Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
1999-12-01
2001-01-09
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S7810CA, C343S840000
Reexamination Certificate
active
06172649
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an antenna with high scanning capacity. It relates more particularly to an antenna for use in a telecommunications system, in particular a telecommunications system using satellites.
Antennas are frequently needed, in various applications, to receive signals from a mobile source and/or to transmit signals to a mobile receiver or target. Such transmit and/or receive antennas are usually active antennas made up of stationary radiating elements in which the direction of the radiation pattern can be varied by varying the phase of the signals feeding the radiating elements.
That technique cannot produce satisfactory radiation patterns for large squint angles, i.e. for directions departing significantly from the mean transmit and/or receive direction.
A source or a receiver can also be tracked using a conventional antenna moved by a motor. That type of antenna with mechanically movable elements and a motor is not suitable for all applications. In particular, it is preferable to avoid the use of any such antenna in space applications, for reasons of reliability, overall size, and weight.
The invention remedies those drawbacks. It provides an antenna with a high scanning capacity and with a satisfactory radiation pattern at large squint angles, and which does not require any moving parts.
The antenna of the invention comprises a set of static radiating elements commanded to perform scanning and reflector means to amplify the scanning angle of the radiating elements. The reflector means include two reflectors having a common focus, the first reflector receiving the beam transmitted by the set of radiating elements and the second reflector receiving the beam reflected by the first reflector.
SUMMARY OF THE INVENTION
According to the invention, the focal length of the first reflector is greater than the focal length of the second reflector so that the exit beam of the antenna has an inclination to a predetermined direction which is greater than the inclination &THgr; relative to the given direction of the beam transmitted by the radiating elements.
The scanning angle of the radiating elements can therefore be reduced in proportion to the amplification provided by the reflector means. Thus the radiating elements are not used for squint angles that are too large. Also, the constraints imposed on radiating elements to scan a small angle are much less severe. In particular, the dimensions of the system are less restricted, which enables a sufficiently large pitch (distance between two adjacent radiating elements) to prevent array lobes without compromising the propagation of the radiation.
The reflector means are in fact analogous to those usually employed to increase the size of the beam, for example in Cassegrain antennas. However, with the invention, the reflector means are used in the opposite way to usual. In a Cassegrain antenna, an increase in the size of the beam corresponds to a reduction in the scanning angle.
In one embodiment of the invention, each reflector is a paraboloid, for example. The scanning amplification gain depends on the ratio between the focal lengths of the two reflectors.
This ratio is 4/1, for example.
The reflectors are disposed so that the output beam is not even partly masked by the first reflector, i.e. the reflector receives the beam from the radiating elements directly.
A preferred application of the invention relates to an antenna for communicating with a plurality of sources or receivers in an extended area, communication having to remain confined within the area despite the changing position of the antenna relative to the area.
This problem arises in particular in a telecommunications system using a network of satellites in low Earth orbit. A system of this kind has already been proposed for high bit rate communication between terrestrial mobile or fixed stations in a particular geographical area covering several hundred kilometers. The altitude of the satellites is in the range from 1000 km to 1500 km.
In the above system, each satellite includes groups of transmit and receive antennas and each group is dedicated to a given area. The receive antennas in each group receive signals from a station in the area and the transmit antennas retransmit the received signals to another station in the same area. The antennas of a particular group point towards the area for all the time it remains within the field of view of the satellite. Accordingly, for each satellite, a region of the Earth is divided into n areas, and as the satellite moves over a region, each area is allocated a group of transmit and receive antennas which point towards it continuously.
While the satellite is moved over a region, which takes around twenty minutes, for example, assigning a single group of transmit and receive antennas to an area avoids switching between antennas, which could compromise the speed or quality of communication.
The low altitude of the satellites also minimizes propagation times, which is beneficial for interactive communications, in particular in multimedia applications.
Clearly, with the above telecommunications system it is preferable for an antenna intended for one area not to suffer from interference due to signals from other areas and for it not to interfere with other areas. Also, the radiation pattern has a shape which varies with the position of the satellite relative to the area. When the areas on the Earth are all circular, the antenna sees the area in the form of a circle when the satellite is at the nadir of the area; in contrast, as the satellite moves away from this position the antenna sees the area in the form of an ellipse that is progressively flattened as the satellite approaches the horizon.
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R. Lenormand et al, “Large Angular Electronic Beam Steering Antenna for Space Application”, Proceedings Of The Antennas And Propagation Society International Symposium (APSIS), Chicago, Jul. 20-24, 1992, vol. 1, Jul. 20, 1992, Institute of Electrical and Electronics Engineers, pp. 2-4, XP000342297.
Croq Fr{acute over (e)}d{acute over (e)}ric
Lenormand R{acute over (e)}gis
Magnin Fr{acute over (e)}d{acute over (e)}ric
Voisin Philippe
Alcatel
Chen Shih-Chao
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wong Don
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