Communications: radio wave antennas – Antennas – Antenna with parasitic reflector
Patent
1992-12-30
1994-05-03
Mintel, William
Communications: radio wave antennas
Antennas
Antenna with parasitic reflector
343895, 343914, H01Q 25000, H01Q 21240, H01Q 15160, H01Q 17000
Patent
active
053091671
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to satellite receiving equipment available in the form of individual receiving stations and more particularly to receiving antennas which can be used with a microwave frequency head and a demodulator in order to constitute such stations.
BACKGROUND
The antenna of a satellite receiving station is conventionally constituted by a parabolic reflector. This reflector is most often of circular or ovoid shape. In all cases, the reception principle remains the same: the electromagnetic waves are focused onto the reception focus. The signal is received by a "source" and then amplified by the microwave frequency head.
There are several types of parabolic antennas. The three main types of antennas are as follows: type whose microwave frequency head is supported by a tripod fixed to the outer edges of the parabola, and directly placed in the proximity of the focus of the reflector: the presence of the head in the active part of the parabola results in a masking effect and in diffraction phenomena. A waveguide (feeder) is sometimes used to convey the signal from the source to the microwave frequency head (placed at the rear in this case). at the rear of the main reflector and receives the waves reflected on a hyperbolic sub-reflector which reconcentrates the signals received by the main reflector towards the low noise amplifier (called LNA "Low Noise Amplifier"); this sub-reflector is the generator of a masking effect. having an off centered focus: the low noise amplifier and the source are offset in such a way as to reduce the masking effect.
The choice of the type of antenna principally depends on the size of the microwave frequency head used: a voluminous microwave frequency head, if it is placed at the center of the parabola, reduces its gain. Furthermore, the materials used for constructing parabolic reflectors are principally of the plastic or metallic (aluminium) type. Finally, the diameter of the parabola is a function of the merit factor, G/T, linking the gain G of the parabola and the overall noise temperature (T) desired on the station. This diameter has been able to decrease considerably in recent years, with a constant G/T, because of technological improvements in the amplifiers which result in a reduction of their noise temperature. The diameter of the parabola defines its beam width and, in addition to their discretion, the major advantage of parabolas of small diameter is the ease of aiming because of the corresponding increase in beam width. However, at the same time, the beam width determines the sensitivity of the system to interference coming from satellites adjacent to the satellite aimed at, which limits the possible reduction in diameter.
Furthermore, most manufacturers are currently developing array antennas, called flat antennas, intended either for receiving television transmissions, or for communications, mobile or fixed, for data transmission for professional use: the entire area of the antenna receives the radio signals transmitted by the satellite; an array of receiving microelements is placed in parallel and the gain is a function of the area of the antenna.
The efficiency of such flat antennas reduces considerably when the antenna area increases because of the loss generated in the summing systems.
But, the use of a flat antenna is capable of simplifying the procedures and therefore of limiting the costs of installations: a flat antenna can be installed almost vertically on a wall, or stuck to a roof. It merges with the decor (greater aesthetic qualities given it by its design): small thickness, reduced dimensions (it is configured in the form of squares of side 35 to 70 cm), lightness, discretion;
In order to be capable of receiving programmes from several satellites a flat antenna should be motorised. Array antennas with electronic aiming are being developed. They will allow the reception of transmissions from several adjacent satellites without movement. But no production for the general public is known at present as each microelement must be
REFERENCES:
patent: 2663016 (1953-12-01), de Pillot de Collieny
patent: 3184747 (1965-05-01), Kach
patent: 4513293 (1985-04-01), Stephens
patent: 4544928 (1985-10-01), Afifi et al.
Patent Abstracts of Japan vol. 6, No. 151 (E-124)(1029) Aug. 11, 1982, & JP-A-57 73506 (Nippon Denshin Denwa Kosha) May 8, 1982.
Cluniat Claude
Delmas Jean-Jacques
Brown Peter Toby
Mintel William
Telediffusion De France S.A.
Thomson-LGT Laboratoire General des Telecommunications
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