WO2017013076A1

WO2017013076A1 - Improved instantaneous wide-frequency-band electronic scanning antenna

Info

Publication number: WO2017013076A1
Application number: PCT/EP2016/067075
Authority: WO
Inventors: Christian Renard; Philippe Freyssinier; Thierry Decaesteke
Original assignee: Thales
Priority date: 2015-07-21
Filing date: 2016-07-18
Publication date: 2017-01-26
Also published as: FR3039327B1; EP3326240A1; FR3039327A1

Abstract

This instantaneous wide-frequency-band electronic scanning antenna (4) comprises a plurality of radiating sources (Si) grouped into sub-arrays (Rj) and means for adjusting the delay that each of the sources needs to introduce in order to point a beam of the antenna at a required pointing angle, the adjustment means comprising a first device for adjusting the delay at each of the sources of a sub-array, and a second device for adjusting the delay at each of the sub-arrays of the antenna, each second adjustment device consisting of a second line with programmable length (21, 22, 23). Said antenna is characterised in that each first adjustment device consists of a first line with programmable length (11 to 19).

Description

IMPROVED ELECTRONIC AND BROADBAND SCANNING ANTENNA

INSTANT FREQUENCY

The present invention relates to electronic scanning and broadband antennas of instantaneous frequency.

An electronic scanning antenna is an antenna consisting of a network constituted by the juxtaposition of a plurality of elementary radiating sources. For the linear antenna shown in Figure 1, the M sources are arranged regularly along a main direction X of the antenna and are spaced apart from each other by a step d. A distance D = (M-1) d thus separates the first source and the ^same source.

In an electronic scanning antenna, the orientation of the pointing direction P of the antenna beam is effected by adjusting the delay between the elementary waves emitted or received by the different sources. Indeed, as shown schematically in FIG. 1, this has the consequence of modifying the angle of the wavefront with respect to the main direction X, and thus of modifying the angle of pointing of the beam of the antenna, c that is, the angle between the pointing direction P and a reference direction Z normal to the main direction X of the antenna.

The delay to be introduced for a given source is related to the desired pointing angle, as well as to the position of the source within the antenna. For the example of Figure 1, the maximum delay to bring between the first source and the ^same source is equal to Dsin (0), where Θ is the desired pointing angle.

An electronic scanning antenna is therefore equipped with adjustment means of the delay that must introduce each of the sources of which the antenna is constituted.

In the case of an electronic scanning antenna operating in a reduced instantaneous frequency band, for example of the order of a few tens of MHz wide, the delay is brought by a phase shift introduced into the transmission or reception signal. from a source to the crossing of a phase-shifter placed just behind this source. This phase shifter is controlled to introduce a suitable phase shift.

In Figure 1, the phase shift Ψ ₀ between two adjacent sources is ₀ = 2n (D / A ₀₎ sin9, where λ ₀ is the wavelength in air of the transmitted wave at the center frequency f ₀ of the instantaneous frequency band.

As it is a phase shift, the concept of modulo 2π applies: a phase shift Ψ and a phase shift Ψ + 2π are equivalent. It suffices to implement phase shifters generating phases in a range between 0 and 2π at the transmission / reception modules associated with each of the sources of the antenna. In the case of an electronic scanning antenna operating in an extended instantaneous frequency band, for example greater than 100 MHz wide, the use of phase shifters introduces a significant misalignment of the antenna between the different frequencies of the frequency band. . Indeed, a phase shifter introduces the constant phase Ψ ₀ for all the frequencies of the frequency band. But the angle of misalignment Θ changes according to the wavelength λ according to the relation: 9 = arcsin [ ₀ A / (2nd)].

In addition to this variation of the pointing angle with the frequency, the use of the concept of modulo 2ττ, valid for a frequency, for example the central frequency f ₀ , leads to a degradation of the lobes of the radiation pattern as a function of the frequency excursion.

For electronic scanning antennas operating in an extended instantaneous frequency band, the solution consists in introducing the delay, not in the form of a phase shift, by means of a phase-shifter, but in the form of a path length. additional, by means of a programmable length line, referred to as LLP in the following.

As illustrated in FIG. 2 for a 6-bit LLP, an N-bit LLP consists of N cascaded controlled switches, each switch for connecting an input terminal to an output terminal by selecting either a direct connection or an indirect connection by a line of length L1 for the first switch, L2 for the second switch ..., LN for the ^Nth switch.

The length L1 is called LSB (for "Least Significant Bit" or the least significant bit). The other lengths are chosen in the following way: L2 = 2L1, L3 = 2L2 = 2 ² L1 ... The length LN = 2 ^N_1 L1 is also called MSB (for "Most Significant Bit") .

Thus, the lengths that can be generated with an N-bit LLP are all lengths from 0 (when all N switches select a direct connection) to 2LN-L1 (when all N switches select an indirect connection), by LSB steps, let: 0, L1, L2, L2 + L1, L3, L3 + L1, L3 + L2, L3 + L2 + L1 ...

A line used to make an indirect connection is for example a line segment TEM (for example a coaxial cable segment) or for example still a circuit called "all-pass filter" known to those skilled in the art.

An LLP has the property of introducing a delay proportional to its length and this whatever the frequency of the signal.

For example, for the linear antenna of Figure 1, taking the origin of delays in the first source, the length L in order on the i ^th source, the position of which is (i-1) d, is : Li = (i - 1) dsin (6). This expression shows that, for an LLP, the angular pointing Θ does not depend on the frequency. LLPs therefore theoretically make it possible to meet this operating requirement of a scanning antenna over an extended instantaneous frequency band.

However, in the case of antennas with a large number of radiating sources, the LLPs are in practice not usable. Indeed, for extended antennas, it is necessary to be able to generate lengths up to a maximum length Lmax important: Lmax = D sin (Omax), where D is a characteristic dimension of the antenna and Omax the angel of misalignment maximum of the antenna. The LLPs to be used in order to adjust the delay of each source to allow the antenna to operate in an extended angular range would thus have a large bulk, in particular because of the large length of the most significant bits.

LLPs are therefore incompatible with close networking of the sources, in particular in the case of planar array antennas, in which the sources are arranged in two orthogonal directions X and Y.

In addition, LLPs introduce relatively large ohmic losses that increase with the length used. The ohmic losses introduced by the most significant bits are thus too important.

For large-scale electronic scanning antennas with an extended instantaneous frequency band, for example for SAR radars (Synthetic Aperture Radar), the solution currently implemented consists of dividing the subnetwork antenna, then to implement for the angular misalignment of the beam, on the one hand, phase shifters at each source of radiant sub-networks and, secondly, a programmable length line at the entrance. each subnet. LLPs provide network pointing stability, while phase shifters provide pointing accuracy and lobe quality of the radiation pattern.

With this arrangement, a stabilization of the beam pointing is indeed observed in comparison with the use of only phase-shifters, but a deterioration of the quality of the antenna transmission / reception diagram when one deviates from the frequency central of the instantaneous frequency band, related to the error made between the applied phase shift and that which should be applied to the frequency in question.

A second cause of degradation of the diagram is related to the use of the concept of phase modulo 2π phase-shifters, which, if valid at the central frequency, introduces an error that increases away from this central frequency. The third cause of degradation of the diagram is related to sub-network subdivision of the network antenna, which generates a certain geometric periodicity on the errors identified above.

The invention therefore aims to overcome the aforementioned problems.

For this purpose, the subject of the invention is an electronic scanning antenna with a wide instantaneous frequency band, comprising a plurality of radiant sources grouped into sub-arrays and delay adjustment means that each source must introduce to point a beam of the antenna at a required pointing angle, the adjustment means comprising, on the one hand, a first delay adjustment device at each of the sources of a sub-array and, on the other hand, on the one hand, a second delay adjustment device at each of the antenna subarrays, each second adjustment device being constituted by a second programmable length line, characterized in that each first adjustment device is consisting of a first programmable length line.

According to particular embodiments, the invention has one or more of the following characteristics, taken in isolation or in any technically possible combination:

a programmable length line at each source of each sub-network being said to rank "1" and a programmable length line at each sub-array of the antenna being said to rank "2", a first range of length introduced by a line of programmable length of rank "1" and a second range of length introduced by a row of programmable length of rank "2" overlap each other;

at least one bit of a line of programmable length of rank "1" introduces a length equal to that introduced by a bit of a row of programmable length of rank "2";

an amplifier is associated with each row of programmable length of rank "1" and / or each row of programmable length of rank "2";

the rows having a programmable length of rank "1" are identical between and / or the lines with programmable length of rank "2" are identical to each other;

the number of bits of a line of programmable length of rank "1" is less than 8, preferably 6, in particular 4; and

antenna adapted to operate on an extended instantaneous frequency band, greater than 100 MHz, preferably of the microwave domain.

The invention will be better understood with the aid of the description which follows, given solely by way of example and with reference to the appended drawings in which: FIG. 1 is a schematic representation of the operating principle of an electronic scanning antenna;

FIG. 2 is a representation of a programmable length line; and,

FIG. 3 is a schematic representation of an embodiment of an electronic scanning antenna according to the invention.

FIG. 3 diagrammatically represents an antenna 4 with electronic scanning capable of operating on an extended instantaneous frequency band.

To generate the delay required to offset the beam of the antenna, it comprises only LLP.

The antenna is subdivided into a plurality of subnets. For example in Figure 3, the linear antenna 4, which is composed of nine sources Si, is subdivided into three subnets Rj, each having three juxtaposed sources.

The LLPs are then distributed over two levels of the transmission / reception chain of the microwave signal: at a level corresponding to each sub-network Rj of the so-called LLPs of rank 2, and at a level corresponding to each source S, of a sub-network Rj of so-called tier 1 LLPs. In FIG. 3, rank 1 LLPs are referenced from 1 to 19 and rank 2 LLPs are referenced from 21 to 23.

For a source If given, the delay to be introduced is the sum of the delay introduced by the rank 2 LLP associated with the subnet Rj at which the considered Si source belongs, and by the rank 1 LLP associated with the source Si considered.

For reasons of simplification of the industrial manufacturing process of the antenna 4, the rank 1 LLPs are identical to each other and the rank 2 LLPs are identical to each other.

The strongest bits of rank 1 LLPs have a shorter length than the highest order bits of rank 2 LLPs.

Tier 1 LLPs can thus be implanted more easily inside the geometrical mesh of the network constituted by the radiating sources. Thus, each LLP of rank 1 is located in the immediate vicinity of the radiating source with which it is associated.

The ohmic losses generated by rank 1 LLPs are low. They are advantageously "masked" by the addition of microwave amplifiers.

The rank 2 LLPs, of greater size, are set back from the sources. Their size then constitutes a less critical constraint.

The ohmic losses that they generate remain important, but have smaller consequences, since away from the sources. These ohmic losses can advantageously be "masked" by the insertion of suitable microwave amplifiers. The invention thus simplifies the implementation of programmable length lines within a network antenna. The directional antenna obtained has improved characteristics compared with antennas of the prior art, especially when it is used on extended instantaneous frequency bands.

The lengths of Tier 1 and Tier 2 LLPs must be mutually adapted. Indeed, the range of lengths to be generated for the proper functioning of the antenna can not be simply distributed between a first range of lengths for the LLPs of rank 1 and a second range of lengths for the LLPs of rank 2, complementary and disjoint from the first,

It is in fact necessary to ensure overlap between these first and second ranges of lengths.

Preferably, this overlap is such that at least one least significant bit of the rank 2 LLPs corresponds to a most significant bit of the rank 1 LLPs.

The determination of the overlap between the first and second ranges and the common bit will now be presented.

In a first step, the characteristics of a sub-network are determined. For this, taking into account criteria relating to the characteristics sought for the global antenna (pointing accuracy for the antenna, granularity of the angular position of the beam, average level of the lobes ...) make it possible to determine the most significant delay. small that can be introduced into a signal. This smallest delay is identified by the length of the least significant bit, LSB1, of rank 1 LLPs.

The choice of the size and the form of the sub-networks is determined by various criteria: total number M = M1 xM2 of sources constituting the antenna (where M1 is the number of sources in a sub-network and M2 the number of sub-networks network subdividing the antenna), form of the mesh of the antenna network, implementation of the components in the mesh, maximum number of bits of the LLP of rank 1 (according to the congestion and the ohmic losses) ... This leads to a set of possible values for N1. It should be noted that N1 must remain as low as possible, for example 6 or 7 bits, so that the corresponding rank 1 LLP can be integrated near the source, and to limit the ohmic losses.

The maximum length Lmax1 to be generated between two sources belonging to the same sub-network is then determined, that is to say the maximum length that the rank 1 LLPs can introduce alone. For example, for a grating antenna which is intended to electronically scan a predefined half aperture cone Gmax, and which is divided into square-side subareas a, the maximum length to be generated is: Lmax1 = V2a sin ( 9max). The most significant bit MSBi of rank 1 LLPs is then defined to reach this maximum length L _max1 : Lmax1 = 2MSB1 - LSB1.

Knowing MSB1 and LSB1, it is then possible to determine N1 for rank 1 LLPs with: MSB1 = 2 ^N1_1 LSB1.

In a second step, the highest MSB ₂ bit of the rank 2 LLPs is defined from the maximum length Lmax that the antenna must be able to introduce. By definition, we have: Lmax = Lmaxl + Lmax2. We can neglect Lmaxl in this last relation, which is to say that the maximum length is the maximum length Lmax2 obtainable by only rank 2 LLPs.

The maximum length Lmax depends on the shape of the antenna. For example, for an antenna intended to electronically scan its beam up to an angle 6max throughout the space, and having a disk shape of diameter D, the maximum length to be generated is:

Lmax = D sin (6max).

For example again, for a rectangular antenna of dimensions AxB, the maximum length to be generated is:

Lmax = VA ² + B ² sin (6max).

From this we deduce Lmax2 and hence a constraint on the LSB2 values, the length of the least significant bit of the rank 2 LLPs, and N2 the number of bits of the rank 2 LLPs:

Lmax2 = 2MSB2 - LSB2; and,

MSB2 = 2 ^{N2 _1} LSB2.

The notion of a common bit between the rank 1 and rank 2 LLPs results in the fact that the length LSB ₂ is chosen to be a power of 2 multiplied by LSB1: LSB2 = 2 ^Q LSB1. This constraint makes it possible to ensure a continuous granularity of the delay introduced by the combination of the LLPs of rank 1 and those of rank 2.

At the end of this step, several pairs LSB2 - N2 are possible.

In a third step, the choice of LSB2 and N2 is optimized and the overlap between the first and second delay ranges introduced by the rank 1 LLPs and the rank 2 LLPs, respectively.

For a pointing angle Θ0, the range of lengths to be generated for a sub-network is calculated: LmaxO-LminO, where LmaxO, respectively LminO, is the maximum length, respectively minimum, to be generated in this subnet from a share of the rank 2 LLP associated with this subnetwork, and secondly rank 1 LLPs associated with each source of this subnetwork, to obtain the pointing angle Θ0. The LminO value is framed by two successive length values, Li and Li + 1, introduced by the rank 2 LLP associated with the subnet. The difference Li + 1 - Li corresponds to the discretization of the LLPs of rank 2, that is to say to LSB2.

A sufficient condition to ensure a good functioning of the global antenna from two levels of LLP is written:

(LmaxO - LminO) + (LminO - Li) <Lmaxl

This condition indicates that the range of lengths LmaxO - LminO to be generated in a subnetwork, which can only be obtained by the different LLPs of rank 1, increased by a reference length LminO - Li introduced by the rank 1 LLP. of the source associated with the minimum length LminO and chosen as a reference for the delays, is necessarily less than the maximum length Lmax1 obtained by the only LLPs of rank 1, Lmaxi having been dimensioned in the first step.

We choose LSB2 so that in all the lengths that a LLP of rank 2 is likely to introduce we can choose a value for Li less than or equal to LminO and to verify the inequality above, while respecting the constraints determined in the second step. In the particular case where LminO is equal to a value obtainable with the LLP of rank 2, this value is chosen for Li.

The determination of LSB ₂ can be performed more easily by maximizing LminO-Li by Li + 1 -Li, or LSB2, in the above inequality:

(LmaxO - LminO) + LSB2 <Lmaxl

This third step is iterated for different value of the pointing angle Θ0 to determine the values of LSB2 and N2 which, given the sometimes complex geometry of the antenna, allow all angles of pointing within the domain predefined angle for the antenna.

In general, this procedure results in the number of common bits between the rank 1 LLPs and the rank 2 LLPs being at least 1. Beyond two bits in common, it is preferable to reduce the number of bits of rank 1 LLPs.

The implementation of this method leads to a reduction in the sampling of the lengths generated with the rank 2 LLPs, thus leading to a recovery of the ranges accessible between the rank 1 LLPs and the rank 2 LLPs.

When using the antenna, for the generation of a length belonging to the overlap interval between the first and second ranges, the bit of the rank 2 LLP will be selected in priority. The implementation of the method is illustrated in Figure 3 where the antenna 4 is composed of M = 9 sources is subdivided into three sub-network (M2 = 3) of three sources (M1 = 3).

For a subnetwork, the smallest LSB1 length bit of a rank 1 LLP is defined for example according to the desired angular precision for the beam pointing, and the most significant bit MSBi of the LLP of rank 1 is defined so that the maximum length that can be generated, for the maximum angular score Gmax, is: Lmax1 = 2MSB1 - LSB1, with Lmax1 equal to 2dsin (0max), where d is the step separating two neighboring sources of the antenna 4.

For a rank 1 LLP, for example: L max = 70 mm, LS B 1 = 10 mm,

N1 = 3 and MSB1 = 40 mm. The lengths accessible by the LLPs of rank 1 are then: 0, 10, 20, 30, 40, 50, 60 and 70 mm.

For the global antenna of length D, the most significant bit MSB2 of a LLP of rank 2 is defined so that the maximum length that can be generated is for example: Lmax2 = 2MSB2 - LSB2 = 230 mm, where Lmax2 corresponds to Dsin (Gmax), with D the total length of the antenna. In addition, we have MSB2 = 2 ^N2_1 LSB2 and LSB2 = 2 ^Q LSB1, which forces the pair LSB2, N2.

Then, for a score Θ0 less than Gmax, we determine the range of length that must introduce a sub-network: LmaxO-LminO. This range is for example 30 mm.

With (LmaxO - LminO) + (Li + 1 - Li) <Lmaxl and LSB2 = 2 ^Q LSB1, one realizes that LSB2 can take the values 10 or 20 mm.

The solution for minimizing this overlap is to have a single common bit between the rank 1 LLPs and the rank 2 LLPs, the 20 mm length bit.

The value 20 mm is chosen for LSB2 and leads to N2 = 4.

There is thus overlap between the lengths accessible by the LLP of rank 2 and those accessible by the LLPs of rank 1: LSB2 <MSB1. Indeed, the third bit of rank 1 LLPs introduces a length of 20 mm just like the first bit of rank 2 LLPs.

Thus, a split of the LLPs into two levels is possible, but requires an adequate choice of the length bits constituting the rank 1 and rank 2 LLPs.

For the simplicity of the presentation, the example chosen is that of a linear network. It is understood that these conclusions apply to a planar or even three-dimensional network.

Those skilled in the art will understand that the distribution of LLPs in two levels as has just been explained, may be generalized to a distribution in K levels, with K an integer greater than or equal to two, for antennas with a large number of sources.

The production of the LLPs is carried out in a conventional manner. They can be realized as MMIC components, for example from elementary cells of the "all-pass" filter type, in particular for rank 1 LLPs. They can also be realized as a combination of cells of the filter type "all-pass" and physical lengths (sections of cable TEM). Finally, they can be made from only physical lengths, for example for rank 2 LLPs.

Claims

1. - Antenna (4) electronic scanning and wideband instantaneous frequency, comprising a plurality of sources (Si) radiating grouped into sub-networks (Rj) and delay adjustment means that must introduce each source to point a antenna beam at a required pointing angle, the adjustment means comprising, firstly, a first delay adjusting device at each of the sources of a sub-array and secondly , a second delay adjustment device at each of the sub-networks of the antenna, each second adjustment device being constituted by a second programmable length line (21, 22, 23), characterized in that each first adjustment device is constituted by a first programmable length line (1 1 to 19).

2. - Antenna (4) according to claim 1, wherein a programmable length line at each source (Si) of each subarray (Rj) being said to rank "1" and a programmable length line at the level of each sub-network (Rj) of the antenna (4) being said to rank "2", a first range of length introduced by a row of programmable length of rank "1" and a second range of length introduced by a row with programmable length of rank "2" overlap each other.

3. Antenna (4) according to claim 1 or claim 2, wherein at least one bit of a line of programmable length of rank "1" introduces a length equal to that introduced by a bit of a line to length. programmable "2".

4. Antenna (4) according to any one of claims 1 to 3, wherein an amplifier is associated with each row of programmable length of row "1" and / or each row of programmable length of rank "2".

5. Antenna (4) according to any one of claims 1 to 4, wherein the lines of programmable length of row "1" are identical between and / or lines of programmable length of rank "2" are identical to each other.

6. Antenna (4) according to any one of claims 1 to 5, wherein the number of bits of a line programmable length of rank "1" is less than 8, preferably 6, in particular 4.

7. Antenna (4) according to any one of claims 1 to 6, adapted to operate on an extended instantaneous frequency band, greater than 100 MHz, preferably the microwave domain.