EP0228743A1 - Plane microwave antenna for the simultaneous reception of two polarizations - Google Patents

Abstract

Multi-element microwave flat antenna for satellite television reception with two simultaneous polarizations. The antenna is provided with two arrays of lines (1) whose ends (3, 30) constitute probes emerging in waveguides (2), the probes of a network (3) being perpendicular to those of l other network (30). Between the two networks of lines is placed only a thin sheet (150) pierced with recesses (6) in the shape of a cross, and provided with spacing pads made of a screen-printed insulating material.

Description

The present invention relates to a flat microwave antenna composed of a plurality of radiating elements (receivers or, according to the principle of reciprocity of antennas, transmitters), designed to operate simultaneously with two different polarizations of the waves, having for this purpose two networks of planar lines each arranged on a dielectric sheet of the type "lines with completely suspended substrate" enclosed between at least locally metallic or metallized devices in which recesses placed opposite one another are drilled to form open elementary waveguides or closed, the ends of the central conductors of the planar lines being arranged inside these waveguides so as to constitute probes which produce a coupling allowing the reception (or the emission) of microwave signals.

Such antennas are used in particular for the reception of television broadcasts by satellite, at a frequency of approximately 12 GHZ, in circular polarization.

A microwave plane antenna comprising a plurality of such elements has been described in the French patent application ^No. 2544920. It is described including an arrangement for maintaining transmission lines compossant or the antenna feed networks. Each of the microwave line networks consists of a printed circuit deposited on a thin sheet of dielectric serving as a substrate sandwiched between two metal plates or in metallized dielectric. Each network is arranged so that the ends of the central conductors of the lines are opposite pierced square recesses respectively in each of the plates which enclose it so as to achieve coupling between the lines and the recesses. Each dielectric sheet carrying the network of central conductors printed with microwave lines is held between the plates which enclose it by positioning studs located on the faces of these plates, facing each other and on either side of this sheet, these pads being further disposed relative to this sheet, in these spaces devoid of printed circuits.

The antenna is intended to operate in circular polarization. Two solutions are known: the first consists in the use of a coupler called "3dB coupler" whose inputs are connected to the two networks sensitive to the two orthogonal linear polarizations; the two directions of circular polarization are thus obtained simultaneously, each on an output of the coupler.
This solution has the drawback of imposing, especially for large antennas, great precision in the realization of the two networks of lines connecting the probes in the waveguides to the inputs of the coupler, because the electrical lengths must be equal, all phase shift degrading the purity of circular polarization. It will therefore only be used for small antennas. In addition, it would still impose the presence of two networks in the antenna even if one wanted to receive only one direction of polarization.

Another solution is provided by the use of a grate depolarizer placed in front of the antenna. This can be of any of the known types: wire, meandering lines, or metal blades. The two directions of circular polarization are then available simultaneously at the output of each circuit. This solution reduces the requirement for precision of line networks and therefore facilitates their manufacture. Furthermore, it would make it possible to use only one network if one wanted to receive only one direction of polarization. To obtain a cross-polar component rate weak, in the present case where two polarizations are received, the two orthogonal networks must be decoupled, or there is a parasitic coupling between the two networks due to the fact that the probes of one network are close to those of the other network. The usual method to reduce this coupling consists in moving the probes away from each other, that is to say to separate the two planes from the line networks from each other, which has the drawback of making it difficult the perfect adaptation of two probes at the same time with the same short-circuit plane behind these probes. In addition, this distance requires an additional block of waveguides between the two networks of lines, which increases the cost and the size of the antenna.

To overcome these drawbacks, the antenna according to the invention is notably remarkable in that the device located between the two networks of lines is of small thickness with respect to the wavelength and the recesses in which it is pierced have a cross shape.

Cross recesses reduce the mutual coupling between two orthogonal probes, by a greater attenuation of the higher modes than with a square recess, because the cross guides have an effect comparable to that of two rectangular guides placed orthogonally, in which the cutoff frequency of TE11 and TM11 modes is higher than in a square guide with a side equal to the long side of the rectangles. Therefore it is possible to reconcile the plans of the two networks, which reduces the cost and congestion.

Advantageously, the device located between the two networks is a thin plate pierced with cross-shaped holes and provided with studs to hold the dielectric sheets at a distance and this plate is a flat sheet on the two faces of which the studs are attached by screen printing. and made of dielectric material.

According to a variant the plate is made of two sheets planes applied one on the other, each sheet being provided on its outer face with studs arranged by screen printing and made of a dielectric material.

This structure is particularly economical since a simple punched sheet is sufficient there to constitute the waveguide device, and furthermore the dots of screen-printed dielectric material are simpler to manufacture and they improve the performance of the antenna.

The antenna according to the prior art also has the disadvantage that the plates, constituting both the main framework of the antenna and the waveguide system, must have good rigidity and high dimensional accuracy. Metal plates with such a complex structure are expensive and also very heavy. Metallized plastic plates have thermal expansion characteristics which are unsuitable for producing a large antenna which must operate as well at -40 ° as in direct summer sun.

To overcome these drawbacks, the antenna according to the invention is particularly remarkable in that the "plates are replaced there by composite devices each consisting of a thin sheet of metal pierced with recesses, on one side of which is applied at least a block constituting a plurality of waveguides, and on the other face of which the spacing pads are located, and in that the set of sheets is held by a single rigid frame.

The block constituting a plurality of waveguides is attached to the sheet and held by it, it is therefore not subject to severe mechanical precision requirements and can therefore be produced in an economical manner. The thin sheet is relatively flexible, and is held in position by the chassis, which therefore constitutes a kind of marble to keep the sheets flat. We now have only one rigid part: the chassis, which holds several sheets, location of the plurality of complex self-supporting plates of the prior art.

• La figure 1 représente en coupe une partie d'une antenne comportant deux réseaux de lignes hyperfréquence, réalisée selon l'invention.
• La figure 2 représente une vue en plan de la même partie d'antenne.
• La figure 3 représente un exemple d'un mode de fixation des constitutants de l'antenne les uns aux autres.
• La figure 4 représente, partiellement en coupe, une antenne complète.

The description which follows, with reference to the appended drawings describing nonlimiting examples will make it clear how the invention can be implemented.

• FIG. 1 represents in section a part of an antenna comprising two networks of microwave lines, produced according to the invention.
• Figure 2 shows a plan view of the same antenna portion.
• FIG. 3 represents an example of a method of fixing the constituents of the antenna to each other.
• Figure 4 shows, partially in section, a complete antenna.

Figure 1 which is a sectional view along line A of Figure 2 shows components of an antenna spaced from each other for better clarity of the figure. The antenna is composed of a network of planar lines arranged on a dielectric sheet 195 and a second similar network arranged on a dielectric sheet 196, these networks are each sandwiched between devices made of metallic or metallized material. The lines carried by the sheets 195 and 196 are not shown because their thickness on the scale is too small to make them visible. There are three devices here in metallic or metallized material; one is placed above the network 195, a second is placed between the networks 195, 196 and the third is placed under the network 196. One of these devices includes the elements references 50 and 156, another of these devices includes the elements referenced 49 and 159. The device located between the two networks of lines is of small thickness with respect to the wavelength. It consists of a thin plate 150 pierced with holes 6 and provided with studs 19, 20 to hold the sheets 195, 196 at a distance. This plate 150 is a flat sheet on the two faces of which are transferred by screen printing the pads 19, 20, made of a dielectric material. In all of these devices, recesses are formed which constitute elementary waveguides 2, into which the ends of the lines open, as will appear more clearly from the description of FIG. 2. Plots, 4, 14, are provided. also on the upper and lower devices for holding the dielectric sheets 195, 196, at a certain distance from said devices. One of these devices consists of a flat sheet 156 pierced with holes 6, on one side of which is applied a block 50 constituting a plurality of waveguides 2, and on the other side of which are located studs Another of these devices is constituted in a similar manner by the sheet 159 pierced with holes 6, the block 49, constituting wave guides, and separation pads 14.

The sheets 156, 150, 159 are made of aluminum and have a thickness of 1 mm, the blocks 49, 50 are molded for example in thermoplastic plastic material, called "ABS" and metallized, and the dielectric sheets carrying the networks of lines. are made from a 70 micron thick "mylar" sheet covered with a 35 micron copper sheet which is etched to form the lines. It is possible to use even smaller thicknesses for the dielectric sheet, with the aim of further reducing the losses; one could use for example a sheet of Kapton of 25 microns, but the latter is more expensive than the sheet of mylar. The material used to constitute the pads is advantageously charged with particles of dielectric material; these particles are for example possibly hollow beads made of glass or plastic. The screen-printed spacers 4, 14, 19, 20 have a thickness of 0.8 mm. They are produced by screen printing using a screen of adequate thickness; the screen consists of a mesh fabric wide enough to let the above balls pass, covered with one or more layers providing the desired thickness, in photo-sensitive material, and the patterns of the studs are obtained, by means of a photographic treatment, on this screen.

Figure 2 shows the same components as Figure 1, but without the upper sheet 156 to reveal the network of lines 1. The latter generally have a width of 1.8 mm. They have narrower parts at the "T" connections to adapt the impedance. The screen-printed studs 19, 29 of the sheet 157 are seen by transparency through the sheet of mylar 195.

The recesses 6 of the sheet 150 have the shape of a cross while the waveguides 2 are of square section. References 7 and 8 respectively indicating a point on the perimeter of the waveguide and a point on the perimeter of the recess show how they are placed relative to each other. The recesses of the upper and lower sheets 156, 159 are also in the form of a cross in order to produce all the sheets with the same tool. The waveguides 2 could also also be in the form of a cross, but for them this is not advantageous, because this unnecessarily complicates the manufacturing tool. This shape is really essential only for the sheet located between the two planes of the lines 195, 196.

The spacing studs 19, seen through the sheet 195 are represented by hatched zones surrounded by dotted lines. The drawing of the silk-screened areas constituting these studs represents practically in negative a drawing similar to that of the network of lines, drawing in which the latter would be widened there. By the words "negative" is meant that the material is absent where the lines are present. Such a drawing can be easily obtained by means of computer-assisted drawing equipment. With this equipment, it is possible to draw bands having the same center line as the microwave lines, but wider, and to add the network of cross recesses. Without of such equipment, it is possible to make the same drawing. It is then necessary to use a cliché of transparent lines on a black background, and to make a negative counter-type by moving the cliché in all directions during the exhibition. The amplitude of this displacement is of course equal to the desired enlargement for the lines. This gives a black drawing of the enlarged lines, which then suffices to superimpose the black drawing of the recesses. In FIG. 2, the drawing of the studs comprises a blank corridor along the block 50, to its left and at the top: this is due to the fact that there is a line, hidden under the edge of the block 50.

The reference 3 indicates the end of a line or network carried by the sheet 195, end which opens into the waveguide 2 to produce a coupling probe allowing the reception of microwave waves; the reference 30 indicates in the same way a probe of the network carried by the dielectric sheet 196. The width of the probes must be slightly increased compared to that of the lines. It is about 2.5 mm.

The interval between two rows of recesses in both directions is 23 mm.

In the figure only a block of waveguides 50 has been shown, so as to leave visible next to this block the network of lines. It is quite obvious that other blocks of waveguides analogous to block 50 must be mounted over the entire surface of the antenna; these blocks are separated from each other which makes it possible to reduce the effect of the different expansions of the plastic material of these blocks on the one hand and of the aluminum constituting the sheets on the other hand. The waveguide blocks 50 are provided with pins such as 5 in FIG. 1, which allow the blocks 50 to be fixed on the sheets. Holes 17, intended to receive these lugs, are visible in FIG. 2.

The repetitive configuration of the line network makes it easy to reconstitute the rest of the antenna which is not shown in the figure. An antenna can be constituted for example by sixteen blocks 50 each comprising sixteen waveguides 2 arranged in a rectangle of eight on two blocks. The drawing of the network of lines carried by the sheet 196 is different from that shown in FIG. 2, so that the lines emerge perpendicular to those of the sheet 195. The drawing of this network (not shown) can be easily imagined from the drawing shown. In addition, schematic examples of these two drawings are given in the patent application cited in the introduction.

It is easy to understand that the spacing pads 4 and 19 or 20 and 24 could also have been deposited by screen printing on the two faces of each sheet 195, 196 instead of being deposited on the metal sheets 156 150, 159. However the deposit on the sheets is much easier.

Figure 4 shows in detail the assembly means of the antenna. There are the sheets 156 and 159, respectively provided with spacing ranges 4 and 14 and enclosing the sheets 195 and 196. This is a variant in which the plate 150 of Figure 1 is made of two sheets planes 157, 158 applied one on the other, each sheet being provided on its outer face with pads 19, 20 respectively.

The lug 18, belonging to the upper open waveguide block is fixed in a hole in the sheets 156 and 157. The lug 5 belonging to the lower closed waveguide block is fixed in a hole in the sheets 159 and 158 The sheet 157 is applied directly against the sheet 158. The use of two sheets can be useful, since it is difficult to find free locations (without silkscreened studs) in the same position on the two faces of the central plate 157, 158, to place a hole for a lug 5, 18. Then the use of two sheets allows to place the holes in different positions on each sheet, without leading to the other side of the plate. In addition, it may be simpler to screen-print the studs 19, 20 on two separate sheets, back-to-back. to each other, only on two sides of the same plate. The entire antenna is mounted on a chassis, a small part of which is shown hatched at 22. A pin 21 is fixed in the material of this chassis and the stack constituting the antenna, provided with suitable holes, is fixed to the chassis using such pins and clips 23 forced into the pins. In the case of a single sheet 150 as in FIG. 1, that is to say if it has been possible to find locations common to the two networks of lines for the holes 17 in FIG. 2, the pins 21 could pass through these holes and fix both the sheets on the frame and the waveguide blocks 50.

FIG. 5 represents a complete antenna: the two parts in section each represent an alternative embodiment. It goes without saying that in practice these two variants would not be present together in the same antenna!

At the bottom left, the variant is the same as that described in FIGS. 1 and 4. There are the same references 22 for the chassis, 49 and 50 for the waveguide blocks. The reference 15 includes the stacking of the sheets previously referenced 150 to 159. The antenna is housed in a protective case, the rear wall 22 of which constitutes the above-mentioned chassis.

At the top right, the waveguide block is constituted by the walls 9, 10 described with reference to FIG. 3. The closed waveguides placed at the rear of the stack 15 are constituted by recesses 23 dug directly into the face of the chassis 22, which is here the rear wall of the housing, applied to the rear face of the rear sheet of the stack 15.

In front of the previously described components is placed a depolarizer 25 with wires, of a known model and a cover 24 for closing the box, cover obviously transparent to electromagnetic radiation, for example made of polyurethane.

The case is made by molding. It can be metallic, but it is more advantageous to realize in the same material as the cover 24, which economically achieves a tight assembly with the latter by gluing.

It is obvious that the application of the invention to the reception of 12 gigahertz television signals retransmitted by satellites is not limiting. On the one hand, the invention is applicable to all kinds of purely terrestrial microwave transmission networks, and on the other hand, the choice of an example of application at the frequency of 12 gigahertz is not exclusive of any other operating frequency, in the microwave range, linked to another envisaged application. The dimensions of the waveguides and their intervals should then of course be modified.

Claims (7)

1) Planar microwave antenna composed of a plurality of radiating elements, designed to operate simultaneously with two different polarizations of the waves, having for this purpose two arrays of planar lines each provided with a sheet of dielectric sandwiched between at least locally metallic devices or metallized in which recesses are drilled to form open or closed elementary waveguides into which open ends of lines, characterized in that the device located between the two networks of lines is of small thickness with respect to the wavelength and the recesses from which it is pierced have the shape of a cross. 2) Antenna according to claim 1, characterized in that the device located between the two networks is a thin plate pierced with cross-shaped holes and provided with studs to hold the dielectric sheets at a distance. 3) Antenna according to claim 2, characterized in that the plate is a flat sheet on the two faces of which the pads are reported by screen printing and made of a dielectric material. 4) Antenna according to claim 2, characterized in that the plate is made of two flat sheets applied one on the other, each sheet being provided on its outer face with pads deposited by screen printing and made of a dielectric material. 5) Antenna according to any one of claims 2 to 4, characterized in that the devices located in front and behind the two networks each consist of at least one thin sheet metal and pierced with recesses, on one side of which is applied to at least one block constituting a plurality of waveguides, and on the other face of which the spacing pads are located, and in that the set of sheets is held by a single rigid frame. 6) Antenna according to claim 6, characterized in that the waveguides formed in said blocks have a square section, although the recesses of the sheets have the shape of a cross. 7) Antenna according to any one of the preceding claims, characterized in that it is placed behind a depolarizer.

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