EP0477102A1 - Directional network with adjacent radiator elements for radio communication system and unit with such a directional network - Google Patents

Abstract

Directional network (10) for radio communications, consisting of a plurality of N adjacent radiating elements (200i) connected in series by a main line (300) and spaced one wavelength (g) apart in the said main line, characterised in that the said directional network consists of an insulating substrate (400) on one face of which are arranged adjacent radiating elements (200i) produced as thin layers, each radiating element comprising a radiating slot (210i) which, from a secondary short-circuited slot line (220i) flares linearly, in that each radiating element (200i) is insulated from an adjacent element by a short-circuited quarter-wave slot line (230i) for decoupling, and in that the said main line (300) is a coaxial cable substantially perpendicular to each secondary slot line (220i) and provided with a central core (320) and with an external conducting sheath (310), the sheath of the said coaxial cable being stripped in the region of each secondary line over a length substantially equal to the width of the said secondary line and connected to two feed points (A, B) of the said secondary line (220i). <??>Application to radio communications antennas. <IMAGE>

Description

The present invention relates to a directional network for radiocommunications, consisting of a plurality of N adjacent radiating elements, connected in series by a main line and spaced apart by a wavelength in said main line. It also relates to a set of such directive networks.

The invention finds a particularly advantageous application in the field of radiocommunication antennas in the UHF band and even in the X band, when high directivity in the network plane and low directivity in the perpendicular plane are sought. For example, if the network is placed vertically, the plane of strong directivity will be the site plan, and the perpendicular plane of weak directivity will be the azimuth plane.

ce qui donne Zc = 200 Ω avec ZT = 50 Ω , valeur caractéristique pour une ligne coaxiale. La ligne principale ne peut être dans ce cas que bifilaire, en raison de l'alimentation en série. Or ces lignes présentent davantage de pertes de puissance et surtout rayonnent un champ parasite important. C'est l'un des inconvénients de ce réseau directif connu, un autre étant lié à la difficulté de réaliser la jonction ou transition entre la ligne principale bifilaire haute impédance et les lignes secondaires coaxiales de faible impédance.A directive network for radiocommunications is known from the state of the art in accordance with the preamble, in which the adjacent radiating elements are four collinear half-wave dipoles supplied in series by a main line of impedance Zc. If ZT is the impedance seen at the input of the secondary lines connecting the main line to the dipoles, the condition for adapting impedances at the input of the network is: $$\text{Zc = 4ZT}$$

which gives Zc = 200 Ω with ZT = 50 Ω, characteristic value for a coaxial line. In this case, the main line can only be two-wire, because of the series supply. However, these lines have more power losses and above all radiate a large parasitic field. This is one of the drawbacks of this known directional network, another being linked to the difficulty of making the junction or transition between the high impedance two-wire main line and the low impedance coaxial secondary lines.

To overcome these drawbacks, the dipoles can be supplied directly two by two by dividers by two, or one by one by a single divider by four. This conventional solution has the advantage of simplicity in terms of design and can give satisfactory radio performance. However, it has a high manufacturing cost (suitable and symmetrical dipoles with interface for attachment to a reflective mast for example) and supply of components (numerous cables and connectors, power dividers).

Also, the technical problem to be solved by the object of the present invention is to propose a directive network for radiocommunications conforming to the preamble which would make it possible to obtain, in a simple and inexpensive manner, good radioelectric characteristics, free in particular of power losses. and stray radiation.

The solution to the technical problem posed consists, according to the present invention, in that said directional network consists of an insulating substrate on a first face of which are disposed, along a first direction, adjacent radiating elements produced in thin metallic layers, each radiating element comprising a radiating slit which, from a short-circuited secondary line with slit of axis perpendicular to said first direction and parallel to a second direction, called main direction of propagation, widens linearly on both sides other of said axis, in that each radiating element is isolated from an adjacent element by a line with a quarter-wave slit short-circuited for decoupling, and in that said main line is a coaxial cable substantially perpendicular to each secondary line slot and provided with a central core and an external conductive sheath, the sheath of said coaxial cable being stripped at the level of each secondary line over a length substantially equal to the width of said secondary line and connected to two points of attack of said secondary line for the first N-1 radiating elements, and the sheath and the central core of the cable coaxial being respectively connected to one and the other of said points of attack for the Nth and last radiating element.

Thus, by an appropriate dimensioning of the radiating slit and of the secondary line, it is possible to reduce the impedance ZT of each radiating element to a value close to 50 / N Ω, where N is the total number of radiating elements, which allows the use of a coaxial cable with a characteristic impedance of 50 Ω as the main line, with the advantage of low energy dissipation and virtually no interference field.

If, for example due to space constraints, the adaptation of impedances cannot be perfectly carried out, the invention provides, in order to complete the adaptation, that said coaxial cable is terminated by a quarter wave transformer. In order to reduce the ratio of said transformer, it is advantageous, in accordance with the invention, for each radiating element to comprise a capacitor constituted by a thin metallic layer deposited on a second face of the substrate, opposite to said first face. This arrangement makes it possible to group the ZT impedance of a radiating element around the value 50 / N. A similar result can be obtained when, according to the invention, two adaptation lines are arranged on either side of said radiating slot.

Because the radiating elements are spaced apart by a wavelength in the main line, the radiating elements emit, or receive, in phase. The main direction of propagation is then perpendicular to the first direction defined by the alignment of the elements along the network. It is nevertheless possible, using the directional network according to the invention, to transmit, or receive, a signal in any direction in the site map. For this purpose, a phase shift is applied to each radiating element so as to define in the plane of said first and second directions a secondary direction of propagation different from said main direction.

Finally, with the aim, for example, of obtaining an azimuth scan in the horizontal plane, provision is made for producing a set of directional networks according to the invention, characterized in that said directive networks are arranged in parallel and equidistant from each other. of the others, and in that a phase shift is applied to each main line so as to define a direction of propagation in the plane perpendicular to said assembly.

The description which follows, with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented.

Figure 1 is a side view of a directional network with adjacent radiating elements, according to the invention.

FIG. 2 is a side view of a current radiating element constituting the directional network of FIG. 1.

FIG. 3a is a front view of the radiating element of FIG. 2.

Figure 3b is a front view of the last radiating element.

FIG. 4 is an equivalent electrical diagram of the directive network of FIG. 1.

Figures 5 and 6 show diagrams taken in the horizontal plane, at the central frequency of the band, corresponding respectively to the main and cross polarizations.

Figures 7 and 8 show diagrams taken in the vertical plane, at the central frequency of the band, corresponding respectively to the main and cross polarizations.

Figure 9 is a perspective view of a set of directional networks according to the invention.

FIG. 1 shows, in side view, a directive network 10 for radiocommunications, fixed for example to a cylindrical or square mast 100 serving as a support and possibly a reflector for the network so as to conform the directivity in the horizontal plane to the intended application. This network includes a plurality of N = 4 radiating elements 200 _i (i = 1, ..., N) connected in series by a main line 300 which is either a supply line when the network is operating in transmission, or a line of collection when the network operates in reception. As shown in Figure 1, the radiating elements 200 _i are spaced by a wavelength λ g in the main line 300, also called guided wavelength. For example, with a central frequency Fo of 925 MHz and a wavelength in a vacuum λ of 320 mm, the guided wavelength λg for a cable with teflon dielectric is approximately 0.7λ or 224 mm.

Figures 1, 2 and 3 show that said directional network consists of an insulating substrate 400, made of epoxy glass for example, on a first face of which are arranged, along a first direction D₁, the radiating elements 200 _i produced in thin metal layers, using printed circuit technology. Each radiating element 200 _i comprises a radiating slot 210 _i which, from a short-circuited secondary line 220 _i with slot of axis d _2i perpendicular to the first direction D₁ and parallel to a second direction D₂, called main direction of propagation, flares linearly on both sides of said axis d _2i . In order to isolate the radiating elements from each other, each element 200 _i has at least one line 230 _i with quarter-wave slot short-circuited for decoupling.

The thin film technology used as well as the configuration chosen for the radiant slit 210 _i and the secondary line 220 _i with short-circuited slit make it possible to obtain a relatively low slit impedance Zf which makes it possible to use a semi coaxial cable. -rigid classic as secondary line 300, said coaxial cable being provided with a central core 320 and an outer conductive sheath 310. This line then has a characteristic impedance Zc of 50 Ω. This is why, to achieve a perfect adaptation, the slit impedance Zf must be equal to 50 / N = 12.5 Ω in the case of N = 4 radiating elements. If it is not possible to reach this ideal value, several means can be used to nevertheless obtain a good adaptation of impedances, in particular by varying the distance between the coaxial cable and the short-circuit of the line. secondary 220 _i , the impedance decreasing when the cable approaches said short-circuit. Provision is also made for each radiating element 220 _{i to} comprise a capacitor 240 _i constituted by a thin metallic layer disposed on a second face of the insulating substrate 400, opposite said first face, at the location of the attack points A, B of the secondary line 220 _i . This capacitor, with a few picofarads of capacitance, has an impedance Z1, in parallel with the slit impedance Zf, as indicated by the equivalent diagram in FIG. 4. We can also burn two lines, or stubs, of adaptation 251 _i and 252 _i on either side of the radiating slot 210 _i . Preferably, these two adaptation stubs have a length equal to or slightly greater than λ / 4. However, if the width of the substrate in the direction d _2i is not sufficient, the adaptation lines 251 _i and 252 _i can be folded symmetrically so as to avoid the creation of a parasitic cross field. The Z2 impedance produced by the adaptation stubs helps to adapt the ZT impedance seen at the input of the secondary lines. Finally, to definitively complete the adaptation of the network impedance, a quarter-wave transformer 500 of adequate ratio, preferably low, is placed at the end of the main line 300.

Thus, the directional network according to the invention takes on the appearance of a metallized substrate plate of very small thickness, the height of which is of the order of N λ g and the width of which is substantially greater than or equal to λ g / 4.

The Applicant has produced a directional network whose secondary line ZT impedance was 18 Ω. To reduce the impedance at the input of cable 300 to 50Ω, it was necessary to give transformer 500 an impedance Z′c of $$\text{Z′c =}\sqrt{\text{50 x 4 x 18}}\text{= 60 Ω.}$$

In practice, the transition between the coaxial cable and the slotted secondary line 220 _i is obtained, as shown in FIG. 3a, by stripping the sheath 310 of the cable at each secondary line over a length substantially equal to the width. of said secondary line and by welding, for example, said sheath in two points of attack A, B of said secondary line for the first N-1 radiating elements. For the Nth and last radiating element, FIG. 3b shows that the sheath 310 and the central core 320 are respectively connected to the points of attack A and B so as to produce a short circuit at the end of the line and thus electrically close the circuit.

Figures 5 and 6 show the diagrams noted by the Applicant in the horizontal plane at the central frequency Fo of the band for respectively main and cross polarizations. We will observe a low level of cross polarization, since it is more than 22 dB lower than the main polarization. On the other hand, the directivity of the main diagrams is low, the attenuation to ± 90 ° of the main direction of radiation being only of the order of 5 dB, which is for example very favorable to the omnidirectionality of the horizontal diagrams in a circular network association of several (2, 4 or 8) directional networks according to the invention.

L étant la longueur totale du réseau directif.Figures 7 and 8 show, similarly, the diagrams recorded at the central frequency Fo in the vertical plane D₁, D₂ containing the network, for respectively main and cross polarizations. It should be noted that the cross polarization is dilated by 10 dB compared to the corresponding main polarization. Examination of these vertical diagrams shows that the 3 dB opening of the beam is close to 17 °, which corresponds to the well known approximate formula: $${\text{ϑ}}_{\text{3db}}\text{\# 51}\frac{\text{λ}}{\text{L}}$$

L being the total length of the directional network.

où d = λ g est la distance entre deux fentes successives du réseau. Cette formule donnerait un dépointage de l'ordre de ± 3° dans une bande de 8 %. Cependant, le réseau selon l'invention n'est pas à ondes progressives mais plutôt à ondes stationnaires et l'inclinaison du front d'onde est moindre, dépendant en fait des impédances individuelles des fentes, des couplages entre les fentes et d'autres phénomènes de diffraction.A deviation of the beam from the horizon is foreseeable due to the very principle of the series connection of the radiating elements. At the central frequency Fo the depointing is zero because all the slits are in phase and the wavefront is vertical. At the frequency Fo + ΔF and for a linear traveling wave network, the inclination of the wavefront would be $$\text{α = Arcsin}\frac{\text{λ}}{\text{d}}\frac{{}^{\text{Δ}}\text{F}}{\text{Fo}}$$

where d = λ g is the distance between two successive slots in the network. This formula would give a deviation of the order of ± 3 ° in an 8% band. However, the network according to the invention is not a traveling wave but rather a standing wave and the inclination of the wavefront is less, depending in fact on the individual impedances of the slots, the couplings between the slots and others. diffraction phenomena.

où ϑ est l'angle polaire compté à partir du zénith. La pondération introduite par le diagramme individuel d'une fente, et la non-uniformité stricte de l'excitation des fentes expliquent les bas niveaux des lobes secondaires, ce qui est évidemment très favorable à une bonne concentration de l'énergie rayonnée dans le faisceau.The side lobes, called secondary, have a level more than 15 dB below the maximum of the main lobe, and, in an 8% band, the level of the side lobes is still more than 12 dB below said maximum. In simplified theory, this level would be 11.5 dB since the normalized network factor here is: $$\text{F4 (ϑ) =}\frac{\text{1}}{\text{4}}\frac{\text{sin [4}\frac{\text{πd}}{\text{λ}}\text{cos ϑ]}}{\text{sin [}\frac{\text{πd}}{\text{λ}}\text{cos ϑ]}}$$

where ϑ is the polar angle counted from the zenith. The weighting introduced by the individual slit diagram, and the strict non-uniformity of the excitation of the slits explain the low levels of the secondary lobes, which is obviously very favorable for a good concentration of the radiated energy in the beam. .

Finally, the level of cross polarization in the vertical plane is extremely low, thanks to the specific design of the network according to the invention.

With radiating elements in phase, the main direction of propagation D₂ is perpendicular to the direction D₁ of the network. To obtain any direction of propagation in the plane D₁, D₂ (vertical plane), it is necessary to apply a phase shift to each successive radiating element, which offers the possibility of electronic scanning of the beam.

Figure 9 shows a set of P directional networks 10 _j with j varying from 1 to P arranged in a parallel and equidistant from each other. In order to define in an horizontal plane P, perpendicular to said set, an azimuthal propagation direction, a phase shift is applied to each main line 300 _j . An azimuth scan is obtained by electronically varying this phase shift.

The isotropic gain of a directional array according to the invention was measured by comparison with a standard antenna. The gain value is very close to 10 dBi. This is explained simply by the fact that four aligned radiating elements, each having approximately 2 dBi of gain, and forming a linear network arranged at a quarter wave distance in front of a reflective mast providing an additional gain close to 3dBi, provide a gain 11 dBi. If one takes into account the technological losses and losses by reflection at the entrance of the network and, on the other hand, that the reflective mast is not infinite, one justifies the measured value.

Claims (6)

Directive network (10) for radiocommunications, consisting of a plurality of N adjacent radiating elements (200 _i ), connected in series by a main line (300) and spaced apart by a wavelength (λ g) in said main line, characterized in that said directional network consists of an insulating substrate (400) on a first face of which are disposed, along a first direction (D₁), adjacent radiating elements (200 _i ) made of thin metallic layers, each radiating element comprising a radiating slot (210 _i ) which, from a short line (220 _i ) short-circuited with an axis slot (d _2i ) perpendicular to said first direction (D₁) and parallel to a second direction ( D₂), said main direction of propagation, flares linearly on either side of said axis (d _2i ), in that each radiating element (200) is isolated from an adjacent element by a line with a quarter slit wave (230 _i ) courtyard t-decoupling circuit, and in that said main line (100) is a coaxial cable substantially perpendicular to each secondary line (220 _i ) with a slot and provided with a central core (320) and an external conductive sheath ( 310), the sheath of said coaxial cable being stripped at each secondary line over a length substantially equal to the width of said secondary line and connected to two points of attack (A, B) of said secondary line (220 _i ) for the first N-1 radiating elements, and the sheath (310) and the central core (320) of the coaxial cable being respectively connected to one and the other of said points of attack for the Nth and last radiating element. Directive network for radiocommunications according to Claim 1, characterized in that the said coaxial cable (300) is terminated by a quarter-wave transformer (500), at the location of the attack points (A, B) of the secondary line (220 _i ). Directive network for radiocommunications according to one of claims 1 or 2, characterized in that each radiating element (200 _i ) comprises a capacitor (240 _i ) constituted by a thin metallic layer deposited on a second face of the substrate, opposite said first face. Directive network for radiocommunications according to one any one of claims 1 to 3, characterized in that two adaptation lines (251 _i , 252 _i ) are arranged on either side of said radiating slot (210 _i ). Directive network for radiocommunications according to any one of Claims 1 to 4, characterized in that a phase shift is applied to each radiating element (200 _i ) so as to define in the plane of said first (D₁) and second (D₂) directions , a secondary direction of propagation different from said main direction (D₂). Set of P directional networks according to any one of Claims 1 to 5, characterized in that said directive networks (10 _d ) are arranged in parallel and equidistant from each other, and in that a phase shift is applied to each main line (300 _j ) so as to define a direction of propagation in a plane perpendicular (P) to said assembly.

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