WO1999028988A2

WO1999028988A2 - Metallized fiber mat, and its use as reflective applique in antenna

Info

Publication number: WO1999028988A2
Application number: PCT/US1998/025705
Authority: WO
Inventors: Steven Witschen; P. Marvin Walden, Iii; Robert W. Martin
Original assignee: Marconi Aerospace Defence Systems, Inc.
Priority date: 1997-12-04
Filing date: 1998-12-04
Publication date: 1999-06-10
Also published as: WO1999028988A3

Abstract

One aspect of the invention relates to a reflective mat useful to provide an electromagnetically reflective surface on a high-frequency radio antenna. In one version of the invention, the reflective mat includes a conductive mat shaped to conform to a signal receiving surface of an antenna core. The conductive mat has a core contacting surface, which is coated with an adhesive material for joining the conductive mat to the receiving surface, and a signal reflecting surface which is coated with a curable finishing material.

Description

ANTENNA METALIZED FIBER MAT REFLECTIVE APPLIQUE

Background of the Invention

The invention relates generally to the field of antennas and more

particularly to durable reflective surfaces useful with antenna cores. Still

more particularly, the invention relates to reflective surfaces which include

metalized fiber mats which are coated with a thermoset resin system that is

partially cured before being applied to the antenna core.

With the ever increasing use of high frequency radio communications

there is a need for low cost, high performance antennas. Generally, high

frequency antennas are constructed in the familiar dish shape to focus the

received radio frequency ("RF") energy by reflection onto an electronic

receiver which then passes the radio signal to other electronic components to

extract the data contained in the signal. In order to maximize the amount of

RF energy reflected by the antenna, the electromagnetically reflecting surface

of the antenna is provided with a metallic material, such as aluminum or

nickel.

Modern low-cost, high-frequency antennas, such as those used in

satellite dishes, are typically constructed as shown in Figure 1. Figure 1

depicts a high frequency antenna 100 in cross-section. The antenna 100 includes a core 102 which forms the parabolic dish shape used to receive and

reflect the high frequency RF signals. As used herein the term "core" refers to

the structural portion of the antenna which supports the antenna's

electromagnetically reflecting surface. Depending on the manufacturing

technology used, the core 102 is normally either attached to, or formed

integrally with, other structural portions of the antenna, such as mounting

structures 104 which are used to attach the antenna to desired positioning

equipment that establishes the core's physical location and orientation.

While numerous variations are possible, conventional low cost cores

are normally constructed from a molded plastic material which is provided

with a signal receiving surface, in this case parabolic surface 106, designed to

reflect selected frequencies of electromagnetic radiation from the parabolic

surface 106 to an electronic receiver 110. The receiver 1 10 then passes the

received signal to other electronic components, such as amplifies,

demodulators, etc., which process the signal into a usable form.

Since plastic is not itself electrically conductive, its electromagnetically

reflective properties are poor. Therefore, the parabolic surface of the core

must be made conductive by either molding a conductive material into the

parabolic surface, or painting the parabolic surface with a conductive material.

Neither of these techniques for manufacturing antennas are completely

satisfactory.

The process of molding in a conductive material requires the use of a

compression molding process. In compression molding, a two piece mold is normally used, and each piece of the mold corresponds to one-half of the

antenna core 102. Therefore, when the molds are assembled, they create a

cavity that is the same size and shape of the desired antenna core. In

operation, the conductive material to be used as the reflective surface, for

instance, a wire mesh, is placed in the mold half that forms the reflecting

surface of the antenna core. An amount of plastic molding material is then

placed into the other half of the mold and the mold halves are compressed

together, forcing the plastic material into the desired core shape. After the

plastic cures, the molds are separated and the plastic antenna core is

removed. It will be clear that the wire mesh is permanently formed into the

reflecting surface of the antenna core during the process.

However, the compression molding process for forming antenna cores

suffers from several drawbacks. First, the process is only suitable with certain

types of plastic materials referred to sheet molding compounds ("SMCs").

These compounds are relatively low in viscosity and precise weights of the

SMC material must be provided for each mold. Generally, pieces of the SMC

material are hand cut to match a given mold. This is a time consuming

process which adds to the cost of the completed antenna core. Moreover,

even after the core is fabricated using the compression molding process, the

reflecting surface must still be painted in order to protect the reflecting

material from environmental pollutants, such as moisture, airborne chemicals,

and the like. The paints commonly used in the art often involve the release of

volatile organic compounds ("VOCs") which are environmentally undesirable. Summary of the Invention

Accordingly, it is one object of the invention to provide a durable,

reflective material which is easily applied to an RF antenna core, such as a

satellite dish. It is another object of the invention to provide a material to be

applied to the antenna core to produce a substantially finished RF reflecting

surface for the antenna, without requiring additional steps for providing an

adhesive to the material, or painting the material. It is still a further object of

the invention to provide techniques for creating a reflective surface on an

antenna core without the use of environmentally harmful materials common in

the art. It is also an object of the invention to provide methods for creating a

reflective surface on an antenna using very low cost tooling to reduce the cost

of the final cost of the antenna. The invention achieves these objects and

provides other improvements and advantages which will become clear in view

of the following disclosure.

One aspect of the invention relates to a mat which is applied to the

signal receiving surface of the antenna core to create an electromagnetically

reflective surface on the antenna core. In one embodiment, the mat is formed

from a material which is naturally conductive, such as a stainless steel mesh.

In an alternate embodiment, the mat is formed from a fabric or fibrous

material which is made conductive by coating the fibers of the mat with a

conductive material. For example, in one particular embodiment, the mat is

formed from a non-woven fiberglass material, and the individual glass fibers are coated with metal to provide the mat with the required electrical

conductivity.

The mat is then covered with a material which provides not only a

smooth protective finish, but also functions as an adhesive to secure the mat

to the surface of an antenna core. In one exemplary embodiment, polyester

powder paints are used to provide both suitable adhesion and finish.

Another aspect of the invention relates to an antenna which has an

electromagnetically reflective surface that includes a mat as described above.

In one embodiment, the mat is permanently applied to an antenna core with

heat and pressure. Since the mat already is coated with a protective finish,

there is no need for the extra production step of painting the reflective

surface. Moreover, since there is no need to paint the surface, the VOCs

often used in the painting process are avoided.

Brief Description of the Drawings

Figure 1 is a cross-sectional view of a conventional high frequency

antenna.

Figure 2 is a cross-sectional view of a segment of a reflective mat

according to an embodiment of the invention.

Figure 3 depicts an apparatus for creating glass fibers useful to an

embodiment of the invention. Figure 4 is a top view of a reflective mat which has been cut to shape

for attachment to an antenna core according to an embodiment of the

invention.

Figure 5 is a schematic diagram illustrating a coating process useful to

create a reflective mat according to an embodiment of the invention.

Figure 6 is a diagram illustrating the application of a reflective mat to

an antenna core according to an embodiment of the invention.

Figure 7 is a cross-sectional view showing an antenna core having an

reflective mat formed on its signal receiving surface according to an

embodiment of the invention.

Detailed Description of Embodiments of the Invention

Referring now to Figure 2, there is shown a cross-sectional view of a

portion of a mat 200 according to an embodiment of the invention. The

reflective mat 200 comprises an underlying conductive mat 202. The

conductive mat 202 is advantageously formed from a plurality of fibers 203

which are either electrically conductive themselves, or are made electrically

conductive by suitable metallic coatings. Each side of the conductive mat 202

is coated with a material 201 a, 201 b which has suitable adhesive and

finishing properties, i.e., will provide a suitable aesthetic and environmentally

protective covering to the underlying conductive mat 202 while at the same

time provide a layer of adhesive material to join the mat to the signal receiving

surface of the antenna core. A number of different conductive mats 202 may be used as matter of

design choice according to different versions of the invention. In general, the

mats 202 will be electrically conductive, fairly lightweight, and capable of

being cut and shaped as required to conform to the parabolic surface of the

antenna core. The conductive mats 202 are constructed from a plurality of

strands, or fibers, which may be woven or non-woven. Suitable woven mats

include metallic meshes, such as a stainless steel or aluminum mesh, or non-

conductive meshes constructed from, for example, fiberglass, in which the

individual fibers or strands of the mat are provided with a conductive coating

in order to make the mat electrically conductive. For example, one

particularly useful mat is formed from nickel coated carbon fibers, and is

commercially available from Technical Fiber Products.

Although useful, woven mats are relatively expensive, and, therefore,

non-woven mats are used according to other advantageous embodiments of

the invention. One particularly useful non-woven mat is formed from

aluminum coated, i.e., "aluminized", glass fibers which are cut to specific

lengths and formed into a mat in a conventional wet lay process.

Figure 3 shows a block diagram of a system for making aluminized

glass fibers. Only the major features of the system are shown for the sake of

clarity. Such systems are well known and are described in detail in various

publications in the art, for example, U.S. Patent No. 2,772,987, to Whitehurst

et al., incorporated herein by reference. A suitable glass material is placed

into a furnace 302 where it is melted. The molten glass is drawn out as a strand, or fiber, of a desired diameter, through an orifice 304 connected to the

furnace 302. The glass fiber 306 is then wound around a take up reel 308 as

it exits the orifice 304. The fiber 306 is drawn across a lip 310 having a

channel 312 formed therein which contains a molten material, such as

aluminum. The surface tension of the molten aluminum allows it to extend

slightly above the upper edge of the lip 310. Thus, the strand 306 can be

lowered into the molten aluminum 312 to any desired depth. This allows the

fiber 306 to be partially or completely coated by the metal, as a matter of

design choice. In one advantageous embodiment, the fiber 306 is only

partially coated with the metal material since coated fibers tend to have

greater mechanical strength than fully coated fibers. However, this is not

critical to the invention, and fully-coated fibers could be used as well.

The material used to make the fibers, may be any material suitable for

making fiberglass. In one advantageous embodiment, the glass material is a

calcium alumino-borosilicate material, referred to in the art under the ASTM

designation "E" glass. Other suitable glasses, referred to by their ASTM

designations, include "C" glass, "S" glass", and "D" glass. Any of these

glasses may be selected as a matter of design choice depending on such

factors as whether chemical resistance or mechanical strength is deemed to

be an important property for the final intended use of the antenna.

Additionally, it will be recognized that any suitable metal can be used to

provide the necessary conductive surface to the glass fiber 306. In one particular useful embodiment, the material is commercially pure aluminum

alloy 1350.

The diameter of the fiber 306 is not critical and may be selected as a

matter of design choice. In one advantageous embodiment, the diameter of

the fibers is between about 0.0004 inches to about 0.001 inches. In an even

more specific embodiment, the fiber 306 is about 0.00073 inches, while the

thickness of the metal coating is about 0.00025 inches.

The fiber 306 is then cut into sections of predetermined length.

According to the present invention, the length of the fiber sections should be

at least one-quarter of the wave length of the electromagnetic signal that the

mat is intended to reflect. However, fiber sections shorter than this will still be

acceptable when combined with other fibers in a matrix.

The fiber sections are then formed into a mat according to processes

which are well known in the art. For example, one category of suitable

processes are "wet lay" process. These processes involve placing the fiber

sections in an aqueous slurry with a lubricant such as a phosphoric acid

material, to prevent fiber to fiber cohesion in the slurry. Aluminum coating

cold end welding is performed chemically in the aqueous solution of the

slurry. Afterwards, the fibers are removed from the first solution, dried on a

wire conveyor, then placed into a second aqueous solution which creates the

actual fiber-to-fiber bonding coherence necessary to form the conductive mat.

The second aqueous solution is then removed by the application of heat,

leaving a mat of bound fibers of the desired thickness. In one advantageous embodiment, the mat is about 0.030 inches thick. More specific details of the

wet lay process will be familiar to those of skill in the art, and will not be

described in greater detail herein.

Another category of processes suitable for forming the mat are "dry

lay" processes. Dry lay processes are well known and will not be described

herein. However, it should be noted that wet lay processes tend to produce

mats with a denser, paper-like texture which is preferred over mats produced

by the dry lay process which produces mats with a loftier or "fluffier" texture.

After the conductive mat 202 has been selected, it is coated with a

material which provides the conductive mat 202 with a protective covering, as

well as a layer of adhesive to join the conductive mat 202 to the parabolic

surface of an antenna core. This material will be referred to herein as the

"finishing" material. Of course, it will be recognized that two separate

materials 201a and 201 b can be applied to the conductive mat 202, one

material providing the adhesive layer, and the other providing the appropriate

protective finish. However, to simplify the manufacturing process, it is

desirable that a single finishing material is used. Thus, desirable finishing

materials will not only have acceptable qualities with respect to aesthetic

finish, environmental durability and protection of the conductive mat 202, but

will also have good adhesive properties to permit the reflective mat to be

permanently affixed to an antenna core.

One category of suitable finishing materials includes polyester

thermosetting powder coat paints, such as PPL9675G, commercially available from Spraylat Corporation. These powder coat paints are applied to

the mat in a manner substantially described with respect to Figure 5.

Referring now to Figure 5, there is shown a process for electrostatically

applying a finishing material to a conductive mat 202 according to an

embodiment of the invention. The figure depicts a simplified powder spray

system, having a tank 204 for containing a suitable powder coating. The tank

204 is connected to a transport line 206 which feeds the coating to a spray

gun 210 under pneumatic pressure. The spray gun 210 is provided with a

nozzle 212 which is connected to a power supply 208. Pneumatic pressure is

used to force the powder coating from the tank 204 to the transport line 206

and out of the spray gun 210. When the powder particles 214 are ejected

from the nozzle 212, they are charged by power supply 208. The mat 202 is

connected to a ground. This causes electrostatic adherence of the particles

214 to the mat 202. Numerous suitable powder spray systems are well

known in the art, for example, the Versa-Spray, commercially available from

Nordson Corporation. The process parameters used in the powder spray

process are, naturally dependent on the particular equipment and powder

coat material selected. In one particular version of the invention, it has been

found that a voltage differential of 40-60 kv is useful.

Those familiar with thermosetting polyester paints will appreciate that

after the finishing material is applied to the conductive mat 202, it must be

"cured" with the application of heat. The heat activates the catalysts

contained in the finishing material which cause the material to harden and set into a final protective layer. Before any curing has been formed, the

conductive mat 202 could be cut into the desired shape and applied to the

antenna core as an applique, as will be discussed in greater detail further

herein. However, if the finishing material is uncured, it is difficult to maintain

the paint presence during handling and transportation of the reflective mat.

At the same time, however, since the polyester powder is also to be

used as an adhesive to attach the reflective mat to the antenna core, it is not

desirable to completely cure the polyester powder before attachment to the

core. Accordingly, in one advantageous embodiment to the invention, after

the powder coat is sprayed onto the aluminized fiberglass mat, the mat is

heated to a point that is high enough to cause the polyester material to begin

to flow, but not high enough to fully activate the catalyst material in the

powder that causes the powder coat to harden. Heating may be performed

by a conventional infrared or direct heat oven. Heating the powder coat

material to a point sufficient to allow it to flow, without completely curing, is

referred to as a "partial cure". As the material begins to flow, minute gaps in

the sprayed coating caused by the roughness of the underlying conductive

mat and imperfections in the coating process are filled in. This creates a

smooth, well-adhered finish to the conductive mat 202. Simultaneously, the

polyester material increases its adherence to the mat so it becomes more

durable and not as likely to be damaged during the handling.

The time and temperatures required for a partial cure will depend on

the polyester powder selected for use, the underlying mat and the conditions and techniques used in coating the mat. It is believed within the skill of those

in the art to select the appropriate time/temperature combination for specific

powder coatings in view of the technical data sheets, cure cycle times, and

other information provided by the powder coat manufacturer. In general, the

temperature should be kept below the initiation of cure temperature for the

material used. The mat is maintained at this temperature until the coating

material has adhered to the mat such that the coating particles do not tend to

fall off when the mat is handled. For example, with respect to Spraylat

PPL9675G, a complete cure is obtained by subjecting the powder coated mat

to a temperature of 395° F for five minutes. An acceptable partial cure is

obtained by subjecting the powder coated mat to a temperature of 300° F for

one minute.

Since it is important to cause the powder coat material to flow during

the partial curing process, the amount of powder coat material placed on the

aluminized fiber mat is important. If the powder coat application is too heavy,

then the partial cure flow is degraded, and there is a waste of powder coat

material. If the powder coating is too light, then again the flow is degraded.

In one advantageous embodiment using an aluminized fiberglass mat

substantially as described earlier, the powder coat material is applied to the

aluminized fiberglass mat until there is approximately a 150% add-on by

weight. In other words, the combined weight of the conductive mat and the

powder coating is 150% heavier than the conductive mat itself. Alternately,

the amount of spray may be measured in terms of the thickness of the powder coating material. In one advantageous embodiment, the powder coat

material is about 0.045 inches thick after it has been sprayed onto the

conductive mat.

In view of the above disclosure, it will be clear to those of skill in the art

that other coating materials capable of providing an environmentally durable

finish, as well as adhesion to the antenna core may be substituted for the

thermosetting polyester powder coating, described above. For example,

another category of suitable finishing materials include thermosetting resin

systems, such as "GELCOAT," commercially available from Reichhold

Corporation. Other suitable thermosetting resin systems will occur to those of

skill in the art and particular systems may be selected as a matter of design

choice. Although there are specific differences, in general these systems

involve binding a thixatropic agent, i.e., a thickening agent, to a coating resin,

then applying the resin to the conductive mat. In this case, a catalyst is also

added to the resin to cause the resin to cure into a hardened finish. The use

of thermosetting resin systems differs from the powder coat system in that the

powder coat systems require the application of heat to achieve the precure

stage. By contrast, with resin systems, precure is obtained by balancing the

catalyst with a retarder so that the resin will flow suitably over the aluminized

fiberglass mat, but will not actually harden into the finished coating, until heat

is applied to the mat during its application to the antenna core.

After the reflective mat has been produced as described above, it is cut

into shape suitable for application to a specific antenna core. Figure 4 is a top view of a mat 400 which has been cut to fit the reflecting surface of a

dish-shaped antenna core. As seen, the mat 400 itself, is not round, but is

oval, and has cuts 402 formed therein to allow the mat 400 to be pressed

tightly against the reflecting surface of the antenna core without wrinkling.

Suitable mats will have sufficient flexibility to conform to the requisite antenna

shape.

Referring now to Figure 6, there is shown a method useful for applying

a reflective mat 200 having a partially cured coating as an applique to an

antenna core 102. Specifically, a reflective mat 200, having an underlying

conductive mat 202, each side of which is coated with a finishing material

201 a, 201 b, is applied to the reflective surface 604 of an antenna core 600 by

means of pressure provided by a heated tool 608. The tool 608 provides a

male heated tooling surface 612 which accurately images the shape of the

parabolic surface 106 of the antenna core 102. The mat contacting surface

612 of the tool 608 is heated by means of a heating element 610 constructed

within the tool 608. The heating element 610 is formed from according to

conventional techniques employing electrically resistive elements, steam or

hot oil elements. The tool 608 presses the mat 200 into the parabolic surface

106 of the antenna core 102, while heating the mat 200 to thermally complete

the curing of the finishing material. As the finishing material cures, the portion

of the coating connecting the mat 200 to the parabolic surface 106 acts as an

adhesive which permanently fixes the mat 200 to the antenna core 102. The

side of the mat 200 which faces away from the core 102 is simultaneously finished to provide a durable, environmentally protective and aesthetically

acceptable coating layer to the underlying conductive mat. The pressure and

temperature applied by the tool 608 during this process will, of course,

depend on the coating selected for the mat 200 and the amount of partial

cure already applied. In one particular embodiment, employing a suitably

precured Spraylat PPL9675G polyester material, the tool 608 is applied to the

mat 200 at a force of about 150 psi at 400 °F for one minute. Afterwards, the

tool 608 is removed, leaving the finished antenna having a permanently

affixed mat 200 on the signal receiving surface 106 of the antenna core 102

as shown as in Figure 7. The adhesive layer 201 a is compressed in this

operation to, for example, about .007 inches. Moreover, a protective finish

layer 201 b is also fully cured and ready for use. No further painting is

required. It will be noted that fewer processing steps are required to attach

mat 200 than required for conventional painting processes. Thus, the

antenna manufacturing process has been simplified, thereby producing a high

quality antenna at a lower cost, while avoiding the use of VOCs which are

required by conventional painting processes.

Although the invention has been described with respect to specific

embodiments, it will be understood by those of skill in the art that various

changes can be made in form and detail without departing from the scope

and spirit of the present invention. All publications discussed herein are

hereby incorporated by reference as those set forth in full.

Claims

What is claimed is:

1. A reflective mat useful to provide an electromagnetically reflective

surface on high frequency radio antennas, the reflective mat comprising:

a conductive mat shapable to conform to a signal receiving surface of

an antenna core, the conductive mat having a core contacting surface, which

is coated with an adhesive material for joining the conductive mat to the

signal receiving surface, and a signal reflecting surface which is coated with a

finishing material.

2. A reflective mat as in claim 1 wherein the adhesive material and the

finishing material comprise a thermosetting polyester.

3. A reflective mat as in claim 1 wherein the adhesive material and the

finishing material comprise a thermosetting resin.

4. A reflective mat as in claim 1 wherein the conductive mat comprises a

woven mesh having a plurality of conductive fibers.

5. A reflective mat as in claim 1 wherein the conductive mat comprises a

material having a plurality of non-conductive fibers which are at least partially

coated with a conductive material.

6. A reflective mat as in claim 1 wherein the conductive mat comprises a

non-woven fiberglass material in which the fibers are at least partially coated

with aluminum.

7. A radio frequency antenna having an antenna core with a signal

receiving surface, the signal receiving surface being at least partially covered

with a reflective mat, the reflective mat comprising:

a conductive mat shaped to conform to a signal receiving surface of an

antenna core, the conductive mat having a core contacting surface, which is

coated with an adhesive material for joining the conductive mat to the signal

receiving surface, and a signal reflecting surface which is coated with a

finishing material.

8. An antenna as in claim 7 wherein the adhesive material and the

finishing material comprise a thermosetting polyester.

9. An antenna as in claim 7 wherein the adhesive material and the

finishing material comprise a thermosetting resin.

10. An antenna as in claim 7 wherein the conductive mat comprises a

woven mesh having a plurality of conductive fibers.

1 1. An antenna as in claim 7 wherein the conductive mat comprises a

coated with a conductive material.

12. An antenna as in claim 7 wherein the conductive mat comprises a non-

woven fiberglass material in which the fibers are at least partially coated with

aluminum.

13. A method for fabricating a radio frequency antenna, the method

comprising:

providing a reflective mat which includes a conductive mat shaped to

conform to a signal receiving surface of the antenna core, the conductive mat

having a core contacting surface, which is coated with an adhesive material

for joining the conductive mat to the signal receiving surface, and a signal

reflecting surface which is coated with a finishing material;

attaching the contacting surface of the conductive mat to the signal

receiving surface of the antenna core with a tool having a heatable surface

configured as a male image of the signal receiving surface, the heatable

surface being joined to the signal reflecting surface of the conductive mat;

heating the heatable surface until the reflective mat is permanently

adhered to the antenna core and the finishing material on the signal reflecting

surface is cured.

14. A method as in claim 13 further comprising the step of partially curing

at least one of the adhesive material or the finishing material.

15. A method as in claim 13 wherein providing a reflective mat comprises

coating the core contacting surface and the signal reflecting surface of the

conductive mat with a thermosetting polyester material.

16. A method as in claim 13 wherein providing a reflective mat comprises

coating the core contacting surface and the signal reflecting surface of the

conductive mat with a thermosetting resin material.

17. A reflective mat useful to provide an electromagnetically reflective

surface on high frequency radio antennas, the reflective mat comprising:

a conductive mat formed from a non-woven fiberglass material in which

the fibers have been provided with a metallic coating, the conductive mat

being shaped to conform to a signal receiving surface of an antenna core, the

conductive mat having a core contacting surface and a signal receiving

surface, each surface being coated with a partially cured finishing material.

18. A reflective mat as in claim 17 wherein the partially cured finishing

material comprises a thermosetting polyester.

19. A reflective mat as in claim 17 wherein the partially cured finishing

material comprises a thermosetting resin.