said potting material to form a thickened flange on the final product.

9. The method of claim 8, wherein said braided sleeve is of fiberglass yarn and said potting resin is an epoxy resin.

10. An antenna structure comprising:

a tubular body of braided filaments having an antenna element lying on its outer surface, and a hardened resinous material impregnating and covering said tubular braided body and said antenna element to provide a rigid support for said antenna element and to bind it to said body.

11. The antenna structure of claim 10, wherein said antenna element extends around and along said body in substantially helical form.

12. The antenna structure of claim 11, wherein the turns of said helical element are spaced from each other along said tubular body.

13. The antenna structure of claim 10, wherein said filaments and said resinous material are both electrically insulating.

14. The antenna structure of claim 13, wherein said filaments are of glass and said resinous material is an epoxy resin.

15. The antenna structure of claim 14, wherein said tubular body is composed of a 45 tubular braid of said filaments.

16. An antenna structure made by the process of any of claims 1-9.