CA1202414A

CA1202414A - Antenna having isotropic electro-conductivity characteristic

Info

Publication number: CA1202414A
Application number: CA000439011A
Authority: CA
Inventors: Kazuharu Shimizu
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1982-10-15
Filing date: 1983-10-14
Publication date: 1986-03-25
Also published as: JPH0380362B2; KR910008947B1; JPS5970005A; KR840006576A; EP0109186B1; EP0109186A1; DE3375259D1

Abstract

ABSTRACT OF THE DISCLOSURE
An antenna including a reflector having a paraboloidal front surface and a primary radiator. The reflector of the antenna comprises a reflecting layer having the paraboloidal front surface and a backing layer attached to the rear surface of the reflecting layer. The reflecting layer is made of a resin in which short car-bon fibers are randomly dispersed, the axis of each fiber being substantially parallel to the paraboloidal front surface. The an-tenna thus constructed has nearly isotropic electro-conductivity and excels in durability.

Description

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The present invention relates to an antenna, specifically to an an-tenna 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 re-flecting surface) and a primary radiator are known. The reflectors have a reflecting layer made of carbon fiber reinforced resin, that is (a) resin reinforced with she~ts in which strands of carbon fil-aments are arranged in parallel in uni-dixection, said sheets being stratified with fiber axes extending orthogonal to one another, or (b) resin reinforced with fabric of strands o 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 trans-mission and reception varies due to anisotropy of the waves being received. Polarization occurs because carbon filaments which im-part 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 di-rections.
A parabolic antenna generally includes a reflector havinga reflecting layer made of 0.5 mm thick carbon fiber reinforced resin, in which four sheets of carbon filaments are arranged par allel in uni-direction and are stratified. If the ~irections of the fiber axes of said four sheets are arranged so as to be at, 0, ~ 2 90, 90 and 0 directions, the rela-tionship between -the angle ~, which is made by the electric vector of an incident wave (linear polarized wave) against the direction of the axis of carbon fila-ments consti-tuting the reflecting layer, and the reflection loss R
can be graphically expressed. The relationship indicates tha-t the reflection los~ is largely dependent on -the direction of arrange-ment of carbon filaments.
To eliminate this drawback, the paraboloidal front sur-face is sometimes laminated with aluminum foil, coated with nickel lQ or flame sprayed with zinc. In this type of antenna, the above-mentioned problem of anisotropy is eliminated because the me-tal 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.
The present invention is directed to providing an an-tenna which is least liable to variations in the efficiency of wave transmission and reception due to polarization of wave and excels in durability to elimina-te the above-mentioned drawbacks in the conventional antenna.
The paraboloidal antenna of the present invention in~
cludes a reflector having a paraboloidal front surface and a pri-mary 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 cornprises a base layer made of a resin and short fibers of carbon fibers (here-ina~ter referred to as "short carbon fibers") dispersed in -the base ~ ~ 3 ~ ~ ~2~

layer and the axis of each fiber is substantially parallel to the paraboloidal front surface. Such short carbon fibers/resin cornpo-site may be a kind of carbon fiber reinforced resin. The primary radiator is disposed at the focal point of the paraboloidal fron-t 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 ~n in average length.
The antenna of the above-mentioned constitution according to the present invention, in which the reflecting layer of the re-flector 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 iso-tropic. Ac-cordingly, the efficiency of wave transmission and reception in this antenna scarcely changes depending on the directions of wave polarisation, in other words, the efficiency of wave transmission and reception is scarcely affected by the direction of wave polari-zation. When short carbon ~ibers of 5-~5 mm in average length are mixed with short carbon fibers of 1-5 mm in averac3e length, the electro-conductivity will be more isotropic and the efficiency of wave transmission and reception will be still more enhancedO
Since short carbon fibers/resin composite is highly re-sistant to the weather and does not deteriorate under exposure to wind, rain and sunshine, the antenna according to the present in-vention is rated extremely durable.

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Further, shor-t carbon fibers/resin composite is ex-tremely easy to be molded. Thus i-t can be mass-produced by drawing and the like at a low cost.
The present invention will become more apparent and more readily appreciated from the foll~winy detailed description of the presently preferred exemplary embodiment of the invention taXen 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 l;
Figure 3 diagrammatically illustrates an apparatus for testing the reflection loss of the antenna;
Figure 4 is a graph showing t.he relationship between the angle ~ of the electric vector of an incident wave (a linear polar-ized wave) against the direction of the axis of short carbon fibers contained in the reflecting layer of the reflector and the reflec-tion loss R;
~ igure 5 is a graph showing the relationship between the length L of short carbon fibers in the reflecting layer and the re-flection loss ~;
Figure 6 is a graph showing the relationship between the fiber content X=W3/(W3~W12), 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 ~n in leng-th, and the reflection loss ~, 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; and 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 shor-t 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 reflec-tion loss R
of the reflecting layer B made of resin in which 100% of short car-bon 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 lO 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 ll is a rear elevation of a backing layer in which a fabric is used.
A particular embodiment of -the present invention is des-cribed below. Figure l illustrates a parabolic antenna of one em-bodiment of the present invention. The antenna l includes a re-flector 2 having a paraboloidal front surface 8 and a primaryradiator 3 which is located at the focal point of -the paraholoidal front surface 8. A waveguide 4 is provided to guide microwaves or millimeter waves from -the primary radiator 3 -to subsequent equip-ment such as a picture tube. A framewor~ 5 supports the an-tenna l.
As shown in Figure 2, the reflector ~ includes (a) a re-flecting layer 9 having the paraboloidal front surEace 8 and made 4~

of short carbon fibers/resin composite and (b) a bac]cing layer 10 attached to -the rear surface of the reflecting layer 9 and made of short glass fiber reinforced resin. Thus the reElector 2 inclu~es a stratification of the re1ecting layer 9 of short carbon fibers/
resin composite and the backing layer 10 of short glass fiber rein-forced resinO
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 polyami~e resin, polyalkyl resin and short carbon fibers 7 of 5-25 mm in average length. The short carbon fibers 7 are dis-persed 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 fi-bers 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 substan-tially parallel to the paraboloidal front surface 8.
The short carbon fibers 7 in the short carbon fibers/resin compo-site serve to impart electro-conductivity to the reflecting la-yer 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 con-ductivity 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 de-crease the conductivity. Thus/ the shor-t carbon fibers 7 are pre-ferably 5-25 mm in average length, more preferably 10-20 mm in ` _ 7 average length. From -the standpoillt of conductivity, the larger the proportion of short carbon fibers 7 contained in -the carbon fi-bers/resin composite, the better. ~n extremely large proportion of short carbon fibers would, however, decrease -the moldability and accordingly, the preferred proportion would be ~0-60~ 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 Eibers oE 5-25 mm in average leng-th would be filled up with short carbon fibers of 1-5 mm in average leng-th.
This mixture would not only reduce -the anisotropy in the conducti-vity bu-t also enhance -the conductivity of the paraboloidal front surface 8. Also, relatively short carbon fibers of 1-5 mm in aver-age length would hardly affect the moldability. For the purpose of securing high moldability, such a mi~ture of carbon fibers is de-sirably such that in terms of weight, fibers of 1-5 mm in averaye length constitute 1-3 against 1 of fibers of 5-25 ~m 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 moldabil-ity 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 stratiEied and the directions o the fiber axes of said sheets may be arranged so as -~o be at about 0, 90 as shown in Figure 9 or about 0, ~5, -45, 9O as shown in Figure 10.
However, use o-f 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 fi-ber fabric and a polyaramide fiber fabric may be used. Instead of fiber reinforced resin, aluminum 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 ta weight per unit area) of the short carbon fiber mat is desirably ~0-100 g/m2. Then an unsaturated polyester resin film not yet hardened is laid on this short carbon fiber mat and the entire com-position is placed in a mold with a paraboloidal surface, to be pressurized and heated for integration~ thereby producing a xe-flector.
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 versa~

kile and available for a number of purposes, for instance, :Eor mi-crowave or millimeter wave communication, broadcasting, radar and TV-broadcast receiving antenna via satellite.
Examples of testing the reflection loss accounting for every critical value indicated above are g.iven 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 Packardls Synthesized Signal Generator HP 8672A (Reference Numeral 12) was transformed into a microwave in the waveguide using a Hewlett Packard's Adapter ~P 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 4041~ 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 microcom-puter "Apple II" (Trade Mark) 19, while the synthesized signal gen-erator 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 re-flection 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 memori~ed and finally the reflected power (dB) o~ the sample minus the reflected power (dB) of the copper plate at each frequency was yiel~ed as the re-flection loss in the sample as an output from the microcornputer.
In the following examples 2,3 and 4, the data at 12 GHz are average values ~or 16 poin-ts taken a-t 100 ~Hz interval from llo 5 GHz to 12.5 GH~. ~s shown in Figure 3, the sample and the blank copper plate 20 were measured as inserted between the ~langes 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 termina-10 tion 22 to suppress a subsequent reflection wave.
The sample 20 was applied with carbon fibers ("Torayca"
(Trade Mark) manufactured by Toray Industries, Inc.) cut to differ-ent lengths with the binder being a polyester resin, by a routine process of paper making. The short carbon iber mat thus produced was impregnated with epoxy resin ~2500, manufac-tured by Toray Industries, Inc., and heated under pressure to mold it into a boara, 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 ac-count for 75~ by weight with the balance of ~5% being the binderO
The parameters in the testing and the results of tes-ting are as follows.
Test example 1 The variations of the reflection loss R with the electro-magnetic vector angle ~ 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 ~ refers -to the previouly menlioned conven-tional antenna. As ~ ll ~

illustrated in Figure 4, the reflector according to -the present in-vention possesses good reflecting characteristic with no dlrect,iv-ity.
Test example 2 The measurement of reflection loss R was conducted with the fibers in the mat cut to 3, 6, 12, 2~ 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 illus-trated in Figure 5, a fairly good reflection characteristic is ex-10 hibited even when the cut length of fiber is less than 25 mm. The measured data is averaged ~or 20 samples.
Test example 3 A measurement was made of a mat tentatively ~roduced 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 charac-teristic is obtained for a system of 50/50~ Eiber mixture.
Test example ~
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 fre~uency was 12 GHz and the data measured is averaged for 20 sam-ples. Figure 7 shows the results, which indicate that the reflec-tion characteristic is better, -the larger the density. The perfor-mance is good at 50 g/m2 of surface densi~y and it begins -to sa turate at about 70 g/m2 of density.

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Test example 5 Variation of reflection loss with frequency was cornpared between a mat A including 506-3 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 resul-ts 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 a-t 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 inven-tion has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the ex-emplary 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

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A paraboloidal antenna comprising: (a) a reflector which has a reflecting layer having a paraboloidal front surface and a backing layer attached to a rear surface of the reflecting layer, and (b) a primary radiator disposed at the focal point of the paraboloidal front surface, the reflecting layer also having a base layer made of a resin with short carbon fibers dispersed therein, the axis of each said fiber being substantially parallel to 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 comprising. (a) a reflector which has a reflecting layer having a paraboloidal front surface and a backing layer attached to the rear surface of the reflecting layer, and (b) a primary radiator disposed at the focal point of the paraboloidal front surface, the reflecting layer also having a base layer made of a resin with 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 said base layer, the axis of each fiber being substantially parallel to the paraboloidal front surface

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 claims 1 or 4, wherein the short carbon fibers are contained in the reflecting layer in the form of a mat and 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 claim 1 or 4, wherein each of the short carbon fibers is substantially independent and free from agglomera-tion, overlapping and intersecting with neighboring fibers.

10. An antenna of claim 1, 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 claim 1, 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 claim 1, 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 sub-stantially 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 claim 1, wherein the backing layer consists of a resin reinforced with filaments selected from the group con-sisting of glass filaments, alumina filaments, silicon carbide fil-aments 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 sil-icon carbide fiber fabric and a polyaramide fiber fabric.

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

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