EP1805848B1 - Multiband printed helical slot antenna - Google Patents

Description

1. Field of the invention

The field of the invention is that of antennas with a large bandwidth having a good left and / or right circular polarization in the useful band. More specifically, the invention relates to helical antennas of this type intended in particular and without limitation to equip different types of positioning systems and / or terrestrial and / or satellite communications requiring the use of separate frequency bands and therefore, the use of multiband antenna to cover all of these different systems and / or communication standards.

In recent years, terrestrial or satellite communications systems have become an economic and strategic issue of considerable importance for large industrial groups. In this sense, a very important craze for satellite positioning systems has appeared, giving rise to many current and future civil applications, including, but not limited to, in fields as diverse as navigation (air, sea, land). ), geodesy, geophysics, cartography, topography, oceanography, time transfer and synchronization, meteorology, engineering, agriculture, space and recreation, for example.

More specifically, the antenna according to the invention finds particular applications in the context of mobile satellite communications between fixed users and / or mobiles of any type, for example aeronautical, maritime or terrestrial, or in the fields service broadcasting and / or satellite positioning. In these different areas, several satellite communication systems are implemented, or are currently under development (eg INMARSAT systems, ...). In addition, among the satellite positioning systems, the following three are currently considered to be the main ones to be used: GPS (for "Global Positioning System" or "Global Positioning System") which corresponds to the US satellite positioning system originally developed for the military field, the Russian GLONASS system, and the European GALILEO system which will be operational by 2008.

For all these different systems, the incidence of signals received or transmitted imposes the use of antennas having a quasi-hemispherical radiation pattern with a good circular polarization (left or right) in the bandwidth (s) used.
This constraint is all the stronger because the various positioning and / or communication systems also require the use of distinct frequency bands and therefore the use of antennas which are also multibanded and therefore adaptive to the coverage of these different systems.

2. The prior art

The printed quadrifilar helix antennas (HQI) known from the prior art have characteristics very similar to those mentioned above.
These antennas are formed of four radiating strands that can be made or not in printed technology. When made in such printed technology, the strands are printed on a thin dielectric substrate ( figure 1 ) and then wound on a radially transparent cylindrical support, as shown in FIG. figure 2 .

In such a configuration, the antenna used requires a power supply circuit which can ensure the excitation of the different printed strands by means of signals of the same amplitude and which are in phase quadrature. Such a function can be performed from 3dB-90 ° coupler structure and a hybrid ring. This assembly can be made in printed circuit and can be positioned at the base of the antennas. This results in a simple but bulky power supply, which goes against the goal of miniaturization of both satellite positioning systems and mobile communication systems. Indeed, to be able to install this type of antenna in a portable terminal, it is imperative on the one hand that it is of small dimensions (weight, volume, etc.), and on the other hand that it has a lowest cost possible.

Several approaches to reduce the dimensions of the antenna and its power system have been proposed. Examples of the solutions presented in patents include: FR-96 03698 and FR-00 11830 , on behalf of the owner of this application, and in the article of B. Desplanches, A. Sharaiha, C. Terret in the article "Parametric study of printed quadrifilar helical antennas with central dielectric rods" (Microwave and Opt.Technol Letters, Vol 20, No. 4, February 20, 1999 ).

These antennas are also of interest in the deployment of personal communications systems (PCS) by geostationary satellites.

These systems are intended to provide terrestrial users with new communications services (multimedia, telephony) via satellites. With the help of geostationary or moving satellites, they make it possible to obtain a global terrestrial coverage. They must be similar to terrestrial cellular systems in terms of cost, performance and size. Thus, the antenna located on the user's terminal is a key element from the point of view of reducing the size.

Such systems are notably described in the documents of Howard Feldman, DV Ramana: "Introduction to Inmarsat's new mobile multimedia service", Sixth International Mobile Satellite Conference, Ottawa, June 1999 , and of JV Evans: "Satellite systems for personal communications", IEEE AP Magazine, Vol. 39, No. 3, June 1997 .

For all these systems, which provide links with geostationary satellites, the very different incidences of signals received or emitted require the antennas to have a hemispherical or quasi-hemispherical coverage pattern. In addition the polarization must be circular (left or right) with a ratio of less than 5 dB in the useful band, while offering good performance on several bands that can be selected on an octave, so that the antenna according to the invention can answer indifferently to several standards of mobile communications, GSM, GPRS, UMTS for example.

More generally, the invention can find applications in all systems requiring the use of several bands and obtaining a polarization circular. We already know, by the patent FR-8914952 on behalf of the same applicant as the present application, a type of antenna particularly suitable for such applications.

This antenna, called printed quadrifilar helix antenna (HQI), has characteristics close to the stated criteria, in a frequency band generally limited to 6 or 8% for an ROS less than two. Broadband or bi-band operation can be achieved using dual-layer HQI antennas. These antennas are formed by the concentric "nesting" of two coaxial, electromagnetically coupled, quadrifilar resonant propellers. The assembly functions as two coupled resonant circuits, the coupling of which separates the resonance frequencies. A two-layer resonant quadrifilar helix antenna is thus obtained, according to the technique described in FIG. FR - 89 14952 .

This technique has the advantage of requiring a single power system, and allowing dual band and wideband operation, up to 15%.

However, it has the disadvantage of requiring the realization of two printed circuits and nested.

Broader band operation was thus made possible by means of a quadrifilar antenna with strands of variable width. A quadrifilar antenna is thus formed of four radiating strands. An exemplary embodiment is described in detail in the document " Analysis of Quadrifilar Resonant Helical Antenna for Mobile Communications "(Analysis of Resonant Quadrifilar Helix Antenna for Mobile Communications), by A. Sharaiha and C. Terret (IEE - Proceedings H, Vol 140, No. 4, August 1993 ).

According to this embodiment, the radiating strands are printed on a dielectric substrate of small thickness, and then wound on a cylindrical support that is transparent from the radio point of view. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end. A wider band operation is then obtained by varying the width of the strands along the helix regularly or not, which allows to obtain accordingly a resonant four-wire broadband resonant helix antenna of up to 14%, according to the technique described in the patent FR - 00 11843 entitled "Propeller antenna with variable width strands".

Another very broadband operation can also be achieved by means of a wideband helical antenna by folding each of the helix strands at its high end and up to the antenna ground. According to the technique described in the patent application document PCT / FR03 / 02774 of the same inventors, entitled "broadband helical antenna", a folded quadrifilar helix antenna achieves a bandwidth of 30%.

All the aforementioned techniques of the prior art, in addition to the fact that they impose the use of a power supply still relatively bulky, further allow to play on the width of the bandwidth to make it more or less wide. However, they do not authorize the antenna to treat different frequency bands allowing it to adapt to any type of positioning system and / or mobile communication. They also impose the implementation of a difficult implementation.

3. Objectives of the invention

The invention particularly aims to overcome these various disadvantages of the state of the art.

More specifically, an object of the invention is to provide a quadrilateral helical antenna with circular polarization having good performance on several bands of frequencies that can be selected on an octave and can cover the transmission and reception band of a set different communication and / or positioning systems, so as to respond to and / or adapt to several standards of communication and / or positioning systems.

In particular, an object of the invention is to provide such a helical antenna that is resonant in a wide frequency range over at least two distinct bands, with radiation patterns that remain constant.

Another object of the invention is to provide such an antenna whose dimensions, performance and cost are adapted (therefore at least similar) to the portable terminals of terrestrial cellular systems.

A further object of the invention is to provide such an antenna using a compact power supply system and not necessarily disposed at the base of the antenna.

Another object of the invention is to provide characteristics that are close to or better than those of double helix antennas (more complex to produce) with a single helix.

4. Main features of the invention

These objectives, as well as others which will appear later, are achieved according to the invention with the aid of a helical antenna according to claim 1.

An additional advantage according to the invention concerns the possibility of being able to place the slot on the conductive strands, preferably at the point where the current is maximum, which eliminates the positioning constraints of the power supply source at the base of the the antenna as imposed by the known systems of the prior art.

Preferably, the sub-strands adjacent to a slot have respectively different heights, so as to allow the resonance of the antenna in at least two distinct frequency bands.

Preferably, the helical antenna according to the invention the sub-strands adjacent to a slot are folded over a predetermined height towards the side opposite to the slot. This variant embodiment of the invention makes it possible in particular to be able to propose antennas of reduced height, in coherence with the current constraints of design and miniaturization of the mobile terminals which they must equip, and this, without stopping at the quality of transmission and / or reception in the various targeted frequency bands.

Advantageously, at least one of the helices is a quadrifilar helix comprising four conductive strands.

Advantageously, the conductive strands are printed on a flexible dielectric substrate. This technique, known per se, allows a simple and effective implementation of the invention.

Preferably, the conductive strands are spaced apart on the substrate by intervals of predetermined width.

In an advantageous embodiment of the invention, the dimensions of the conductive strands are determined so as to provide a relatively wide bandwidth spectrum, so as to be able to benefit from the multiband function in a band of width greater than 8% for a ROS less than 2.

Also preferably, the slots are placed between the high end and a position where the current is highest on each of the conductive strands. Indeed, it is important that the slot is not placed in an area of the antenna where the current flowing is virtually zero. On the contrary, placing the slot in an area where the current is maximum will promote the resonance of the antenna.

Preferably, the number of slots separating each of the conductive strands is determined according to the number of distinct frequency ranges in which the antenna must be able to operate, so as to provide a multiband operation.

The antenna thus obtained has a larger bandwidth (in one or more subbands) than the conventional antenna, with strands of constant width, hereinafter referred to as a reference antenna, without increasing the manufacturing complexity or the cost price.

5. List of figures

• Les figures 1.a et 1.b illustrent le principe d'une antenne quadrifilaire imprimée (HQI) respectivement dans une position antenne hélice développée et antenne hélice enroulée sur un support cylindrique;
• les figures 2.a et 2.b illustrent un exemple d'antenne HQI bi-bandes à fentes respectivement dans une position développée et enroulée;
• la figure 3 donne une description du courant classique d'une antenne HQI de longueur d'onde voisine de (3λ/4) ;
• la figure 4 illustre une antenne HQI tri-bandes à fentes, dans sa configuration antenne enroulée / antenne développée;
• la figure 5 présente une comparaison de l'évolution de la courbe de coefficient de réflexion en dB entre une antenne HQI classique connue de l'art antérieur et une antenne HQI bi-bandes à fentes;
• les figures 6 et 7 présentent les diagrammes de rayonnement dans le cadre d'une antenne bi-bandes à fentes, en polarisation circulaire, à de fréquences différentes ;
• la figure 8 donne une illustration pour une antenne tri-bandes à fentes de l'évolution de la courbe de coefficient de réflexion en fonction de la fréquence, comparativement à une antenne HQI conventionnelle;
• la figure 9 présente les diagrammes de rayonnement d'une antenne tri-bandes à fentes en polarisation circulaire obtenus relativement à trois fréquences de résonance de antenne;
• la figure 10 propose enfin une illustration pour une antenne quadribande-bandes à fentes de l'évolution de la courbe de réflexion en fonction de la fréquence, comparativement à une antenne HQI conventionnelle.
• La figure 11 donne un exemple d'antenne bi-bandes dont les sous-brins séparés de fentes sont repliés partiellement le long du brin principal.
• La figure 12 donne un exemple d'antenne bi-bandes dont les sous-brins séparés de fentes sont repliés sur la presque totalité de leur longueur le long du brin principal.

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment of the invention, given by way of a simple illustrative and nonlimiting example, and the appended drawings among which:

• The Figures 1.a and 1.b illustrate the principle of a printed quadrifilar antenna (HQI) respectively in a position propeller antenna developed and antenna coil wound on a cylindrical support;
• the Figures 2.a and 2.b illustrate an example of a dual-band HQI antenna with slots respectively in a developed and wound position;
• the figure 3 gives a description of the conventional current of an HQI antenna of wavelength close to (3λ / 4);
• the figure 4 illustrates a slotted tri-band HQI antenna in its coiled antenna / developed antenna configuration;
• the figure 5 presents a comparison of the evolution of the reflection coefficient curve in dB between a conventional HQI antenna known from the prior art and a slotted bi-band HQI antenna;
• the Figures 6 and 7 present the radiation patterns in the context of a two-band antenna with circular polarization slots at different frequencies;
• the figure 8 gives an illustration for a slit tri-band antenna of the evolution of the reflection coefficient versus frequency curve compared to a conventional HQI antenna;
• the figure 9 presents the radiation patterns of a tri-band antenna with circular polarization slots obtained with respect to three antenna resonant frequencies;
• the figure 10 Finally, we propose an illustration for a quad-band antenna with slits of the evolution of the reflection curve as a function of frequency, compared to a conventional HQI antenna.
• The figure 11 gives an example of a dual-band antenna whose sub-strands separated slots are partially folded along the main strand.
• The figure 12 gives an example of a two-band antenna whose sub-strands separated by slots are folded over almost their entire length along the main strand.

More specifically, Figures 1.a and 1.b have a conventional quadriliform helix antenna, as already discussed in the preamble, which comprises four strands 10 ₁ to 10 ₄ of length 12 and width d ( figure 1.a ). These radiating strands are printed on a dielectric substrate 11 of small thickness then wound on a radially transparent cylindrical support 12 ( figure 1.b ), of radius r, of circumference c and axial length 11, and description of the conventional current of an antenna HQI of length close to α (alpha) being the angle of winding or elevation of the antenna.

Conventionally, the antenna requires a power supply circuit which ensures the excitation of the different strands by signals of the same amplitude and in phase quadrature. This function can be obtained from 3dB - 90 ° coupler structures and a hybrid ring, made in a printed circuit and placed at the base of the antennas. This gives a simple but bulky power supply.

To be able to install this type of antennas in a mobile terminal, it is imperative that the antenna is of small dimensions (weight, volume, ...) and moreover, that it has the lowest possible cost, in coherence with the constraints of the corresponding markets. The bandwidth of this type of antenna is generally of the order of 6% to 8% for an ROS less than two. For example, the antenna configuration presenting on the Figures 11 and 12 minimizes the height of antennas to be developed and integrated into new generation mobile devices. Indeed, in such a geometrical configuration of the antenna, the sub-strands are folded over a predetermined height which may be more or less important. This trick offers another advantage of not impairing the quality of reception and / or transmission of the antenna in the targeted frequency bands. It is also simple and inexpensive to implement on a large scale in terms of manufacturing antennas according to the invention.

As mentioned above, the invention particularly aims a circular polarization quadrifilar helix antenna with good performance on several bands that can be selected on an octave. This antenna can in particular meet a multitude of standards in the field of positioning systems and / or mobile communications.

So and as illustrated on the Figures 2.a and 2.b , a multiband HQI antenna thus consists of 4 conductive strands (20 ₁ to 20 ₄ ) printed on a flexible dielectric substrate 21, regularly spaced, width Wa. One or more slots 22 are placed from the high end to the maximum current 23 of each strand (20 ₁ to 20 ₄ ), of fixed or variable widths. Each strand separated by the slot or slits 22 give rise to two or more sub-strands 24 of fixed or variable widths and different heights thus allowing the resonance at several frequencies. The height 23 of the slot 22 must necessarily be placed at a current belly 30 to allow multi-band operation, as shown in FIG. figure 3 . The antenna is then wrapped around a transparent support to obtain the antenna in its operating state. The figures 2 and 3 represent an example of HQI antenna with a slot 22 corresponding to a dual-band operation. More specifically, the figure 3 gives a description of the conventional current of an HQI antenna of wavelength close to (3λ / 4). For example, for a given resonance frequency, the entire antenna can radiate on a single sub-strand.

The figure 4 represents the geometry of the antenna according to the invention for a multiband operation, in this case for a tri-band operation.

It is important to emphasize that there is not necessarily a direct correspondence between the number of slots 22 cut on the printed strands and the number of desired frequency bands. Everything depends on the coupling achieved at the antenna.

The Figures 5 to 10 give examples of multi-band operation of the antenna according to the invention.

For example, Figures 5 to 9 show an example of dual-band operation of an antenna according to the invention, for an antenna designed according to a preferred embodiment and having a strand length 0.83λ ₀ , diameter 0.18λ ₀ .

So, the figure 5 presents a comparison between the evolution of the reflection coefficient measured at the entrance of one of the four conducting strands (20 ₁ to 20 ₄ , figure 2 ) as a function of frequency, the other strands being loaded at 50Ω, for a split-band antenna according to the invention (solid line 51) and for a conventional HQI antenna according to the known techniques of the prior art (dotted line 52).

In a complementary way, the figure 6 gives an illustration of the radiation pattern of the dual-band antenna of the figure 2 and 3 in circular polarization, for a frequency of F1 = 1.3GHz and the figure 7 the same illustration when the frequency is increased to F2 = 1.45GHz. We then note, for central frequencies equal to 1.3Ghz and 1.45GHz, an adaptation of the HQI antenna less than -10dB on two bandwidths (53, 54) which reach 5% each, whereas with HQI antennas according to the prior art it was possible to obtain a single adaptation of the HQI antenna less than -20 dB on a single bandwidth 55, for a center frequency of 1.3 GHz.

Thus, very advantageously, the HQI antenna according to the invention makes it possible to obtain radiation diagrams which are equivalent in the two bandwidths (53 and 54) to the diagrams obtained for a conventional antenna with a wide aperture and good Rejection of the reverse bias in the aperture. This new approach according to the invention therefore advantageously makes it possible to extend the possibilities of using such antennas and to adapt the communication systems that they equip to several types of communication standards, which goes against prejudices of the person skilled in the art.

In a variant of the aforementioned embodiment of the invention, the figure 8 presents a tri-band operation of an antenna according to the invention and having the same geometrical parameters as in the aforementioned two-band embodiment, the strands always having a length of 0.83λ ₀ and a diameter of 0.18λ ₀ . Thus, we see for the three resonance frequencies considered F1 = 1.22GHz, F2 = 1.48GHz and F3 = 1.68GHz, an adaptation of the HQI antenna less than -10dB on three bands (81, 82, 83). The radiation patterns are equivalent in bandwidths to the diagrams obtained for a conventional antenna with always a wide aperture and a good quality of polarization, as shown in the diagram. figure 9 for the frequency values F1, F2 and F3 of the figure 8 . For these resonance frequency values, there are then observed radiation patterns of the tri-band antenna with circular polarization, cross-polarization (91) and main polarization (92) polarization.

Similarly, the figure 10 gives another example of adaptation of the antenna HQI according to the invention less than -10dB on four bands (101, 102, 103 and 104) for four resonance frequencies considered F1 = 1.22GHz, F2 = 1.48GHz, F3 = 1.68GHz and F4 = 1.81GHz, while maintaining good performance in terms of polarization values at the resonant diagrams at these different frequencies.

In these different embodiments of the invention, it is important to note that the multi-band quadrupole slotted printed antenna antenna makes it possible to obtain in a wide frequency range one operating in two or more bandwidths with radiation diagrams that remain constants, which consequently allows a new adaptability of the communication systems covered by the invention to several standards, unlike the existing technical solutions of the prior art.

Of course, other variants of the invention can be envisaged, as illustrated on the Figures 11 and 12 which give examples of geometrical configuration of the sub-strands of the antenna. The variants of the antenna envisaged in the Figures 11 and 12 have as their main objective to allow a significant reduction of the height of the antenna. This reduction in height is allowed by folding the sub-strands (110) along the main strand (112), either partially as illustrated in Figure 13, or over most of its length, as shown in Figure 14. Although obviously, in these last two configurations, it is necessary to maintain a slot height (113) sufficient to maintain a good quality of reception and / or emission of the antenna.

A particular embodiment of the invention will now be described in detail. Of course, this is only a simple example of a dual band antenna, and many variations and adaptations are possible, depending on the needs and applications.

• Longueur du premier brin: 166 mm;
• Longueur du deuxième brin: 156 mm;
• Rayon de l'antenne : 18 mm;
• Angle d'enroulement ou d'élévation: 50°;
• Largeurs des brins: 16 mm;
• Profondeur de la fente: 36 mm;
• Largeur de la fente: 8 mm;
• Permittivité du substrat sur lequel sont reportés les brins: 2.2;
• Épaisseur du substrat sur lequel sont reportés les brins: 0.127 mm.

Thus, the dual-band antenna mentioned as a detailed example of implementation has the following characteristics:

• Length of the first strand: 166 mm;
• Length of the second strand: 156 mm;
• Radius of the antenna: 18 mm;
• Angle of winding or elevation: 50 °;
• Widths of the strands: 16 mm;
• Depth of the slot: 36 mm;
• Width of the slot: 8 mm;
• Permittivity of the substrate on which the strands are reported: 2.2;
• Thickness of the substrate on which the strands are reported: 0.127 mm.

The technique of the invention therefore gives a significant increase in the number of bandwidth ranges made accessible. This produces a printed quadrifilar helix antenna operating in several bandwidths. By also varying the width of the strands, it also becomes possible to increase the bandwidth of the antenna without reducing strand lengths.

Many variations of this embodiment are possible. In particular, it should be remembered that the variation of the width of the strands and / or sub-strands, and / or slots can be regular following a linear law, exponential, double exponential, stepped ... or not regular.

Moreover, it should be noted that the invention can be applied to any type of helical antenna, and not only to quadrilateral antennas.

It may also be envisaged that the strands, and / or the sub-strands, and / or the slits do not all have identical dimensions.

According to the embodiment described, the antenna is printed flat, then wound on a support to form the antenna. According to another even faster embodiment, the substrate for receiving the printed elements can be made directly in its final cylindrical form. In this case, the printing of the strands is performed directly on the cylinder and the point of connection with a current belly is determined in coherence with the number of bandwidths to be accessed by the antenna

Furthermore, it should be noted that, although it can be used individually, the antenna of the invention also lends itself to the realization of antenna arrays.

Claims (5)

1. Multiband helical antenna comprising at least one helix formed by at least two conductive strands (201, 202) with open upper ends,

   - each of said conductive strands (201, 202) being formed by a main strand (112) and at least two parallel sub-strands (24) extending the main strand from its upper end (113) at the upper end of said strand (201, 202);

   - said at least two sub-strands (24) being separated by a slot (22) extending parallel to said strand (201, 202) from its upper end and respectively having different heights, so as to enable the antenna to resonate in at least two distinct frequency bands,

   - said main strand (112) having a width (wa) equal to the sum of the widths of said sub-strands and of the width of said at least one slot,

   - said at least one slot (22) being connected at the upper end (113) of said main strand (112) at at least one point to a current antinode (30), said current antinode corresponding to a location where the electrical supply current (23) on said strand is maximum.
2. Helical antenna according to claim 1, characterised in that at least one of said conductive strands (20₁, 20₂) has at least three sub-strands separated by at least two slots having different heights and/or widths.
3. Helical antenna according to any one of claims 1 to 2, characterised in that said two sub-strands (24) adjacent to a slot (22) are folded at a predetermined height towards the side opposite the slot.
4. Helical antenna according to any one of claims 1 to 3, characterised in that at least one of said helices is a quadrifilar helix, comprising four conductive strands.
5. Helical antenna according to any one of claims 1 to 4, characterised in that said conductive strands (20₁, 20₂) are printed on a flexible dielectric substrate.

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