The present invention relates to a microwave plane antenna formed from a number of radiating elements (receivers or, according to the principle of reciprocity of antennas, transmitters), provided in order to operate simultaneously with two different polarizations of waves, having for this purpose two systems of planar lines each placed on one sheet of dielectric of the "completely suspended substrate lines" type enclosed between devices that are at least locally metallic or metallized in which cut-outs placed opposite each other are bored in order to form elementary open or closed waveguides, the ends of the central conductors of the planar lines being placed inside these waveguides in order to form probes which produce a coupling enabling the reception (or transmission) of microwave signals.
Such antennas are used in particular for receiving satellite television transmissions at a frequency of about 12 GHz and circularly polarized.
A microwave plane antenna including an assembly of such elements has been described in the French patent application No. 2544920 corresponding to U.S. Pat. No. 4,614,947. In it there is in particular described an arrangement enabling the transmission lines forming the antenna feed system or systems to be supported. Each of the systems of microwave lines is formed by a printed circuit deposited on a thin sheet of dielectric serving as a substrate enclosed between two metallic plates or between two metallized dielectric plates. Each system is placed in such a way that the ends of the central conductors of the lines are facing square cut-outs bored in each of the plates which enclose it respectively in order to produce the coupling between the lines and the cut-outs. Each sheet of dielectric carrying the system of printed central conductors of the microwave lines is supported between the plates which enclose it by positioning studs located on the surfaces of these plates, facing each other and on either side of this sheet, these studs alos being placed, with respect to this sheet, in spaces having no printed circuits.
The antenna is intended to operate in circular polarization. Two solutions are known: the first consists in the use of a coupler known as a "3 dB coupler" whose inputs are connected to the two systems sensitive to the two orthogonal linear polarizations; both directions of circular polarization are thus obtained simultaneously, each on one output of the coupler. This solution has the disadvantage, particularly for large antennas, of imposing a high precision of production on the two systems of lines connecting the probes in the waveguides to the inputs of the coupler, as the electrical lengths must be equal, any phase shift degrading the purity of circular polarization. It will therefore only be used for small antennas. Also, it would still impose the presence of two systems in the same antenna even if only one single direction of polarization was required to be received.
Another solution is provided by the use of a grid depolarizer placed in front of the antenna. This can be one of several known types: formed from wires, from meandering lines or from metallic strips. Both directions of circular polarization are then simultaneously available at the output of each circuit. This solution reduces the accuracy requirement on the systems of lines and therefore facilitates their manufacture. Also it would enable the use of only a single system if only one single direction of polarization was required to be received. In order to obtain a weak contrapolar component ratio, in the present case in which two polarizations are received, the two orthogonal systems must be decoupled; there does however exist a parasitic coupling between the two systems due to the fact that the probes of one system are close to those of the other system. The usual method for reducing this coupling consists in moving the probes away from each other, i.e. moving the two planes of the systems of lines away from each other, which has the disadvantage of making it difficult to perfectly match both probes at the same time with the same short-circuit plane behind these probes. Also, this separation requires an additional set of waveguides between the two systems of lines, which increases the cost and dimensions of the antenna.