US 3631504 A
Description (OCR text may contain errors)
ates Mei Inventors Kunihiro Suetaki 10-1 1 Minami 3 ehome, Mequro-ku, Tokyo; Yoshiyuki Naito, No. 261-44 Suenaga, Kawassaki-shi, both of Ja Appl. No. 884,832
Filed Dec. 15, 1969 Patented Dec. 28, 1971 PARABOLIC ANTENNA WITH WAVE ABSORBER AT CIRCUMFERENTIAL EDGE 5 Claims, 8 Drawing Figs.
US. Cl 343/840 Int. Cl ..i-101q 19/12 Field of Search 343/781, 834, 836, 840, 912, 909
 References Cited UNITED STATES PATENTS 2,276,497 3/1942 Kroger 343/836 2,281,196 4/1942 Lindenblad 343/836 2,460,869 2/1949 Braden 343/840 3,101,473 8/1963 Fenlon 343/840 3,314,071 4/1967 Lader et a1 343/840 FOREIGN PATENTS 1,048,298 1/1959 Germany 343/840 Primary ExaminerEli Lieberman Attorney-Leonard H. King ABSTRACT: An antenna having a parabolic reflector including a wave absorber in the vicinity of the circumferential edge of the reflector. The absorber is composed of layers of different absorbing materials. The edge of the reflector is inserted into one of the layers. The absorber extends beyond the edge of the reflector, thereby providing an improved front-toback ratio of wave transmission.
PATENIED IIEC28 IHTI 3 1531,- 504 SHEET 1 UF 3 Fig.
I 2 3 f receiving am fie transmitting antenna p r antenna receiving transmitting antenna antenna ig, 3 (b) YOSH/YUK/ /\//9/ r0 1/4 1 flaw) A ORNEY standing wave ratio PATENTED UEC28 197i SHEET 3 BF 3 for 2200 MHZ I800 lebo 2000 2| 0o 2200 f( M H,)
INVENTOR Kw/H/eo fair/m5 Yaw/rum Mum ATTORNEY PARABOLIC ANTENNA WITll-I WAVE ABSORBER AT CIRCUMFERENTIAL EUGE BACKGROUND OF THE INVENTION This invention relates generally to a parabolic antenna, and more particularly to a parabolic antenna which is subject to microwaves being received and transmitted from a common station.
In long distance microwave systems, there exists the problem of providing microwave signals with sufficient energy to travel over the entire propagation path. As a result of losses suffered along the transmission path, frequently there is not suflicient reserve energy for the amplifier connected to the receiving antenna to reconstruct the signals. Although the transmitting system can be made to provide the signals with additional energy, there are practical limitations to this. A more common method of insuring the sufficiency of energy is to provide repeater stations at predetermined terminal span positions in the path. Each repeater station commonly contains a receiving antenna, an amplifier and a transmitting antenna. The weak incoming signals are received by the receiving antenna, amplified, and retransmitted as strong signals to the next repeater station.
In each of the repeater stations the receiving and trans mitting antennas are usually arranged in close proximity. Since directional antennas and reflectors are generally used, the major part of the transmitted wave is received by the following station. However, a part of the wave is turned backwardly to be received by the receiving antenna of the transmitting station.
The term A,,, is generally used to represent the front-to-back signal ratio of an antenna. This term represents the ratio of the energy of the wave which is transmitted by the antenna in the direction of propagation, to the energy of the wave received by the same antenna from the opposite direction. For good communications, it is desired to have the ration A greater than 60 db.
In most repeater stations, the antennas are of the conventional parabolic type constructed of metal, wherein the frontto-back ratio is represented by:
A =G|-(l to 13) db. where G is the gain of the antenna.
It is therefore possible to obtain an A, of more than 60 db. by increasing the gain, G. This can be done by increasing the diameter of the parabolic antenna. However, by doing so, the weight of the antenna is also increased so that the steel tower required for supporting the system becomes inevitably larger and moreover, it is difficult to keep an accurate surface on the antenna when the diameter is large. Therefore, both technically and economically it is not feasible to enlarge the diameter of the antenna.
Another solution is to use a horn antenna rather than the conventional parabolic type, since horn antennas have better directivity and greater front-to-back ratios. However, horn antennas are generally very heavy requiring large steel towers to support them.
Accordingly, it is an object of this invention to provide a parabolic antenna system having an improved front-to-back ratio.
Another object of this invention is to provide a transmitting antenna which can be used in a repeater station in conjunction with a receiving antenna.
A further object of this invention is to improve the front-toback ratio of a conventional parabolic antenna by placing a wave absorber at the circumferential edge of the parabola.
A still further object of this invention is to improve the parabolic antenna by absorbing the incident waves on a transmitting antenna.
BRIEF DESCRIPTION OF THE DRAWING This invention is described in greater detail in the following description when read in conjunction with the drawing in which:
FIG. 1 is a block diagram of a repeater system used in a long-distance microwave communication system as is shown in the prior art;
FIG. 2 shows the general arrangement of the elements of the system of FIG. 1;
FIG. 3 (a) shows in a transverse section a prior art parabolic antenna dish;
FIG. 3 (b) shows in a transverse section a prior art parabolic antenna dish having a metal ring in the circumference thereof;
FIG. 4 shows in a partial transverse section a parabolic antenna dish having a wave absorber at its circumferential edge;
FIG. 5 shows in a partial transverse section a parabolic antenna in accordance with this invention;
FIG. 6 (a) shows graphically the relationship between the standing wave ratio and frequency; and
FIG. 6 (b) shows in section the wave absorber of this invention.
Referring to FIGS. 1 and 2, a repeater station is shown which comprises a receiving antenna 1, an amplifier 2, and a transmitting antenna 3. As can be seen from FIG. 2, the transmitting antenna is within close proximity of the receiving antenna, and as a result a part of the wave incident upon the system is not only received by the receiving antenna 1, but also by the transmitting antenna 3.
In order to eliminate some of the microwaves incident on the receiving antenna, it has been known in the prior art to introduce a metal ring in the circumference of the outer surface of the parabolic antenna. FIG. 3 (a) shows the conventional parabolic antenna without the metal ring and FIG. 3 (b) shows the same antenna equipped with a metal ring 5. The metal ring 5, is generally placed as at the outer edge of the parabolic antenna 4 and extends the cone shape of the antenna in a substantially cylindrical direction.
The scheme as shown in FIG. 3 (b) does improve the frontto-back ratio, but only to a limited degree. For certain directions, the metal ring does prevent incident waves from arriving at the receiving antenna, Considering an angle 6, defined as the angle between the approaching waves and the axis of the parabolic surface, it can be seen that for waves approaching at an angle of 0=90, the metal ring will improve the front-to-back ratio A by interfering with the arriving incident waves. However, for waves approaching at an angle of 0=1 i.e., when the radiation field is in the back of the parabolic antenna, the metal ring does not improve the front-to-back ratio, but on the other hand, because of its metallic composition, degrades the value A The reason is that the electric current flowing on the back surface of the parabolic antenna turns at its circumference to flow into the front side whereby a radiation field is produced which degrades the front-to-back ratio.
The invention as hereinafter described provides a novel parabolic antenna having, instead of the metal ring of the prior art, a wave absorber covering the metallic circumference of the antenna.
Referring to FIG. 4, a portion of the outer surface of the metallic parabolic antenna is shown at 4, the outer rim of the circumference is covered by a wave absorber. Although the absorber can be of any known wave-absorbing material, it is shown as being composedof two such layers, 6 and 7, different from each other. The absorber is generally composed of dielectric material which is designed to reduce the attenuation of the waves at specific frequency ranges.
If the attenuation of the wave is no more than 20 db. after the wave has passed through the layers 6 and 7, a considerable electric current can flow on the metallic surface, and hence it is advisable to obtain more than 20 db. attenuation through the layers 6 and 7.
According to the fundamental feature of this invention, the electric current flowing in the neighborhood of the edge of the parabolic antenna is attenuated by the wave absorber 6 and 7 provided in the vicinity of the internal edge of the circumference of the parabolic antenna.
Although the wave absorber as shown in FIG. 4 attenuated the waves appearing at edge of the antenna, indicated by region A, frequently waves are incident upon the antenna which turn on to the inside surface from just beyond the antenna in the region indicated at B. In the later region, since there is no wave absorber, the front-to-back ratio will not be affected. This problem can be eliminated by extending the wave absorber beyond the end of the parabolic antenna as shown in FIG. 5.
In FIG. 5 the circumference of the parabolic antenna 4 is shown inserted into the wave absorber so that no wave can turn into the opposite surface from the region B. In other words, the absorbing layers 8 and 9 are thick in the part A as shown in FIG. 5 and an absorbing layer 10 covers the opposite surfaces of the circumference part of the parabolic antenna 4. The required thickness of wave absorber can be decreased by utilizing wave-absorbing magnetic materials such as ferrite. If the thickness is not a consideration, the wave absorber can be formed only of dielectric materials. The layer 9 of the part A is designed such that the attenuation in this layer is more than db. as previously mentioned, and hence a metal plate on the back surface is not necessary. Moreover, the front-to-back ratio is still more improved because the wave turning into the back surface is attenuated by the wave absorber 10.
FIG. 6 (a) shows graphically experimental results of the wave absorber shown in FIG. 6 (b), wherein the voltage standing wave ratio is plotted against frequency. In FIG. 6 (a) the curve 1 is the characteristics of the wave absorber having the metal plate 13 while the curve 2 shows performance of the same wave absorber with the metal plate 13 removed.
It is easily understood from these curves that the standing wave ratio less than 1.2 in high-frequency region (1,900 to 2,300 MHz.) is obtained by the above-mentioned wave absorber, regardless of the presence of the back plate 13.
The parabolic antenna 4 shown in FIGS. 4 and 5 can be regarded as equivalent to the metal plate 13 of FIG. 6 (b), hence the part A of FIGS. 4 and 5 corresponds to the curve 1 of FIG. 6 (a) while the part B corresponds to the curve 2.
Therefore, if the respective length of the parts A and B in FIGS. 4 and Sis about 5 cm., the property of the parabolic antenna can be much improved. And moreover, in FIG. 5, a part of the layer 8 corresponding to the part A is thicker than the other part of the layer 8 corresponding to the part B and covers the back surface of the parabolic antenna 4. so that the front-to-back ratio of the antenna can be improved.
There has been disclosed heretofore the best embodiment of the invention presently contemplated and it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the spirit of the invention.
1. A parabolic antenna including a reflector having wave absorbing means in the vicinity of the circumferential edge of the reflector, said means comprising a plurality of layers of absorbing material wherein said edge is inserted into one of said layers such that said means covers the front and back surfaces of said edge and wherein said means extend beyond said edge of the reflector.
2. A parabolic antenna as in claim 1 wherein said edge is inserted into the outermost layer of said means.
3. A parabolic antenna as in claim 1 wherein said material is of a dielectric type.
4. A parabolic antenna as in claim 1 wherein said material is of a ferrite type.
5. In a repeater station for a long range communication system including a transmitting antenna, an amplifier system and a receiving antenna, said antennas including parabolic reflectors having wave absorbing means in the vicinity of the circumferential edge of the reflectors, said means comprising a plurality of layers of absorbing material wherein said edge is inserted into one of said layers such that said means covers the front and back surfaces of said edge and, wherein said means extend beyond said edge of the reflectors.