EP0109186B1

EP0109186B1 - Antenna

Info

Publication number: EP0109186B1
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: 1988-01-07
Also published as: DE3375259D1; KR910008947B1; CA1202414A; EP0109186A1; JPS5970005A; JPH0380362B2; KR840006576A

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 Casegrainian antenna including a reflector having a paraboloidal front surface (a radio wave reflecting surface) and a primary radiator have been known in the past. Known reflectors can 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 unidirection, said sheets being laminated 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 known 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 and are laminated. 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 8, 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.
Other types of antennas and related prior art is disclosed in the following documents.
German Specification No. 2,008,266, with respect to which claim 1 is delimited, discloses a parabolic reflector antenna. As shown in the drawing, the reflector is made up of two layers, a reflecting surface layer which is about 1 mm thick and a substantially thicker layer of glass fibre reinforced synthetic resin which forms the actual carrier-layer being stiffened by ribs. An exciter is situated at the focal point of the reflector. The reflecting surface layer is made of a mixture of 65% by weight of aluminium grit and 35% by weight of cold-setting synthetic resin.
German specification No. 3,106,506 relates to metallized carbon fibres and composite materials containing these fibres. Carbon filament yarn and carbon fibres are provided with a metal coating by means of a currentless process, in order to provide structures having excellent adhesion properties relative to synthetic plastic materials without prejudicing their tensile strength.
An article published in "Frequenz", Vol. 35 No. 6 at pp 155-162, discloses a parabolic antenna having a reflecting surface layer which is made of a synthetic resin reinforced by fabrics or filaments or carbon fibres.
U.S. Specification No. 3,716,869 relates to a millimeter wave antenna system which is mounted on a satellite. It includes a parabolic reflector made of carbon fibre reinforced plastic composite material to enable the shape of the reflector to be maintained within 3% of 1 mm wavelength despite large temperature fluctuations of the order of 167°C between portions which are illuminated by the sun and those which are in the umbra. A honeycomb structure is sandwiched between layers of carbon fibre reinforced plastic material.
U.S. Specification No. 4,388,623 discloses an antenna which has a large area reflector formed of a plurality of conductive slats of carbon fibre reinforced plastics material having air-gaps therebetween. The slats are mounted on a supporting .and shaping framework which is of similar material.
Our prior U.S. Specification No. 4,169,911 relates to porous carbon fibre material with a thin film covering each fibre. Porous material comprises a number of intersecting cut carbon fibres each of which has a diameter of 3-20 pm. The fibres are covered with a thin metal film and are completely and randomly dispersed accumulated and bound with a binder at portions of intersection of the fibres, in order to form a porous structure through which a plurality of pores extend from one surface of the material to the other.
U.S. Specification No. 3,137,000 relates to a radio wave reflecting plate wherein skins of plastics reinforced by glass fibres are provided at both surfaces of a paper honeycomb and copper wires are held in parallel spaced relation to each other.
It is therefore an object of the present invention to provide an antenna which overcomes the above disadvantages and which is substantially free from variations in the efficiency of wave transmission and reception due to wave polarization.
According to the present invention there is provided a paraboloidal antenna including:

(a) a reflector comprising
- (a-1) a base layer made of a resin and forming a reflecting layer having a paraboloidal front surface; and
- (a-2) a backing layer attached to the rear surface of the reflecting layer;
(b) a primary radiator located at the focal point of the paraboloidal front surface; characterized by carbon fibers dispersed in the base layer, the axis of each fiber being substantially parallel to the paraboloidal front surface.

The carbon fibres may consist of a mixture of carbon fibers of 5-25 mm in average length with carbon fibers of 1-5 mm in average length.
The mixing ratio of the carbon fibers having an average length of 5-25 mm to the carbon fibers having an average length of 1-5 mm, may lie in the range of 1:1 to 1:3.
The electro conductivity of the above constructed antenna is substantially isotropic. Accordingly, the efficiency of wave transmission and reception does not substantially change in accordance with the direction of wave polarization.
Moreover, since the above construction of 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.
Furthermore, since the above construction of carbon fibers/resin composite is extremely easy to mold, it can thus be mass-produced by drawing or the like technique at a low cost.
The present invention will now be described in greater detail by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective 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+W,₂), where W₃ is 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. 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 laminate 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 or 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. In said glass fiber reinforced resin 10, 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 caron 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. On the other hand, 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 proportions 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, the ratio of fibers of 1-5 mm in average length to the fibers of 5-25 mm in average length lies in the range of 1:1 to 3:1.
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 may be laminated 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 unsaturatated 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. r.
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 20 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 4041 B 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 II" 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 bland 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 θ 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 previously 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 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/m² of surface density and it begins to saturate at about 70 g/m² of 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.

Claims

1. A paraboloidal antenna including

(a) a reflector (2) comprising:

(a―1) a base layer made of a resin and forming a reflecting layer (9) having a paraboloidal front surface (8); and

(a-2) a backing layer (10) attached to the rear surface of the reflecting layer;

(b) a primary radiator (3) located at the focal point of the paraboloidal front surface;

characterized by carbon fibers (7) dispersed in the base layer, the axis of each fiber being substantially parallel to the paraboloidal front surface

2. An antenna according to claim 1, characterized in that said carbon fibres consist of a mixture of carbon fibres of 5-25 mm in average length with carbon fibres of 1-5 mm in average length.

3. An antenna according to claim 2 characterized in that a mixing ratio of the carbon fibers having an average length of 5-25 mm to the carbon fibers having an average length of 1-5 mm lies in the range 1:1 to 1:3.

4. An antenna according to any one of the preceding claims, characterized in that the carbon fibers are contained in the reflecting layer within the range of 40-40-60% by volume based on the total volume of the reflecting layer.

5. An antenna according to any one of the preceding claims, characterized in that the resin is a thermosetting resin.

6. An antenna according to claim 5, characterized in that the thermosetting resin is epoxy resin, unsaturated polyester resin, phenolic resin or polyimide resin.

7. An antenna according to any one of the preceding claims 1 to 4, characterized in that the resin is a thermoplastic resin.

8. An antenna according to claim 7, characterized in that the thermoplastic resin is polyamide resin or polyalkyl resin.

9. An antenna according to any one of the preceding claims, characterized in that the backing layer consists of a resin reinforced with fibers of 10-50 cm in average length.

10. An antenna according to claim 9, characterized in that said fibers are glass fibers, alumina fibers, silicon carbide fibers or polyaramide fibers.

11. An antenna according to any one of the preceding claims 1 to 8 characterized in that the backing layer consists of a resin reinforced with glass filaments, alumina filaments, silicon carbide filaments or polyaramide filaments.

12. An antenna according to any one of the preceding claims 1 to 8, characterized in that the backing layer is a resin reinforced with a glass fiber fabric, an alumina fiber fabric, a silicon carbide fiber fabric or a polyaramide fiber fabric.

13. An antenna according to any one of the preceding claims 1 to 8, characterized in that the backing layer is an aluminium honeycomb or a synthetic paper honeycomb.