EP0288988B1 - Adaptive antenna system for high frequencies, especially for ultra-high frequencies - Google Patents

Abstract

The antenna includes at least one group of antenna elements (EA1, EA2, . . . , EAp). Microwaves emitted by a central emitter (EH) reach a particular element (EA1 for example) by passing through a controllable phase shifter (VE1), a laser (LE1) which modulates a carrier light signal at a frequency specific to said antenna element with the microwaves, via an optical deflector (FIE) which injects said light signal into an optical waveguide (GE) which is common to all of the antenna elements in the group, via an optical deflector (FPE) which directs said light signal to a detector which is specific to said antenna element (EA1), and which reconstitutes the microwaves, and via a microwave amplifier (AE1) which applies the microwaves to said antenna element. The system is applicable to telecommunications and to radar.

Description

The present invention relates to an adaptive antenna system for radio waves, in particular of microwave frequencies.

It is known that an antenna system is said to be adaptive when, from a fixed antenna used in transmission, it is possible to modify the direction of the beam which is radiated by this antenna. If the latter is used for reception and can receive beams from various sources, only one of these beams being selected by a central organ of the system, it is the direction of the beam thus selected that an adaptive system makes it possible to modify. The antenna can of course also be mobile. It is then the direction of the beam relative to the antenna that an adaptive system makes it possible to modify. This adaptation in direction can be supplemented by an adaptation relating to the shape of the radiation diagram.

The advantage of such adaptations results in particular from the fact that electromagnetic waves, in particular microwaves, are widely used for telecommunications and that they are also used for electromagnetic detection of the position and shape of objects by systems that we call radars. In these two important classes of application, it appears useful to have antennas whose radiation can be adapted according to the evolution of the task to be accomplished over time.

For example, a telecommunications satellite must allow information to be transmitted between points in a given area of the earth. The antenna must continuously illuminate or target this area despite the translational and rotational movements of the satellite. To obtain an optimal efficiency of the telecommunication system it is necessary to make evolve the beam of the antenna so that it permanently lights the surface inside which one wants to establish the communications.

Radar will be more effective if the antenna beam can be directed in a flexible and rapid manner towards the various targeted targets, that is to say those which we want to observe more particularly.

It is therefore desirable, at least in these two types of applications, to have an adaptive antenna system. It is moreover often desirable that this system is self-adaptive, that is to say that its adaptation takes place automatically under the action of signals emitted or returned by the targeted target.

Various adaptive antenna systems are known for which it is possible to adapt the radiation pattern of a network to a given mission, by acting on the amplitude and the phase of its radiating sources (HUDSON, J. E, " Adaptive array principles "IEE Electromagnetic Waves Series ^No. 11, 1981 Peter Peregrinus Ldt). A particularly important application for space techniques is the rejection of jammers (COHEN, M, "Theoretical and experimental study of an adaptive network antenna". Doctoral thesis Engineer School Nat. Sup. Aeronautics Space n ^o 82, 1983). (COHEN, M, COMBES, PF and MAGNAN, JC, "Adaptive arrays antenna performances". Reports of the 4th Int. Conf. On Antennas and Propagation Apr. 1985. Warwick p. 241-245 IEE. Public Conf.) .

In this case, the link is characterized by the ratio (Q) signal (S) to noise (B) plus interference (I), the source of interference being assumed to be in the field of view of the antenna, that is:

There are adaptation methods which make it possible to find, for each interference configuration, a law for supplying n sources of the antenna which minimizes the degradation of the useful signal and makes the ratio Q optimal (APPLEBAUM (S), " Adaptive arrays "IEEE Trans. Ant and Prop (USA) AP.24 ^No. 5, September 1976).

These systems have the disadvantage of being relatively complex, expensive, and heavy.

The present invention aims in particular to provide an adaptive antenna system for radio waves, in particular of microwave frequencies, simpler and / or lighter and / or less expensive than known systems.

This object is achieved according to the invention by the system as defined in the main claim. With regard to preferred characteristics of implementation of this invention, reference is made to the secondary claims.

With the aid of the attached diagrammatic figures, a description will be given more particularly below, by way of nonlimiting example, how the present invention can be implemented within the framework of the description which has been given thereof. above. When the same element is represented in several figures, it is designated by the same reference sign.

FIG. 1 represents a block diagram of a composite transmission channel of a first system according to the invention.

FIG. 2 represents a block diagram of a composite reception channel of the same system.

FIG. 3 represents a block diagram of a peripheral part partially common to these two channels.

FIG. 4 represents a block diagram of a group of composite transmission channels of the same system.

FIG. 5 represents a block diagram of a group of composite reception channels of the same system.

FIG. 6 represents a block diagram of an optical part of a composite channel of a second system according to the invention with an optical phase control member, as a variant.

The present invention uses, for example in the case of the emission of microwave radiation, a new method of distributing the amplitude and the phase of the electromagnetic field over the surface of the antenna, this in order to make it possible to realize the 'self-adaptation of the radiated beam. It takes advantage of the properties of optical waveguides and semiconductor lasers, the frequency of which can be chosen by adapting the composition of the material.

In the case of a telecommunications satellite, the self-adaptive beam control system must be light despite the size of the antenna which may be large. Its reliability must be high and its price axxeptable. In all applications these characteristics are essential.

According to the invention, an optical method of distributing the amplitude and the phase of the microwave wave leads to light, efficient systems and of a cost which will often prove suitable.

• on produit un signal ou "onde" hyperfréquence dans un organe central situé dans la zone "interne" précédemment mentionnée, et on distribue cette onde sur la surface de l'antenne par l'intermédiaire de guides d'ondes optiques.

The principle used is as follows:

• a microwave signal or "wave" is produced in a central member located in the "internal" zone mentioned above, and this wave is distributed over the surface of the antenna by means of optical waveguides.

The phase and possibly the amplitude of the field at each point of the antenna are developed in this internal area either by acting directly on the microwave wave or by passing through an optical wave.

An essential novelty of the invention is to take advantage of possibilities offered by optics to distribute in a simple, light and inexpensive manner the field of the microwave wave on the surface of the antenna. The amplitude and the phase of the wave are worked out in the internal zone by methods which can be microwave or optical. The amplitude and phase of the wave are controlled by electronic methods which allow rapid self-adaptivity of the radiated beam to be obtained.

We will first calculate the number of elementary sources to be excited independently on the radiating surface as a function of the wavelength L, the diameter a of the surface and the angle A within which we must be able to choose l beam orientation. Each of these sources is constituted by one of said antenna elements. Then we will describe the structure of a composite path corresponding to an element then we will describe the complete system.

The number n of elementary sources to be excited on the radiating surface is determined as follows:
If all the elements of the antenna are excited in phase, the radiation is maximum in the direction normal to the plane of the antenna. The width 2Bo of the angle inside which the energy is radiated is given by the laws of diffraction: We have

$\text{2B₀ = L / a (1)}$

Divide the surface of the antenna into square elements of side b. Let us choose the phase of the center of these elements so that the radiation of the antenna is oriented in a direction making the angle B₁ with the normal. In order for radiation in this direction to be possible with a suitable quality of the radiation diagram, the Rayleigh condition must be respected. The wave surface produced from the elementary sources on side b must not deviate by more than L / 4 from a plane perpendicular to the direction defined by the angle B₁. We must therefore respect the condition:

$$\frac{\text{b}}{\text{2}}\text{x B₁ less than or equal to L / 4}$$

The minimum number of elementary sources will therefore correspond to:

It can therefore be seen that the radiation properties of an antenna can be characterized by two parameters:
2B₀: width of the radiated beam
2B₁: width of the angle within which the direction of the radiation can be displaced.

The ratio (B₁ / B₀) ² is given by the relation (2). It is equal to the number n of elementary sources which can be supplied independently.

Let us consider for example a radiating antenna at L = 5cm whose diameter is a = 1m. The width 2B₀ of the emitted beam is:
2B₀ = 0.05 radian or about 3 degrees.

The relation (5) makes it possible to determine the ratio B₁ / B₀ by the equality (B₁ / B₀) ² = n.
if
n = 10 2B₁ = 0.15 rad. = 10 °
n = 100 2B₁ = 0.45 rad. = 30 °
n = 10³ 2B₁ = 1.5 rad = 90 °.

In the case of this typical antenna, the beam can be moved within a range of 10 ° if n = 10 and 30 ° if n = 100. These orders of magnitude correspond to sufficiently large angles to allow important applications. We will particularly consider the cases where n = 10 and n = 100.

The control of the amplitude and the phase of an elementary source answers the following considerations:
The diagrams of FIGS. 1 and 2 represent the means of excitation of an elementary source and of reception from an elementary receiver with an amplitude and a phase that can be controlled electrically, this source and this receiver both being constituted by the same antenna element EA1. All of these means constitute the composite transmission and reception channels previously mentioned and corresponding to this element.

Regarding the transmission (see fig. 1), a microwave transmitter EH constitutes the central organ previously mentioned. In reception it is an RH receiver which constitutes this organ (see fig. 2).

The transmitted wave propagates from this microwave transmitter to the element EA1 of the antenna where it is radiated. The wave received in EA1 propagates to the RH receiver. In said peripheral zone, that is to say near the antenna, the waves transmitted and received are oriented on different paths by a non-reciprocal junction CI, called a circulator and containing for example ferrites. These paths of the waves transmitted and received are shown diagrammatically in FIG. 3.

• un émetteur de signal hyperfréquence EH modulé par le signal informatif à transmettre,
• un varactor d'émission VE1 commandant la phase de ce signal hyperfréquence et constituant le dit organe de commande de phase,
• un laser d'émission LE1 émettant une lumière modulée par ce signal hyperfréquence et constituant un dit organe de transformation interne,
• un guide d'onde optique interzonal d'émission GE,
• un détecteur d'onde optique DE1 pour restituer le signal hyperfréquence, ce détecteur consituant un dit organe de transformation périphérique
• et un amplificateur d'émission AE1 pour alimenter l'élément d'antenne EA1, le gain de chacun des amplificateurs analogues AE1, AE2...AEp étant choisi et éventuellement commandable pour réaliser une adaptation du diagramme de rayonnement.

The amplitude and phase control of an EA1 antenna element requires:
In the emission channel:

• an EH microwave signal transmitter modulated by the informative signal to be transmitted,
• an emission varactor VE1 controlling the phase of this microwave signal and constituting said phase control member,
• an emission laser LE1 emitting light modulated by this microwave signal and constituting a said internal transformation member,
• an interzonal optical emission waveguide GE,
• an optical wave detector DE1 for reproducing the microwave signal, this detector constituting a said peripheral transformation member
• and a transmission amplifier AE1 for supplying the antenna element EA1, the gain of each of the analog amplifiers AE1, AE2 ... AEp being chosen and possibly controllable to carry out an adaptation of the radiation diagram.

In addition, an internal microwave guide HIE1 from the transmitter EH to the emission laser LE1 is required, and a peripheral microwave guide HPE1 from the detector DE1 to the amplifier AE1. This amplifier is connected to the antenna element EA1 by a guide assembly HP1 comprising the members described using FIG. 3. It It should be understood that the organs mentioned above with the number 1 at the end of their reference designation constitute examples corresponding to the antenna element EA1. Each antenna element EAi corresponds to equivalent bodies whose reference designations end in the number i.

• un amplificateur de réception AR1 recevant le signal hyperfréquence capté par cet élément d'antenne, ceci par l'intermédiaire de l'ensemble de guidage HP1,
• un guide hyperfréquence périphérique de réception HPR1,
• un laser LR1 constituant un dit organe de transformation périphérique,
• un guide d'onde optique interzonal de réception GR,
• un détecteur de réception DR1 constituant un dit organe de transformation interne,
• un guide hyperfréquence interne de réception HIR1 avec un varactor VR1 constituant ledit organe de commande de phase,
• et un réception hyperfréquence RH constituant ledit organe central. Ce récepteur additionne les signaux reçus des diverses voies avec des pondérations convenables éventuellement commandables pour adapter la forme du diagramme de réception du système d'antenne.

The reception channel includes analogous bodies in the reference designations of which the letter E is replaced by the letter R. These are notably, for the antenna element EA1:

• a reception amplifier AR1 receiving the microwave signal picked up by this antenna element, this via the guide assembly HP1,
• a peripheral HPR1 reception microwave guide,
• a LR1 laser constituting a said peripheral transformation member,
• an interzonal optical reception waveguide GR,
• a reception detector DR1 constituting a said internal transformation member,
• an internal microwave reception guide HIR1 with a varactor VR1 constituting said phase control member,
• and a RH microwave reception constituting said central organ. This receiver adds the signals received from the various channels with suitable weights which can be controlled to adapt the shape of the reception diagram of the antenna system.

We will now examine the control of the amplitude and phase of n elementary sources, for example in the case of transmission.

It is conceivable to control n elementary sources by putting n transmission and reception channels in parallel. To make such a system, a number n of each of the chain components would be required: n transmitters, n varactors, n modulators, etc. In particular, 2 n optical waveguides are required.

The increase in the number of components as n increases is a drawback which should not be overlooked. It is true that all of these components, except the waveguides, can be produced by collective methods which make them reliable and inexpensive. However, it is of great interest to seek to reduce the number of components in order to decrease the cost of the system. It is particularly useful to decrease the number of inter-zonal waveguides which are relatively long and occupy a substantial space if their number is high. The system according to the invention reduces the number of certain components including that of these waveguides. The diagrams of the paths followed by the wave at transmission and at reception are shown in FIGS. 4 and 5.

The n antenna elements EA1, EA2 ... EAn are grouped into groups of p elements each, such as the elements EA1, EA2 ... EAp.

For transmission, a microwave transmitter EH is common to all the antenna elements EA1, EA2 ... EAp of the same group. It emits a microwave signal which is modulated by the informative signal to be transmitted and which is received by p varactors of transmission VE1, VE2 ... VEp. The latter apply phase shifts corresponding to these antenna elements, respectively. Each signal thus phase shifted modulates a semiconductor laser of emission LE1, LE2 ... LEp whose power can correspond to the amplitude of the field which must radiate the corresponding antenna element EA1, EA2 ... EAp. The emission frequencies of all these lasers are different and each corresponds to an antenna element.

They emit in optical waveguides GIE1, GIE2, ... GIEp, respectively, which converge on a frequency filter FIE. This filter constitutes said internal emission deflector. It transmits light from these various guides to a common guide GE which connects the central area in which the EH transmitter is located, to a peripheral antenna area where said circulating amplifiers and antennas are located. This guide is said interzonal guide.

At the outlet of this guide, the lights of the various wavelengths are directed by a peripheral FPE emission deflector, also constituted by a filter, towards various corresponding optical guides GPE1, GPE2, ... GPEp which direct them towards as many detectors DE1, DE2, ... DEp which are followed by as many microwave amplifiers AE1, AE2, ... AEp. These feed the antenna elements EA1, EA2, ... EAp.

On reception the signals received by these antenna elements are amplified into AR1, AR2, ... ARp and modulate a number p of corresponding lasers LR1, LR2, ... LRp which emit at the same frequencies as previously indicated in guides optical waves GPR1, GPR2, ... GPRp. These the latter converge on a filter constituting a peripheral FPR reception deflector which injects the corresponding lights into a common interzonal optical guide GR. A filter constituting an internal FIR reception deflector directs the lights of the various frequencies on as many guides GIR1, GIR2, ... GIRp.

The light signals are detected in detectors DR1, DR2 ... DRp, and the resulting microwave signals are phase shifted by varactors VR1, VR2 ... VRp applying the phase shifts corresponding to the antenna elements EA1, EA2 ... EAp. These phase shifts are chosen so that the signals thus phase-shifted then rediscover the mutual phase relationships that they had when they were transmitted by an external transmitter, which is far from the present antenna system and which is targeted by the latter. . These signals are received by the common microwave receiver RH. The latter restores the information carried by the signals received by the antenna elements from the targeted external transmitter.

As for the realization of lasers LE1, LE2 ... LEp, LR1, LR2 ... LRp we can notice the following:
It is known that by suitably adapting the composition of the materials which constitute the semiconductor lasers, sources are available whose frequency can be chosen in the wavelength range 0.5 - 2 micrometers. In the current state of our knowledge we can obtain about 20 frequency sources V₁, V₂ ... V _p . We can therefore choose p = 20.

Two successive frequencies are separated by a deviation dV. We will have dV / V = 0.01 approximately.

The selectivity required for FIE, FPE, FPR, FIR filters is therefore modest. They can be achieved by simple and conventional techniques using networks.

The simplification provided by the invention is substantial since it makes it possible to divide by p or more the number of microwave emitters EH, receivers RH and long wave guides. Thanks to this simplification, the system is feasible under satisfactory economic conditions in a large number of cases.

Assuming p = 20 we will evaluate the number of components of the system in cases where n = 10 and n = 100. We can for example admit that the antenna has a diameter a = 1m, the wavelength being L = 5cm. The values of B₀ and B₁ are given by the relations 1 and 1 ′.

This case where n = 10 corresponds to an excursion 2B₁ = 0.15 rad. = 10 ° in the vicinity of normal.

We need an EH transmitter, an RH receiver and two long inter-zonal optical waveguides, one for transmission and the other for reception.

10 varactors, 10 modulated lasers, 10 detectors, 10 amplifiers are needed along the transmission path.

Along the path of the received wave there must be 10 amplifiers, 10 modulated lasers, 10 detectors, 10 varactors, The case where n = 100 corresponds to an excursion 2B₁ = 0.45 rad. = 30 ° in the vicinity of normal.

To connect the EH transmitter to the antenna you need q long waveguides with q = n / p = 5.

It takes just as much to connect the receiver to the antenna.

10 long optical waveguides must therefore be used to make the system. This modest number does not entail severe constraints of cost, size and weight.

This number would be 200 if the possibilities offered by the invention were not used, which would pose sometimes insurmountable problems. Thanks to this, 5 EH transmitters are needed instead of 100. Similarly 5 RH receivers must be implemented instead of 100.

On the other hand, along the emission path, 100 varactors, 100 modulated lasers, 100 detectors, 100 amplifiers are required.

Along the path of the received wave there must be 100 amplifiers, 100 modulated lasers, 100 detectors, 100 varactors.

Thus the electrical control of the amplitude and of the phase of an antenna element EAi by the modulation and the detection of a laser wave of a frequency Vi which can be chosen from p frequencies makes it possible to make the system self-adaptive. The number of optical waveguides, transmitters and receivers is divided by p while the number of other components remains the same.

We have described above a phase shift which is carried out by a method electronics in a varactor. The microwave phase shifted thus modulates a laser LEi of frequency Vi. The amplitude of the wave radiated in EAi can be determined by the power of the laser, the phase being determined by the varactor VEi.

Alternatively, these two operations can be carried out by an optical method. This method is shown diagrammatically in FIG. 6 which relates to the case of transmission and must be compared with FIG. 1, the more or less similar elements bearing the same references with the letter A or B instead of the number 1. A LEA laser emits light at a suitable frequency (for example the frequency Vi previously considered). This light is divided and transmitted on the one hand to an electrically controlled optical phase shifter VEA which applies the appropriate phase shift to it, on the other hand to an amplitude modulator LEB which modulates it by a microwave signal itself modulated by the informative signal to issue.

The two resulting light beams are combined in a long GEA optical guide at the output of which the light signal is detected by a DEA detector. The latter restores the microwave signal applied to the LEB modulator, with the phase shift provided by the VEA phase shifter. This microwave signal can therefore be used like that provided by the detector DE1.

A similar method can be applied at reception.

If the optical modulator introduces the phase shift which has been chosen for the elementary source EAi, a phase shift of the microwave wave is obtained at the desired value.

This possibility must be appreciated when we have to solve a particular problem.

Claims (3)

1. An adaptive antenna system for radio waves, said system comprising:

   - an antenna constituted by a plurality of antenna elements (EA1, EA2, ... EAp) distributed over a surface in a "peripheral" zone of the system, each of said elements being capable of emitting and/or receiving a fraction of the wave energy propagating in the external free space at at least one common predetermined radio frequency and along aiming directions distributed in three dimensions, each of said elements coupling said wave energy to a peripheral radio signal at the same frequency propagating within the system and corresponding to said element;

   - a peripheral radio waveguide (HP1, HP2, ... HPp) also corresponding to said element for transmitting said radio signal;

   - a peripheral transformation member (DE1, DE2, ... DEp) corresponding to said antenna element and disposed on the corresponding peripheral waveguide to couple said peripheral radio signal to a light signal corresponding to said antenna element, said coupling being performed by modulating or demodulating said signal;

   - an interzone optical waveguide (GE) connecting said peripheral zone to an "internal" zone of the system for transmitting said light signals;

   - an internal transformation member (LE1, LE2, ... LEp) corresponding to said antenna element in order to couple said light signal by demodulation or modulation to an internal radio signal also corresponding to said antenna element;

   - an internal radio waveguide also corresponding to said element for transmitting said internal radio signal, said internal radio waveguide and said internal transformation member, said peripheral optical waveguide, said peripheral transformation member and said peripheral radio waveguide constituting portions of a composite line corresponding to said element; and

   - a central member (EH) for emitting and/or receiving radio signals to or from said internal radio waveguides, thereby coupling said central member to each of the antenna elements via a corresponding composite line;

   - said system further including, on each of said composite lines, at least one phase control member (VE1, VE2, ... VEp) corresponding to the same antenna element and controlling the phase of said peripheral radio signal relative to said internal radio signal to enable a particular aiming direction to be selected from a plurality thereof and to adapt the selected aiming direction of the system on command, said adaptation being due to the fact that only external wave energy propagates along said direction for which the various fractions of said wave energy passing through the various antenna elements are coupled in-phase at said central member;

   - said system being characterized in particular in that said interzone optical waveguide (GE) is common to at least one group of said antenna elements (EA1, EA2, ... EAp), with the light signals corresponding to the various antenna elements of said group being at different frequencies, and the system further including two light deflectors, namely a peripheral deflector (FPE) and an internal deflector (FIE), which deflectors deflect light through an angle depending on its frequency and are common to all of the antenna elements of the group for coupling the peripheral and internal ends of said common optical waveguide (GE) to the various peripheral and internal transformation members (DE1, DE2, ... DEp; LE1, LE2, ... LEp) respectively which correspond to the various elements of said group; and in that said antenna elements (EA1) are mixed elements capable both of emitting and of receiving said external wave energy, the system including in conjunction with each of said antenna elements:

   - a mixed peripheral radio waveguide (HPM) connected to said element;

   - an emission peripheral radio waveguide (HPE, HPE1);

   - a reception peripheral radio waveguide (HPR, HPR1); and

   - a circulator (CI) for coupling said emission waveguide to said mixed waveguide for transmitting radio signals, and for coupling said mixed waveguide to said reception waveguide for receiving radio signals, with two paths referred to as composite paths, namely an emission path and a reception path, corresponding to said element and having in common mixed peripheral radio waveguide and said circulator, with the other members (HPE1, DE1) of said two paths being distinct for transmission and reception.
2. A system according to claim 1, wherein said radiations, signals, and radio waveguides are microwave radiations, microwave signals and microwave waveguides, respectively.
3. A system according to claim 1, characterized in that said phase control member is an optical phase shifter (VEA) placed on an optical phase-shifting length of each of said composite paths, said length receiving a light signal at a frequency specific to said path, said internal transformation member (LEB) modulating or demodulating an equivalent light signal on a transformation optical length connected in parallel with said phase-shifting length.

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