EP0542595B1 - Microstrip antenna device especially for satellite telephone transmissions - Google Patents

Abstract

An antenna comprises a first dielectric layer (D1) including on one side an earth-plane (PM1), and on the other a first conductive slab (P1) of chosen shape. A second dielectric layer (D2) surmounts the first on the first slab side, and supports, on the other side, facing the first slab, a second conductive slab (P2) of chosen shape. A third dielectric layer (DR) surmounts the second. The second slab (P2) is of smaller size than that of the first slab (P1), and this first slab (P1) is fed from the bottom, at at least one chosen point (FR1) situated between its centre and its periphery. Advantageously, the first slab (P1) is connected to a run-through (TR1) of the first-plane joining up with a feed circuit (DL1, DL2) implanted in the dielectric substrate of three-plate structure (SDH, SDB). <IMAGE>

Description

The invention relates to microstrip antenna devices or "microstrip".

Many antenna structures have already been described in this field. The simplest microstrip radiating structure includes a dielectric layer, carrying on one side a block conductor of the chosen shape, and on the other a conductive plane that we call ground plan. To obtain an antenna, you must define the energy supply mode of this structure microwave.

The idea of providing a stack of overlapping pavers has been described in the article by LONG & WALTON, "A dual-frequency stacked circular disk antenna ", IEEE Transactions on Antenna and Propagation, Vol. AF 27, N ° 2, March 1979. Other proposals have been formulated since.

Regarding the feeding of two-block antennas superimposed, we must distinguish two very different cases from the point from a functional point of view, depending on whether the power is supplied to the level of the upper or lower block (closest to the ground plan).

In the case where feeding is carried out at the level of the pavement most often it is a connection to the periphery of this pavement. In addition, we systematically plan an upper block larger than that of the lower block (see in particular the article by TULINTSEFF, ALI, and KONG, "Input impedance of a probe-fed stacked circular microstrip antenna ", IEEE Transactions on Antennas and Propagation, Vol. 39, No. 3, March 1991).

Those skilled in the art know that the development of cobblestone antennas superimposed is particularly tricky. Attempts have been made to model the properties. We will quote as example the article by COCK and CHRISTODOULOU, "Design of a two-layer, capacitively coupled, microstrip patch antenna element for broad band applications ", IEEE Symposium on antenna propagation, 1987. Despite these attempts, it remains extremely difficult to predict by modeling and understand the behavior of microstrip structures with two or more overlapping pavers.

The Applicant notably posed the problem of carrying out a antenna conforming to electronic scanning, intended for the communication with mobiles such as aircraft (system called SATCOM).

This system is intended to work with the group of satellites geostationaries managed by the INMARSAT organization. In what concerns at least aircraft applications, the proposed telecommunications is governed by an international standard known as ARINC 741.

Technically, it is a question of developing an antenna capable to operate on the one hand in transmission, on the other hand in reception, in two very close bands, a little over 1.5 gigahertz for reception, and just over 1.6 gigahertz for transmission.

The electronic scanning function is necessary for this antenna, due to the movement of the mobile carrier, which is assumed here be an aircraft. There is also a choice between roof antenna, or two side antennas. In the case of two side antennas, the aforementioned ARINC standard defined two acceptable official fingerprints, delimiting the volume in which must register the planned antenna.

The antenna must also be compliant, i.e. susceptible to adapt to the exact wall shape of the carrier mobile. She should still be thin, to minimize streaking aerodynamic, and of course designed to respect the mechanical characteristics required for the structure of the aircraft.

During the research it carried out, the Applicant noted that it was possible to design a microstrip antenna going practically the opposite of the solutions accepted so far by those skilled in the art.

The present invention therefore proposes an antenna element basically different from those known so far.

This antenna element is of the type comprising a first layer dielectric comprising on one side a ground plane, and on the other a first conductive pad of selected shape, a second layer dielectric, which overcomes the first layer, on the side of the first block, and supports on the other side, opposite the first block, a second conductive pad of selected shape, a third layer dielectric overcoming the second, as well as means microwave power supply from one of the conductive blocks.

According to the invention, the second block is smaller than that of the first block, and only the first block is physically connected to the microwave power supply means, the connection feed from below, at least one point chosen from the first block, located between its center and its periphery.

With this structure, it appeared possible to build a operational antenna, subject to choosing the position of the point in question, depending on the respective sizes of first and second blocks, dielectric characteristics of first and second dielectric layers, as well as those of the third dielectric layer, which preferably has dielectric constants significantly higher than those of two others.

According to another aspect of the invention, the first block is connected at a crossing of the ground plane joining a supply circuit implanted in a dielectric substrate of typical structure triplate. More particularly, the three-ply structure includes a substrate layer located between the ground plane already mentioned and a low ground plane; between the two ground plans are provided conductive bushings defining a peripheral shielding of the feed part of the antenna element. Preferably, we provides a Wilkinson divider to power the pavement lower in two points forming with its center a triangle substantially isosceles rectangle, while the respective signals brought to these two points are in quadrature. The divisor of Wilkinson is implanted at an intermediate level of the layer substrate, in accordance with the three-ply structure. This level intermediate serves in practice as a level of distribution of power between a central connector for all of the antenna, and the different antenna elements that go constitute it, in the application as a network antenna.

In an advantageous embodiment, the two blocks are of general circular shape, and these two blocks are substantially coaxial, that is, they are located on the same perpendicular to the planes of the dielectric layers.

• la figure 1 est un schéma de principe général d'un élément d'antenne, en perspective éclatée ;
• la figure 2 est une vue en coupe partielle fragmentée d'un élément d'antenne ;
• la figure 3 est une vue partielle détaillée (superposée) du branchement du pavé inférieur à son alimentation par diviseur de Wilkinson ;
• la figure 4 est une vue de dessous des 24 diviseurs de Wilkinson, pour une antenne à 24 éléments, interconnectés à un connecteur central ;
• la figure 5 est une vue de dessus de 24 pavés inférieurs, correspondant précisément à la figure 4 ; et
• la figure 6 est un diagramme représentant le coefficient de réflexion de l'antenne en fonction de la fréquence.

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the attached drawings, in which:

• Figure 1 is a general block diagram of an antenna element, in exploded perspective;
• Figure 2 is a fragmentary partial sectional view of an antenna element;
• Figure 3 is a detailed partial view (superimposed) of the connection of the lower block to its supply by Wilkinson divider;
• Figure 4 is a bottom view of the 24 Wilkinson dividers, for a 24-element antenna, interconnected to a central connector;
• Figure 5 is a top view of 24 lower blocks, corresponding precisely to Figure 4; and
• FIG. 6 is a diagram representing the reflection coefficient of the antenna as a function of the frequency.

Those skilled in the art know that form is important in devices microstrips. Furthermore, the drawings are, for the essential, of a certain character. They can therefore be incorporated into the description, not only to do better understand this, but also to contribute to the definition of the invention, if applicable.

In Figures 1 and 2, the reference PM0 designates a ground plane lower, which can be assembled with an insulating adhesive on a sheet to be incorporated into the wall of the aircraft. This ground plan lower is surmounted by two dielectric layers SDB and SDH (respectively low and high). The SDH layer is in turn surmounted by another PM1 ground plane. The whole forms a triplate structure, with appropriate metallizations engraved between the SDB and SDH layers, or more exactly on one of these layers.

Basically, these metallizations include a line supply L, which is then subdivided like a Wilkinson divider, shown schematically in Figure 1, but better visible in Figures 3 and 4. This divider includes two DL1 and DL2 branches which first move away from each other, to join at a level where they are connected to a resistance RLL implanted in the thickness of the SDB layer, but without join the lower ground plane PM0. Then the two branches DL1 and DL2 move apart again to join points respective supply EL1 and EL2.

These points EL1 and EL2 are connected by crossings TR1 and TR2 (not connected to the ground plane PM1) to supply points FR1 and FR2 provided on the lower block P1, or pilot block, engraved on the upper face of a dielectric layer D1 placed above of the ground plane PM1.

As can be seen in Figures 3 and 4, the end parts of the DL1 and DL2 engravings are of different lengths, so that, electromagnetically, the signals available at the points FR1 and FR2 are substantially in quadrature, one with the other. Correlatively, the power points FR1 and FR2 of block P1 are substantially located at right angles to one of the other.

These two points are located at distances d1 and d2 from the center of block P1 which are in principle equal. We will come back later on the choice of these distances. But it is possible to indicate immediately that these distances d1 and d2 are in principle between 50% and 100% of the radius of block P1 (noted DP1 / 2 on Figure 3).

A second dielectric layer is provided above block P1 D2, with the same dielectric constant as layer D1, but thicker, as shown in Figure 2. Partly layer D2 receives a second block by etching conductor P2 (coupled block), which is in principle circular and coaxial with block P1, but has a diameter smaller than that of block P1.

The antenna element is completed with a dielectric layer additional DR, forming a radome, and in principle having a dielectric constant significantly higher than that of the layers D1 and D2.

Figures 2 and 4, it further appears that the line L is continues until passing through a metallic hole, towards a general CCH microwave connector, coaxial type, located behind the metal sheet underlying the lower ground plane PM0.

Furthermore, comparing Figures 2 and 4, it appears that this connector is provided for each pad with peripheral shielding horseshoe-shaped, crossing the entire layer SDB dielectric. This shielding could be defined by a layer conductive continues. The Applicant has found that it is sufficient to provide a certain number of through studs, surrounding the location of the CCH crossing, with spacing between these studs which remains sufficiently less than the length of the microwave signals processed.

Similarly, the peripheral pads such as BP11, BP12 and BP13 define an antenna element power shield considered, compared to neighboring antenna elements, and by report to the outside.

On the other hand, it will be noted that above the ground plane PM1, it is not provided no isolation of the antenna element from its neighbors.

Figure 5 shows how you can arrange 24 elements antenna to form an electronic scanning antenna compliant, satisfying the conditions of the problem posed. As already indicated, these antenna elements are connected to a connector general, 24 pins (at least). Upstream of this connector, it a treatment is provided for each antenna element individual reciprocal phase shift, using DPH phase shifters, as shown in Figure 2.

• la hauteur et la constante diélectrique des trois couches DR, D2 et D1;
• les diamètres des pavés P1 et P2; et
• le rayon d = d1 = d2 des deux points d'alimentation du pavé inférieur P1.

The main parameters involved in such an antenna are:

• the height and the dielectric constant of the three layers DR, D2 and D1;
• the diameters of the blocks P1 and P2; and
• the radius d = d1 = d2 of the two supply points of the lower block P1.

a) un comportement bi-fréquences comportant une très bonne adaptation (meilleure que -20 décibels), sur deux fréquences F1 et F2;

b) un comportement large bande, assurant au moins une adaptation de -10 décibels entre des fréquences F3 et F4 contenant l'intervalle de fréquence F1 et F2.

The problem posed, in the particular application targeted, is to obtain a dual behavior from the unitary antenna element (Figure 6):

a) dual-frequency behavior with very good adaptation (better than -20 decibels), on two frequencies F1 and F2;

b) broadband behavior, ensuring at least an adaptation of -10 decibels between frequencies F3 and F4 containing the frequency interval F1 and F2.

The Applicant has observed that, provided that the frequencies F1 and F2 are not too far from each other, and as soon as the parameters of heights and dielectric constants are fixed of the above three layers, there is practically only one solution, in terms of the radii of the two blocks and the radius power supply for block P1, which makes it possible to satisfy the conditions which have just been recalled.

Any modification of one of the parameters causes it to become very difficult to find a situation likely to satisfy said conditions.

Although the phenomena in question are not yet completely understood, it seems that, in general, everything is happening as if only one of the two blocks P1 and P2 resonates at the frequency of work. On the other hand, there is a very small area, in the antenna definition parameters, for which the two blocks interact, while exhibiting typically dual-frequency behavior, as wished. And we still have to look for the point optimal of this dual-frequency behavior, to respond to desired operating conditions for the antenna, such as recalled above.

In particular, it turned out that it is practically very difficult to operate the antenna element without add a top layer of DR radome.

• épaisseur de la couche DR : 1,5 à 2,5 mm;
• constante diélectrique relative de la couche DR : de 4 à 5 ;
• épaisseur de la couche D2 : environ 4,8 mm;
• épaisseur de la couche D1 : environ 1,6 mm;
• constantes diélectriques relatives des couches D1 et D2 ainsi que SDB et SDH : environ 2 ;
• diamètre du pavé P1 : environ 70 mm;
• diamètre du pavé P2 : environ 60 mm;
• rayon des points d'alimentation FR1 et FR2 : entre 0,5 et 0,7 fois le rayon du pavé P1.

The Applicant has thus been able to produce antennas meeting the following parameters:

• thickness of the DR layer: 1.5 to 2.5 mm;
• relative dielectric constant of the DR layer: from 4 to 5;
• thickness of layer D2: approximately 4.8 mm;
• thickness of layer D1: approximately 1.6 mm;
• relative dielectric constants of layers D1 and D2 as well as SDB and SDH: approximately 2;
• diameter of paver P1: approximately 70 mm;
• diameter of the block P2: approximately 60 mm;
• radius of supply points FR1 and FR2: between 0.5 and 0.7 times the radius of block P1.

• coefficient de réflexion meilleur que -20 dB à la fréquence centrale de réception (1,545 GHz) ;
• coefficient de réflexion meilleur que -20 dB à la fréquence centrale d'émission (1,645 GHz) ;
• comportement passe-bande à mieux que -10 dB entre 1,53 et 1,66 GHz.

Such antennas can satisfy the conditions laid down for the SATCOM working band, namely:

• reflection coefficient better than -20 dB at the center reception frequency (1.545 GHz);
• reflection coefficient better than -20 dB at the central emission frequency (1.645 GHz);
• bandpass behavior at better than -10 dB between 1.53 and 1.66 GHz.

We will now focus on the constitution of the network of elements antennas as illustrated in Figures 4 and 5.

First, it was indicated above that each lower block is fed at two points located on two rays substantially perpendicular to each other.

It appeared interesting to distribute the two properly feeding points, and this in a different way for the 24 antenna elements shown. The Applicant has found that this reduces the antenna ellipticity rate, account given that it operates in circular polarization, and with electronic scanning. To this end, it is possible either to distribute the feeding points substantially at random, i.e. to experimentally seek an optimal configuration of the point of view of this ellipticity rate (for example as on the figure 5).

The electronic scanning array antenna thus obtained proven capable of operating for deflection angles up to 60 °, with secondary lobe levels sufficiently low, and a gain of at least 12 decibels compared to to an isotropic antenna.

A good compromise between loss of gain and lobe level secondary has been obtained by applying a law of illumination weakly weighted in amplitude. It may be a Taylor law, circular type 20 decibels, these indications being understandable for the skilled person.

The phase shifters associated with each of the antenna elements can be integrated into the beam orientation unit (or BSU for "Beam Steering Unit"), housed inside the aircraft.

Advantageously, phase shifters with lines switched by PIN diodes, controlled by 4-bit binary words, this which provides a resolution of 22.5 °.

The distributor, integrated into the phase shift block, ensures the weighting in amplitude according to the aforementioned law.

In the particular application targeted, the antenna must operate simultaneously in transmission and reception, at frequencies relatively close. Regarding the calibration of electronic scanning phase shifters, there is reason to set phase or "phase" the network, on a band of about 8%.

Rather than calculating the phase law at the mid frequency of the tape, the Applicant has found that it is preferable to consider the use of two frequency bands separate, as well as the quantification and nature of phase shifters (switched lines). For this purpose, it uses the calibration process described below.

Let be an element Ai of a conforming antenna, therefore not planar, with coordinates (in the center) Xi, Yi, Zi. When we want to spot the main beam in the direction U, V at the frequency f, it is necessary to apply to this antenna element Ai a theoretical phase shift DPi, which is a function of f, U and V that knows those skilled in the art: DPi (f, U, V)

In practice, we use a calibration table TC (n, F) where n is an integer (or other discrete variable) representing the required state of the phase shifter, with 0 <= n <= N, while we also limit ourselves at discrete values of the frequency F. This is written: DQi (F, n)

In the example in question, we take 101 frequency points in the band 1.53 - 1.66 GHz; and N = 15, with n defined on 4 bits. This process only "phases" the network correctly for one frequency. However, the antenna has essentially dual-frequency behavior.

The Applicant then established a "distance" between the theoretical phase and the tabulated phase, for the two frequencies f1 and f2, in particular of the form: DDi = ¦ DQi (F1, n) - DPi (f1, U, V) ¦ + ¦ DQi (F2, n) - DPi (f2, U, V) ¦ where ¦ denotes the absolute value.

The calibration then consists in seeking a priori, for each direction of sight and each antenna element, the value of n which minimizes this function DDi.
The phase shifters are controlled accordingly. Of course, this calibration can be stored.

The present invention is not necessarily limited to the mode of the described embodiment, or to the intended application. The element antenna can itself be used for other applications, provided that we keep the new structure. Is also at consider the combination of a microstrip element and a tri-plate power supply, in the same dielectric stack.

Polarization can be other than circular polarization of the described embodiment.

Another feature of the invention is that it can avoid, for layers D1 and D2, the use of dielectrics of weak constant, or porous, even made up of a gas.

Claims (13)

1. Antenna device, of the type comprising a first dielectric layer (D1) comprising on one side an earth plane (PM1), and on the other a first conducting pad (P1) of chosen shape, a second dielectric layer (D2), which surmounts the first layer, on the same side as the first pad, and supports on the other side, opposite the first pad, a second conducting pad (P2) of chosen shape, a third dielectric layer (DR) surmounting the second, as well as microwave-frequency supply means for one of the conducting pads, characterized, in combination, in that the second pad (P2) is of smaller size than that of the first pad (P1), and in that only the said first pad (P1) is physically connected to the said microwave-frequency supply means, the said supply connection being made through the bottom, at at least one chosen point (FR1) of the said first pad (P1), situated between its centre and its periphery.
2. Device according to Claim 1, characterized in that the first pad (P1) is linked to a duct (TR1) of the earth plane joining up with a supply circuit (DL1, DL2) implanted in a dielectric substrate of three-plate structure (SDH, SDB).
3. Device according to Claim 2, characterized in that the two pads (P1, P2) are of circular general shape.
4. Device according to Claim 3, characterized in that the two pads (P1, P2) are substantially coaxial.
5. Device according to one of Claims 2 to 4, characterized in that the dielectric materials of the first and second layers (D1, D2) and of the substrate (SDH, SDB) have a dielectric constant of the order of 2, and in that the ratio of the thicknesses of the second (D2) and first (D1) layers is of the order of 3.
6. Device according to Claim 5, characterized in that the dielectric material of the third layer (DR) has a dielectric constant of the order of 4.
7. Device according to one of Claims 2 to 6, characterized in that the three-plate structure comprises a substrate layer (SDH, SDB) implanted between the earth plane (PM1) and a bottom earth plane (PMB), with conducting ducts (BP11-BP13) defining a peripheral shield, and a Wilkinson divider (DL1, DL2, RLL), implanted at an intermediate level of the substrate layer, and able to supply the lower pad at two points (FR1, FR2) forming with its centre a substantially right-angled isosceles triangle.
8. Device according to Claim 7, characterized in that the dielectric material of the substrate layer (SDH, SDB) possesses substantially the same dielectric constant as those of the first and second layers (D1, D2).
9. Device according to one of the preceding claims, characterized in that it comprises an array of first (P1) and second (P2) pads respectively implanted on the same first and second dielectric layers.
10. Device according to Claim 9, characterized in that it is associated with controlled phase-shifters (DPH) which impart an electronic scanning function thereto.
11. Device according to Claim 10, taken in combination with one of Claims 7 and 8, characterized in that the pairs of supply points (FR1, FR2) of the lower pads are distributed according to a predetermined configuration so as to improve the ellipticity factor of the antenna at large pointing deviations.
12. Device according to Claim 11, characterized in that the predetermined configuration is of substantially random type or obtained experimentally.
13. Device according to one of Claims 10 to 12, characterized in that the phase-shifters (DPH) are calibrated on the basis of a function of distance between theoretical values and actual values, for both of the two central frequencies of the antenna.

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