EP0427654B1 - Tuned helical antennae consisting of two quadrifilar antennas fit into each other - Google Patents

Description

The present invention relates to a new antenna structure, having a quasi-hemispherical radiation pattern, and capable of having a relatively wide passband, so for example that two neighboring transmission subbands can be released there.

This type of antenna finds, for example, application in the context of satellite communications between fixed users and aeronautical, maritime or land mobiles. In this area, several satellite communication systems have been or are under development in L-band (INMARSAT, MSAT, PROSAT, NAVSTAR G.P.S, ...).

The first three systems mentioned correspond to links with geostationary satellites. The specifications of the antennas intended to equip the mobiles, in these systems, require that these antennas have a radiation diagram with quasi-hemispherical coverage, due to the very different incidences, or significant variations in the incidence of the signals received or transmitted.

In addition, the polarization of the antennas must be circular with an ellipticity ratio better than 5 dB. (insulation 20 dB.) and special attention must be paid to the fight against multi-path phenomena for land and aeronautical mobiles. This latter specification also requires that for low elevations, the preponderant component of the electric field be vertical.

As regards the antennas usable for the reception of signals by traveling satellites used in systems of the type of the American system NAVSTAR, the specifications impose that they are operational in a bandwidth of approximately 10%, or in two sub - neighboring bands.

In the current state of knowledge, the only antenna structure compatible with a type of specification (essentially a quasi-hemispherical radiation diagram and circular polarization) is the resonant quadrifilar helix.

This type of known antenna, as shown in FIGS. 11A, 11B, is formed by two two-wire helices 111, 112, arranged orthogonally and excited in phase quadrature.

The exemplary embodiment represented in FIGS. 11A, 11B is cited in the work "UHF satellite array nulls adjacent signals" Microwaves & R.F., March 1984.

The antenna is a resonant quadrifilar helix with strands 111A, 111B; 112A, 112B short-circuited at their non-excited end 113. The passband is of the order of 10% with an opening at -3 dB of 140 ° for a strand length equal to λ₀ / 2 and a helical winding on a turn around. This type of antenna is not to be confused with certain helical antennas of the type disclosed for example in patent document US-4148030 (FOLDES) and which have the purpose of providing very directional axial radiation diagrams (and not almost hemispherical as in the invention) and with great gain. Their operation is of the traveling wave type, and not in resonant mode. These known antennas moreover have a different structure. In particular, they have a length of several times the wavelength λ of the antenna operating. In addition, each helical strand is cut into resonant dipoles to operate at a specific frequency.

Another embodiment of a quadrifilar helical antenna is also known, used in the mobile satellite communications system INMARSAT STANDARD-C where the antenna must operate correctly in two sub-bands (1530-1545 MHz) and (1631.5 -1646.5 MHz) corresponding respectively to reception and emission (KM KEEN "Developing a standard-C Antenna" (Development of an antenna in standard C) MSN Communication Technology, June 1988).

In this known embodiment, the antenna is a resonant quadrifilar helix with printed strands open at their unexcited end.

Although meeting the required specifications, resonant quadrifilar antennas have a number of drawbacks.

The main problems posed by this known type of structure are due to the constraints of adapting the impedances of the antenna with those of the feed coaxials, while achieving adequate excitation of the two-wire helices. orthogonal.

In narrowband systems, the power supply / adaptation module can be placed outside the antenna, around the working frequency. But when the antenna must operate in broadband, as discussed here, a power supply / internal adaptation module is generally used in the antenna structure. The most common is the so-called "balun" system (sometimes also called a balun) or its "folded balun" variant with asymmetrical input and symmetrical output.

Such an arrangement is shown in FIG. 11, where, given the excitation and the symmetry of the antenna, the two orthogonal helices 111 and 112 have the same input impedance. Each two-wire propeller 111A, 111B; 112A, 112B is supplied by a coaxial balun of the "folded balun" type. The two two-wire cables are then excited in phase quadrature using a 115 hybrid coupler (90 °, -3 dB.). Each coaxial (asymmetrical) input therefore sees in parallel the impedance of the two-wire helix and an adapter of length close to λ / 4.

The balun / adapter assembly used in this type of antenna is produced for example by means of a coaxial section of length λ / 4, the core and the sheath of which form a dipole; to avoid the problems due to the radiation of the sheath, the dipole can be closed between the core and an additional coaxial sheath (bazooka system) so as to avoid the circulation of a current on the sheath of the coaxial.

However, this type of arrangement has the drawback of forming a sort of bandpass filter with a band which is still too narrow.

We then imagined more complex systems in which a compensated line is used by means of a solid conductor, or else a dead coaxial cable, forming a plug circuit (see CC KILGUS "Resonant quadrifilar helix" Microwave quadrifilar helix) Journal, December 1970).

In all cases, an adaptation device must be added between the hybrid coupler and the "baluns" to adapt the antenna. This is clear in particular from the abacus of SMITH in FIG. 12, where we clearly see for two embodiments, that the operating windows 121, 122 are essentially outside the adaptation zone 123.

However, the use of adaptation devices introduces losses and often limits the band of use of the antenna. In addition, in these exemplary embodiments, and no doubt for reasons of space, the "folded balun" is placed in the body of the excited antenna at its upper end. This then produces a diffraction disturbance of the radiation patterns, particularly at high frequencies.

The invention aims to overcome these drawbacks.

More specifically, the invention provides a new antenna structure having a quasi-hemispherical radiation pattern, and circular polarization, in particular (but not exclusively) in the L band.

Another objective of the invention is to provide such a structure avoiding the introduction of complex adaptation devices between the antenna and its excitation.

The invention also aims to provide an antenna having a widening of the bandwidth, or a dual band operation (dual frequency), in particular either in a bandwidth ≈ 10%, or in two neighboring sub-bands.

A complementary objective of the invention is to provide an antenna of low cost, and with energy consumption compatible with the constraints of on-board systems on land, sea, air or space mobiles.

These objectives, as well as others which will appear subsequently, are achieved according to the invention using a resonant helical antenna with quasi-hemispherical radiation, of the type comprising a quadrifilar helix made up of two bifilar helices arranged orthogonally and excited. in phase quadrature, this antenna comprising at least a second quadrifilar helix, coaxial and in electromagnetic coupling with said first quadrifilar helix, each of said quadrifilar helices being wound on a separate cylinder, of constant radius.

The superposition of these two resonant quadrifilar propellers allows to obtain a quasi-hemispherical radiation pattern over a wide frequency band, or over two neighboring frequency bands, depending on the settings chosen for their electromagnetic coupling.

Advantageously, the length of the strands is less than the wavelength λ of operation of said antenna, and preferably between λ / 2 and λ, so as to obtain the desired hemispherical radiation pattern, with operation in standing waves.

According to a preferred characteristic of the invention, the strands of said second quadrifilar helix are in a situation of exact or close radial superposition with the strands of said first quadrifilar helix.

According to another characteristic of the invention, said coupled quadrifilar propellers are connected in parallel to a common power supply. Advantageously, said common supply comprises, on the one hand, a coupler element, for the excitation in phase quadrature of the two orthogonal bifilar helices of each quadrifilar helix, and on the other hand a balancing element for the supply in phase opposition of each of the strands of the two-wire propellers.

Preferably, the strands of at least one of the two quadrifilar propellers are open or short-circuited at their non-excited end.

Advantageously, at least one of the quadrifilar propellers is produced in technology printed on a dielectric support.

• contrôle de l'écart radial de superposition desdites hélices quadrifilaires;
• contrôle du décalage angulaire entre lesdites hélices quadrifilaires ;
• contrôle du pas d'hélice de chacune desdites hélices, de façon notamment à adapter l'impédance présentée par chaque brin.

According to an advantageous characteristic of the invention, the coupling of said quadrifilar propellers is controlled using at least one of the following means:

• control of the radial overlap of said quadrifilar propellers;
• control of the angular offset between said quadrifilar propellers;
• control of the helix pitch of each of said helices, so as in particular to adapt the impedance presented by each strand.

According to a first embodiment, said coupling of said quadrifilar helices is carried out so as to obtain radiation from the antenna in a single wide bandwidth.

According to a second embodiment, said coupling of said propellers quadrifilaires is carried out so as to obtain radiation from the antenna in at least two disjoint bandwidths.

It is clear that thanks to the invention the coupling control can be optimized, without degrading any of the other characteristics of the antenna, and in particular the circular polarization and the radiation pattern.

• la figure 1 est une vue en perspective d'un mode de réalisation avantageux d'une structure d'antenne à double hélice quadrifilaire selon l'invention;
• la figure 2 est une vue en développé d'une des deux hélices quadrifilaires superposées, réalisée sous forme de rubans de cuivre imprimés sur substrat de kapton ;
• la figure 3 est une vue en plan de la base des cylindres supports de l'antenne des figures 1 et 2, portant des segments conducteurs de connexion des brins rayonnants ;
• la figure 4 schématise une structure d'alimentation classique pour l'antenne des figures 1 à 3 ;
• les figures 5, 6, 7 représentent respectivement le diagramme de SMITH, la valeur du ROS et le diagramme de rayonnement en polarisation circulaire copolaire et contrapolaire d'un prototype de l'invention dimensionné pour fonctionner en double bande (antenne bifréquence) ;
• les figures 8, 9, 10 représentent respectivement le diagramme de SMITH, la valeur du ROS et le diagramme de rayonnement en polarisation circulaire copolaire et contrapolaire d'un prototype de l'invention dimensionné pour fonctionner à large bande.
• les figures 11A, 11B et 12 illustrent respectivement une vue de face et de dessus, et l'abaque de SMITH de la courbe d'impédance d'une hélice quadrifilaire monocouche de type connu.

Other characteristics and advantages of the invention will appear on reading the description of a preferred embodiment of the invention, given by way of illustration and not limitation, and of the appended drawings in which:

• FIG. 1 is a perspective view of an advantageous embodiment of an antenna structure with a quadrifilar double helix according to the invention;
• FIG. 2 is a developed view of one of the two superimposed quadrifilar helices, produced in the form of copper ribbons printed on a kapton substrate;
• Figure 3 is a plan view of the base of the antenna support cylinders of Figures 1 and 2, carrying conductive segments for connection of the radiating strands;
• Figure 4 shows schematically a conventional feed structure for the antenna of Figures 1 to 3;
• FIGS. 5, 6, 7 respectively represent the SMITH diagram, the value of the ROS and the radiation diagram in co-polar and cross-polar polarization of a prototype of the invention dimensioned to operate in double band (dual-frequency antenna);
• FIGS. 8, 9, 10 respectively represent the SMITH diagram, the value of the ROS and the radiation diagram in co-polar and cross-polar polarization of a prototype of the invention dimensioned to operate at broadband.
• FIGS. 11A, 11B and 12 respectively illustrate a front and top view, and the SMITH chart of the impedance curve of a single-layer quadrifilar propeller of known type.

A preferred embodiment of the antenna structure of the invention is shown in Figure 1. It is formed by two quadrifilar propellers 11 and 12 concentric, wound on cylindrical insulating supports 13 and 14, coaxial, of diameters d₁, d₂ distinct. It is clear that the structure of the antenna of the invention can be generalized obviously to more than two concentric helices.

Each quadrifilar helix 11 and 12 has four strands 11₁, 11₂, 11₃, 11₄ and 12₁, 12₂, 12₃, 12₄ respectively, regularly spaced and wound on the cylindrical supports 13, 14.

Each strand 11₁, 11₂, 11₃, 11₄; 12₁, 12₂, 12₃, 12₄ is formed by a continuous ribbon of electrically conductive material such as copper, of width W, printed on a kapton substrate, as shown in FIG. 2. The kapton substrate can have a thickness of 50 »M, for a width W of copper tape of 35» m.

The length of each strand is advantageously between λ / 2 and λ and in all cases less than or equal to λ, to operate in resonant mode and obtain a quasi-hemispherical radiation diagram.

In the case where the are strands of length slightly greater than a radial radiation diagram is obtained, and no longer quasi-hemispherical. This type of operation can however be of interest in certain particular applications.

The four strands of each helix 11,12 are open at one end 15 (upper end in FIGS. 1 and 2) and electrically connected to the other end 16 (lower end in FIGS. 1 and 2) with conductive segments 31, 32, 33, 34, arranged on the base 30 of the lower part 16 of the support cylinders 13, 14 as shown diagrammatically in FIG. 3. These flat segments 31, 32, 33, 34 are advantageously made up of ribbons printed on kapton, in the form of portions of segments of decreasing width from the edge to near the center of the base 30 of the cylinders 13, 14. Each of these conductive segments is connected to the central core of one of the four 50 Ω coaxial cables supplying the antenna structure. The two quadrifilar propellers 11,12 are thus fed in parallel, strand by strand (11₁, 12₁; 11₂, 12₂; 11₃, 12₃; 11₄, 12₄).

The four strands of each propeller 11,12 are excited across the segments 31, 32, 33, 34 according to the supply configuration shown diagrammatically in FIG. 4, using a conventional device consisting of a hybrid coupler module 41 (3 dB, 90 °) and two symmetrical modules 42,43 (3 dB, 180 °). One of the inputs 41₁, 42₁, 43₁, of each of these modules 41,42,43 is connected to ground through a 50 Ω resistor 44. The coupler module 41 is arranged so that the two outputs 41₃, 41₄ supply the other input 42₂, 43₂ of the two balancing modules 42,43. The outputs at 180 ° 42₃, 43₄ of the baluns are connected so as to supply two segments 31.34, the outputs at 0 ° 42₄ and 43₃ exciting the other two segments 33.34. In this way, one obtains an excitation in phase quadrature of the two bifilar helices 31,33 and 32,34 of each quadrifilar helix 11,12, and an excitation in phase opposition of each of the strands 31 and 33 on the one hand, 32 and 34 on the other hand, of each two-wire propeller.

This set can be made compactly in printed technology and placed directly at the base of the antenna structure.

Given the value close to 50 Ω of the input impedance of each of the strands of the double helical quadrifilar structure, no additional impedance adjustment is necessary.

Of course, other supply configurations can be envisaged, as well as other technical means of implementation, as will be apparent to those skilled in the art. Thus, in another embodiment of the excitation of the antenna structure, not shown, it is possible not to supply one of the two quadrifilar propellers, which then functions as a parasitic element with respect to the second.

The coupling between the two quadrifilar propellers can be checked in several ways. It is in particular possible to act on the radial difference between the two helices, on the angular offset of the antennas around the axis of revolution of the antenna, with respect to an exact position of radial overlap strand by strand, or still on the propeller pitch of each of the propellers.

The electromagnetic coupling of each antenna strand adapted in impedance, for example to 50 Ω, is of course controlled, so as not to degrade, or as little as possible, the other characteristics of the antenna, and in particular the circular polarization and the radiation pattern.

We will now present the results obtained with two prototypes of implementation of the antenna structure of the invention, corresponding respectively to a double-band configuration (fig. 5, 6, 7) with wide band (fig. 8, 9, 10).

Dual-frequency antenna (or double band) .

In the first embodiment calculated and tested, the antenna parameters are presented in table I, (with C: circumference; Le: length of a radiating strand; Lax: axial length; with reference to the notations in FIG. 2 )

A series of measurement readings was taken on each propeller taken separately, then in simultaneous supply in parallel. In what follows, the impedance presented is the impedance calculated at the input of a radiating strand of the propeller in the presence of the others, this impedance being half that of a two-wire propeller.

In the case of the measurements of the quadrifilar antennas taken in isolation, a bandwidth was noted for an R.O.S <2 equal to 60 Mhz (indoor antenna) and 50 Mhz (outdoor antenna).

The parallel supply of the two helices leads to the impedance curve of the SMITH abacus of FIG. 5 where the curve shown for F1 = 1.480 to Ff = 1.730 has two frequency bands 51.52 disjoint in the domain d adaptation of the antenna. It is also possible, using an impedance transformer, to center the impedance curve on the chart. A suitable dimensioning of the antenna parameter also makes it possible to obtain a coincidence of the portions 51 and 52. The curve marks a double resonance due to the coupling between the two quadrifilaries. As it appears on the diagram ROS of fig. 6, the assembly operates as two coupled resonant circuits whose coupling deviates the resonant frequencies 61,62. The ROS is less than 1.5 in two separate frequency bands: 1.54 Ghz <f <1.5666 Ghz and 1.602 Ghz <f <1.64 Ghz.

In addition, the antenna being practically adapted to 50 Ω around the two resonant frequencies, the excitation device does not require any specific mounting of additional adaptation, which overcomes the antenna of the drawbacks of the simple quadrifilar antenna.

FIG. 7 represents the radiation diagram of the coupled antenna, which differs little from the radiation diagrams of the quadrifilar propellers taken in isolation.

This implementation can of course be generalized to more than two concentric quadrifilar propellers, so as to obtain as many distinct frequency bands as there are propellers.

Broadband antenna .

By modifying the parameters of the antennas and the distance between the layers, the electromagnetic coupling between the two superimposed quadrifilar helices makes it possible to obtain a single passband wider than with a monolayer propeller of the same parameters.

Such a configuration is obtained for example by choosing the values of the parameters from Table II.

For these parameter values, the initial bandwidth is 65 Mhz for a ROS <2.5 for the indoor antenna, and 56 Mhz for a ROS < 2 for the outdoor antenna.

In coupled operation, the bandwidth for the two-layer antenna is equal to 86 MHz for an R.O.S <2. The diagram of the R.O.S. and the SMITH chart of the corresponding impedance curve are shown in Figures 8 and 9.

The R.O.S is less than 1.75 on a continuous frequency band from 1.535 to 1.595 GHz approximately, with a resonance frequency of 1.59 Ghz. The impedance curve in fig. 9 extends for F1 = 1.5 Ghz to Ff = 1.63 Ghz practically entirely in the adaptation zone of the abacus (with the possibility of more exact centering on the abacus as for the previous embodiment).

In general, the structure of the antenna of the invention thus makes it possible to "reduce" the imaginary part of the impedance and to bring its real part around 50 Ω.

There are no significant modifications to the radiation diagrams, FIG. 10 representing the diagram for the coupled two-layer antenna.

Because of these characteristics, and the possibility of a dual-frequency and broadband embodiment, the antenna structure of the invention finds numerous fields of application.

Thus, it applies to satellite communication systems under development in L-band, for example those used by the "International Maritime Satellite Organization (INMARSAT)" in the field of maritime communications on a global scale.

We can also cite, in the United States, the "Mobile Satellite System (MSAT)" which pursues the development of its own communication service for land vehicles; similarly, different concepts have been proposed for communications and aeronautical traffic control. (see J. HUANG and D. BELL "L-Band satellite communication antennas for U.S coast boats, land vehicles and aircraft" IEEE, AP-S INT.SYMP. Digest 1987 (AP 22-1)).

In Europe, the ESA PROSAT program (European Space Agency) provides, for data transmission (PRODAT), the development low G / T terminals (-24 dB / K) for air navigation (elevation between 10 ° and 90 °), maritime navigation (elevation between -25 ° and 90 ° to take into account ± 30 ° movements of the ship due to roll and pitch), and terrestrial (elevation between 15 ° and 90 °) in which the antenna structure of the invention finds an advantageous application.

Claims (10)

1. Resonant helical antenna with quasi-hemispherical radiation, of the type comprising a quadrifilar helix (11) consisting of two bifilar helices (11₁, 11₂; 11₃, 11₄) arranged orthogonally and excited in phase quadrature, characterized in that it includes at least a second, coaxial excited quadrifilar helix (12) electromagnetically coupled with the said first quadrifilar helix (11), each of the said quadrifilar helices (11, 12) being wound on a cylinder of distinct constant radius.
2. Antenna according to Claim 1, characterized in that the length of the wires (11₁, 11₂, 11₃, 11₄, 12₁, 12₂, 12₃, 12₄) is less than the wavelength λ of operation of the said antenna, and preferably between λ/2 and λ.
3. Antenna according to either one of Claims 1 and 2, characterized in that the wires (12₁, 12₂, 12₃, 12₄) of the said second quadrifilar helix (12) radially overlap the wires (11₁, 11₂, 11₃, 11₄) of the said first quadrifilar helix (11).
4. Antenna according to any one of Claims 1 to 3, characterized in that the said coupled quadrifilar helices (11, 12) are connected in parallel to a common feed.
5. Antenna according to Claim 4, characterized in that the said common feed includes, on the one hand, a coupler element (41), for the phase quadrature excitation of the two orthogonal bifilar helices of each quadrifilar helix (11, 12) and, on the other hand, a balancer element (42, 43) for the feeding in phase opposition of each of the wires of the bifilar helices.
6. Antenna according to any one of Claims 1 to 5, characterized in that the wires of at least one of the two quadrifilar helices are open or short-circuited at their non-excited end.
7. Antenna according to any one of Claims 1 to 6, characterized in that at least one of the quadrifilar helices (11, 12) is made by the technology of printing on a dielectric support.
8. Antenna according to any one of Claims 1 to 7, characterized in that coupling of the said quadrifilar helices (11, 12) is controlled with the aid of at least one of the following means:

   - control of the radial separation of overlap of the said quadrifilar helices (11, 12);

   - control of the angular offset between the said quadrifilar helices (11, 12);

   - control of the helical pitch of each of the said quadrifilar helices (11, 12).
9. Antenna according to Claim 8, characterized in that the said coupling of the said quadrifilar helices (11, 12) is effected so as to obtain radiation of the antenna in a single wide passband.
10. Antenna according to Claim 8, characterized in that the said coupling of the said quadrifilar helices (11, 12) is effected so as to obtain radiation of the antenna in at least two disjoint passbands.

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