EP0109186A1

EP0109186A1 - Antenna

Info

Publication number: EP0109186A1
Application number: EP83306201A
Authority: EP
Inventors: Kazuharu Shimizu
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1982-10-15
Filing date: 1983-10-13
Publication date: 1984-05-23
Also published as: DE3375259D1; KR910008947B1; CA1202414A; EP0109186B1; JPS5970005A; JPH0380362B2; KR840006576A

Abstract

An antenna (1) including a reflector (2) having a paraboloidal front surface (8) and a primary radiator (3). The reflector (2) of the antenna comprises a reflecting layer (9) having the paraboloidal front surface and the backing layer (10) attached to the rear surface of the reflecting layer (9). The reflecting layer is made of a resin in which short carbon fibers (7) are randomly dispersed with the axis of each fiber substantially parallel to the paraboloidal front surface. The antenna thus constructed has nearly isotropic electro-conductivity and excels in durability.

Description

The present invention relates to an antenna, specifically to an antenna including a reflector having a paraboloidal front surface for use in transmission and reception of microwaves or millimeter waves, such as a parabolic antenna or a Cassegrainian antenna.
A parabolic antenna or a Cassegrainian antenna including a reflector having a paraboloidal front surface a radio wave reflecting surface ) and a primary radiator have been known in the past. The reflectors have a reflecting layer made of carbon fiber reinforced resin, that is (a) resin reinforced with sheets in which strands of carbon filaments are arranged in parallel in uni-direction, said sheets being stratified with fiber axes extending orthogonal to one another, or (b) resin reinforced with fabric of strands of carbon filaments. However, such conventional antennas have a drawback in that the anisotropy of the paraboloidal front surface with respect to electro-conductivity is so large that the efficiency of transmission and reception varies due to anisotropy of the waves being received. Polarization occurs because carbon filaments which impart electro-conductivity to the paraboloidal front surface and radio-wave-reflectivity to the reflector are arranged with the axes of the filaments extending in two directions, i.e., 0° and 90° directions.
A parabolic antenna includes a reflector having a reflecting layer made of 0.5 mm thick carbon fiber reinforced resin, in which four sheets of carbon filaments are arranged parallel in uni-direction and are stratified. If the directions of the fiber axes of said four sheets are arranged so as to be at, 0°, 90°, 90° and 0° directions, the relationship between the angle e, which is made by the electric vector of an incident wave ( linear polarized wave ) against the direction of the axis of carbon filaments constituting the reflecting layer, and the reflection loss R can be expressed by a broken line shown in Figure 4 mentioned later. The relationship indicates that the reflection loss is largely dependent on the direction of arrangement of carbon filaments.
To eliminate this drawback, the paraboloidal front surface is sometimes laminated with aluminium foil, coated with nickel or flame sprayed with zinc. In this type of antenna, the above-mentioned problem of anisotropy is eliminated because the metal is isotropic with respect to electro-conductivity. However, this type of antenna lacks durability because the metal is less resistant to the weather and the coating or flame sprayed metal is liable to be damaged.
An object of the present invention is to provide an antenna which is least liable to variations in the efficiency of wave transmission and reception due to polarization of wave and excels in durability to eliminate the above-mentioned drawbacks in the conventional antenna.
This object is attained in the present invention by an antenna including a reflector having a paraboloidal front surface and a primary radiator. The reflector includes a reflecting layer having a paraboloidal front surface and a backing layer attached to the rear surface of the reflecting layer. The reflecting layer comprises a base layer made of a resin and short fibers of carbon fibers ( hereinafter referred to as "short carbon fibers" ) dispersed in the base layer and the axis of each fiber is substantially parallel to the paraboloidal front surface. Such short carbon fibers/resin composite may be a kind of carbon fiber reinforced resin. The primary radiator is arranged at the focal point of the paraboloidal front surface. The short carbon fibers are desirably 5-25 mm in average length. Preferably the short carbon fibers are separated from each other and are free
from the agglomeration. The short carbon fibers may be a mixture of fibers of 5-25 mm in average length and fibers of 1-5 mm in average length.
The antenna of the above-mentioned constitution according to the present invention, in which the reflecting layer of the reflector is made of short carbon fibers/resin composite in which short carbon fibers are dispersed and extend in random directions substantially parallel to the paraboloidal front surface, has its electro-conductivity least anisotropic or nearly isotropic. Accordingly, the efficiency of wave transmission and reception in this antenna scarcely changes depending on the directions of wave polarization, in other words, the efficiency of wave transmission and reception is scarcely affected by the direction of wave polarization. When short carbon fibers of 5-25 mm in average length are mixed with short carbon fibers of 1-5 mm in average length, the electro-conductivity will be more isotropic and the efficiency of wave transmission and reception will be still more enhanced.
Since short carbon fibers/resin composite is highly resistant to the weather and does not deteriorate under exposure to wind, rain and sunshine, the antenna according to the present invention is rated extremely durable.
Further, short carbon fibers/resin composite is extremely easy to be molded. Thus it can be mass-produced by drawing and the like at a low cost.
These and other objects of the present invention will become more apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings, of which:

Figure 1 is an oblique view of a paraboloidal antenna as an embodiment of the present invention;
Figure 2 is a partial sectional view of the antenna of Figure 1;
Figure 3 is a system diagram showing an apparatus for testing the reflection loss of the antenna;
Figure 4 is a graph showing the relationship between the angle ₈ of the electric vector of an incident wave ( a linear polarized wave ) against the direction of the axis of short carbon fibers contained in the reflecting layer of the reflector and the reflection loss R;
Figure 5 is a graph showing the relationship between the length L of short carbon fibers in the reflecting layer and the reflection loss R;
Figure 6 is a graph showing the relationship between the fiber content X=W₃/(W₃+W₁₂), where W₃is the weight of short carbon fibers of 3 mm in length and W₁₂ is the weight of short carbon fibers of 12 mm in length, and the reflection loss R;
Figure 7 is a graph showing the relationship between the density D of the short carbon fiber mat in the reflecting layer and the reflection loss R ;
Figure 8 is a graph showing the relationship between the frequency F and the reflection loss R of the reflecting layer A made of resin in which 50 % of short carbon fibers of 3 mm in length and 50 % of short carbon fibers of 12 mm in length are dispersed and the relationship between the frequency F and the reflection loss R of the reflecting layer B made of resin in which 100 % of short carbon fibers of 24 mm in length are dispersed;
Figure 9 is a rear elevation of a backing layer in which glass filaments are arranged in two directions so as to cross at an angle of about 90 degrees;
Figure 10 is a rear elevation of a backing layer in which glass filaments are arranged in four directions so as to cross at an angle of about 45 degrees; and
Figure 11 is a rear elevation of a backing layer in which a fabric is used.

An embodiment of the present invention is to be described below. Figure 1 illustrates a parabolic antenna of one embodiment of the present invention. The antenna 1 includes a reflector 2 having a paraboloidal front surface 8 and a primary radiator 3 which is located at the focal point of the paraboloidal front surface 8. A waveguide 4 is provided to guide microwaves or millimeter waves from the primary radiator 3 to subsequent equipment such as a picture tube. A framework 5 supports the antenna 1.
As shown in Figure 2, the reflector 2 includes (a) a reflecting layer 9 having the paraboloidal front surface 8 and made of short carbon fibers/resin composite and (b) a backing layer 10 attached to the rear surface of the reflecting layer 9 and made of short glass fiber reinforced resin. Thus the reflector 2 includes a stratification of the reflecting layer 9 of short carbon fibers/resin composite and the backing layer 10 of short glass fiber reinforced resin.
The short carbon fibers/resin composite consists of a thermosetting resin 6 such as epoxy resin, unsaturated polyester resin, phenolic resin, polyimide resin, or a thermoplastic resin 6 such as polyamide resin, polyalkyl resin and short carbon fibers 7 of 5-25 mm in average length. The short carbon fibers 7 are dispersed in a base layer made of said resin 6 with the axis of each fiber 7 substantially parallel to the paraboloidal front surface 8. Meanwhile, in said glass fiber reinforced resin, short glass fibers 11 of 10-50 cm in average length are used. The short glass fibers 11 are likewise dispersed in a resin with the axis of each fiber substantially parallel to the paraboloidal front surface 8. The short carbon fibers 7 in the short carbon fibers/resin composite serve to impart electro-conductivity to the reflecting layer 9. To secure high electro-conductivity, it is theoretically obvious that the longer the fibers 7, the better. However, fibers which are too long would result in uneven dispersion, lower conductivity and difficulty in molding. Therefore, the short carbon fibers 7 are desirably 25 mm or less in length. To the contrary, fibers which are too short would improve the moldability but decrease the conductivity. Thus, the short carbon fibers 7 are preferably 5-25 mm in average length, more preferably 10-20 mm in average length. From the standpoint of conductivity, the larger the proportion of short carbon fibers 7 contained in the carbon fibers/resin composite, the better. Extremely large proportion of short carbon fibers would, however, decrease the moldability and accordingly, the preferable proportion would be 40-60% by volume based on the total volume of the reflecting layer 9.
In the short carbon fibers/resin composite, short carbon fibers of 5-25 mm in average length may be mixed with short carbon fibers of 1-5 mm in average length. In such a mixture, the space left by short carbon fibers of 5-25 mm in average length would be filled up with short carbon fibers of 1-5 mm in average length. This mixture would not only reduce the anisotropy in the conductivity but also enhance the conductivity of the paraboloidal front surface 8. Also, relatively short carbon fibers of 1-5 mm in average length would hardly affect the moldability. For the purpose of securing high moldability, such a mixture of carbon fibers is desirably such that in terms of weight, fibers of 1-5 mm in average length constitute 1-3 against 1 of fibers of 5-25 mm in average length.
Glass fiber reinforced resin in which short glass fibers are used serves to impart mechanical strength to the antenna. In the illustrated embodiment, from the standpoint mainly of moldability glass fibers 11 of 10-50 cm in average length are adopted. However, the glass fibers of other structure may be adopted. The glass fibers may be in the form of a mat bonded with a binder. The preferable weight per unit area of the mat is 3-100 g/m². The sheets of glass filaments 12 which are arranged parallel in uni-direction may be stratified and the directions of the fiber axes of said sheets may be arranged so as to be at about 0°, 90° as shown in Figure 9 or about 0°, 45°, -45°, 90° as shown in Figure 10. However, use of glass fibers or filaments is not mandatory. Fibers or filaments of alumina, silicon carbide or polyaramide may be used as well as glass fibers or filaments. Further, filaments may be used in the form of a fabric 13 as shown in Figure 11. That is, a glass fiber fabric, an alumina fiber fabric, a silicon carbide fiber fabric and a polyaramide fiber fabric may be used. Instead of fiber reinforced resin, aluminium honeycomb or synthetic paper honeycomb ( for example, honeycomb of paper made of poly-m-phenylene isophthalamide )may be employed.
The antenna according to the present invention can be manufactured by various methods, one of which is illustrated here.
On a glass fiber SMC ( Sheet Molding Compound ) of several millimeters in thickness is formed a layer of short carbon fibers bonded with a binder, that is a layer of short carbon fiber mat, by a routine process of paper making. Thereby the density ( a weight per unit area ) of the short carbon fiber mat is desirably 30-100 g/m². Then an unsaturated polyester resin film not yet hardened is laid on this short carbon fiber mat and the entire composition is placed in a mold with a paraboloidal surface, to be pressurized and heated for integration, thereby producing a reflector.
When a waveguide, a primary radiator and a framework are fitted to this reflector, an antenna is manufactured.
The antenna according to the present invention is available for versatile purposes, for instance, for microwave or millimeter wave communication, broadcasting, radar and TV-broadcast receiving antenna via satellite.
Next, examples of testing the reflection loss accounting for every critical value indicated above are given below.
In the test, the reflection loss was measured as follows. The measuring system was constituted as shown in Figure 3. A high-frequency signal generated by Hewlett Packard's Synthesized Signal Generator HP 8672A ( Reference Numeral 12 ) was transformed into a microwave in the waveguide using a Hewlett Packard's Adapter HP X281 ( Reference Numeral 13 ). The wave propagating through the waveguide and reflected from a sample or a blank copper plate was split by the directional coupler 14 into two parts, one of which went through the isolator 15, impedance-matched by E-H tuner 16, and was transformed into a current signal by the crystal mount 17 and detected by YHP 4041B pA-meter ( pico-ammeter ) 18. The isolator and the directional coupler used here were the products of Shimada Rika K.K.
The whole measuring system is controlled by a microcomputer "Apple 1" 19, while the synthesized signal generator 12 and said pA-meter 18 are coupled by means of GP-IB. The frequency was swept at every 100 MHz by the synthesized signal generator 12. In the first sweeping, the measured power of a reflection wave from the blank polished copper plate 20 and, in the second sweeping, the measured power of a reflection wave from the sample, as detected by the pA-meter 18 were memorized and finally the reflected power (dB) of the sample minus the reflected power (dB) of the copper plate at each frequency was yielded as the reflection loss in the sample as an output from the microcomputer. In the following examples 2,3 and 4, the data at 12 GHz are average values for 16 points taken at 100 MHz interval from 11.5 GHz to 12.5 GHz. As shown in Figure 3, the sample and the blank copper plate 20 were measured as inserted between the flanges of the waveguide. As sectionally shown, they were fixed to the flanges by bolts and nuts with holes 21 bored at 4 peripheral points. The rear of the sample was terminated with a nonreflective termination 22 to suppress a subsequent reflection wave.
The sample 20 was applied with carbon fibers ( " Torayca " manufactured by Toray Industries, Inc. ) cut to different lengths with the binder being a polyester resin, by a routine process of paper making. The short carbon fiber mat thus produced was impregnated with epoxy resin #2500, manufactured by Toray Industries, Inc., and heated under pressure to mold it into a board. When the density of the mat is about 50 g/m², the molded product will be about 0.2 mm thick. In the mat, carbon fibers account for 75% by weight with the balance of 25% being the binder.
The parameters in the testing and the results of testing are as follows.

Test example 1.

The variations of the reflection loss R with the electromagnetic vector angle 0 of the incident wave ( linear polarized wave ) were measured, the results being shown in Figure 4, in which the solid line C refers to the present invention and the broken line D refers to the previouly mentioned conventional antenna. As illustrated in Figure 4, the reflector according to the present invention possesses good reflecting characteristic with no directivity.

Test example 2.

The measurement of reflection loss R was conducted with the fibers in the mat cut to 3, 6, 12, 24 and 48 mm in length, the frequency being 12 GHz and the density of mat being about 50 g/m². Figure 5 shows the results of measurement. As illustrated in Figure 5, a fairly good reflection characteristic is exhibited even when the cut length of fiber is less than 25 mm. The measured data is averaged for 20 samples.

Test example 3.

A measurement was made of a mat tentatively produced with a mixture of carbon fibers cut to 3 mm and to 12 mm. Density of this mixed fibers mat was about 50 g/m² and the frequency was 12 GHz. The measured data is averaged for 20 samples. Figure 6 shows the results, which indicate that the best reflection characteristic is obtained for a system of 50/50% fiber mixture.

Test example 4

The reflection loss in a mat of 12 mm fibers when the density was varied 10, 30, 50, 70 and 90 g/m² was measured. The frequency was 12 GHz and the data measured is averaged for 20 samples. Figure 7 shows the results, which indicate that the reflection characteristic is better, the larger the density. The performance is good at 50 g/m²of surface density and it begins to saturate at about 70 g/m²of density.

Test example 5

Variation of reflection loss with frequency was compared between a mat A including 50%-3 mm length fibers and 50%-12 mm length fibers and a mat B including 100%-24 mm length fibers, the density being about 50 g/m². Figure 8 shows the results. The reflection loss is desirably more than -0.2 dB. The test results indicate that in the mat B, the values are around -0.2 dB line whereas in the mat A, the values are above this line of -0.2 dB at practically all frequencies. This proves the excellent performance of the mat A as a reflector for the paraboloidal antenna.
Although only an exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.

Claims

1. A paraboloidal antenna including:

(a) a reflector comprising:

(a-1) a reflecting layer having a paraboloidal front surface and

(a-2) a backing layer attached to the rear surface of the reflecting layer;

(b) the reflecting layer comprising:

(b-1) a base layer made of a resin and

(b-2) short carbon fibers dispersed in the base layer, the axis of each fiber being substantially parallel to the paraboloidal front surface; and

(c) a primary radiator disposed at the focal point of the paraboloidal front surface.

2. An antenna of claim 1, wherein the average length of the short carbon fibers is 5-25 mm.

3. An antenna of claim 2, wherein the average length of the short carbon fibers is 10-20 mm.

4. A paraboloidal antenna including:

(a) a reflector comprising:

(a-l) a reflecting layer having a paraboloidal front surface and

(a-2) a backing layer attached to the rear surface of the reflecting layer;

(b) the reflecting layer comprising:

(b-1) a base layer made of a resin and

(b-2) short carbon fibers having an average length of 1-5 mm and short carbon fibers having an average length of 5-25 mm dispersed in the base layer, the axis of each fiber being substantially parallel to the paraboloidal front surface; and

5. An antenna of claim 4, wherein a mixing ratio of the short carbon fibers having an average length of 5-25 mm to the short carbon fibers having an average length of 1-5 mm is 1:1-3 by weight.

6. An antenna of claims 1 or 4, wherein the short carbon fibers are contained in the reflecting layer in the form of a mat.

7. An antenna of claim 6, wherein a weight per unit area of the mat is 30-100 g/m².

8. An antenna of claims 1 or 4, wherein the short carbon fibers are contained in the reflecting layer within the range of 40-60% by volume based on the total volume of the reflecting layer.

9. An antenna of claims 1 or 4, wherein each of the short carbon fibers is substantially independent and free from agglomeration, overlapping and intersecting with neighboring fibers.

10. An antenna of claims 1 or 4, wherein the resin is a thermosetting resin.

11. An antenna of claim 10, wherein the thermosetting resin is selected from the group consisting of epoxy resin, unsaturated polyester resin, phenolic resin and polyimide resin.

12. An antenna of claims 1 or 4, wherein the resin is a thermoplastic resin.

13. An antenna of claim 12, wherein the thermoplastic resin is selected from the group consisting of polyamide resin and polyalkyl resin.

14. An antenna of claims 1 or 4, wherein the backing layer consists of a resin reinforced with short fibers selected from the group consisting of glass fibers, alumina fibers, silicon carbide fibers and polyaramide fibers.

15. An antenna of claim 14, wherein the short fibers randomly disperse in the backing layer and the axis of each fiber is substantially parallel to the paraboloidal front surface.

16. An antenna of claim 14, wherein the average length of the short fibers is 10-50 cm.

17. An antenna of claims 1 or 4, wherein the backing layer consists of a resin reinforced with filaments selected from the group consisting of glass filaments, alumina filaments, silicon carbide filaments and polyaramide filaments.

18. An antenna of claim 17, wherein the filaments are arranged in two directions so as to cross each other at an angle of about 90 degrees and the axis of each filament is substantially parallel to the paraboloidal front surface.

19. An antenna of claim 17, wherein the filaments are arranged in four directions so as to cross each other at an angle of about 45 degrees and the axis of each filament is substantially parallel to the paraboloidal front surface.

20. An antenna of claims 1 or 4, wherein the backing layer is a resin reinforced with a fabric selected from the group consisting of a glass fiber fabric, an alumina fiber fabric, a silicon carbide fiber fabric and a polyaramide fiber fabric.

21. An antenna of claims 1 or 4, wherein the backing layer is an aluminium honeycomb.

22. An antenna of claims 1 or 4, wherein the backing layer is a synthetic paper honeycomb.