WO2006045769A1

WO2006045769A1 - Multiband printed helical slot antenna

Info

Publication number: WO2006045769A1
Application number: PCT/EP2005/055489
Authority: WO
Inventors: Ala Sharaiha; Yoann Letestu
Original assignee: Universite De Rennes 1
Priority date: 2004-10-25
Filing date: 2005-10-24
Publication date: 2006-05-04
Also published as: EP1805848A1; FR2877148A1; EP1805848B1; FR2877148B1

Abstract

The invention relates to a helical antenna comprising at least one helix which is formed by at least two radiating conductive strands (201,202), wherein at least one of said conductive strands (201,202) extends into at least two sub-strands (24) which are separated by a slot (22) with a pre-determined height (23) and width. According to the invention, the aforementioned slot (22) is connected at at least one point to a current antinode (30) such as to authorise the multiband operation of the antenna.

Description

Printed multi-band slotted propeller antenna. 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 the main ones to be used: GPS (for "Global Positioning System" or "Global Positioning System") which corresponds to the position American satellite originally developed for the military field, the Russian system GLONASS, and the European GALILEO system which will be operational in 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 (FIG. 1) and then wound on a radially transparent cylindrical support, as illustrated in FIG. configuration, the antenna used requires a power circuit that can ensure the excitation of 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 cumbersome 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. By way of example, mention may be made of the solutions presented in patents FR-96 03698 and FR-OO 11830, in the name of the holder of the present application, and in the article by B. Desplanches, A. Sharaiha, C. Terret in the article "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 Howard Feldman's papers, DV Ramana: "An Introduction to Inmarsat's New Mobile Multimedia Service," Sixth International Mobile Satellite Conference, Ottawa, Junel999, and 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. It is already known from patent FR-89 14952 in the name 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 by using 2-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 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", by A. Sharaiha and C. Terret (IEE - Proceedings H, 140, 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 wide-band variable width resonant quadrifilar helix antenna, up to 14%, according to the technique described in the patent FR-00 11843 entitled "variable-width helix antenna".

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 patent application 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 using a helical antenna comprising or less a helix formed of at least two radiating conductive strands, at least one of which one of the conductive strands advantageously extends into at least two sub-strands separated by a slot of predetermined height and width, said slot being connected in at least one point to a current belly. Of course, different geometries of strands and / or sub-strands and / or slots may be envisaged for the invention in terms of the constraints related to the dimensioning of the antenna and / or related to the sizing in frequency bands in reception and / or or in transmission required by the antenna to be implanted. Various examples of geometry of strands, sub-strands and / or antenna slots are presented through FIGS. 2 and 11 to 14.

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.

Advantageously, each of the slots is of variable width, depending on the width of the frequency range to be made accessible to the antenna in reception and / or transmission. Of course, the slit widths can also be fixed in a predetermined manner as needed. Also advantageously, in a variant of the invention, the helical antenna comprises at least two slots having different widths.

Preferably, the sub-strands are of variable width. According to an advantageous embodiment of the invention, the width or the width of the variable width varies monotonically between a maximum width and a minimum width. The widths of the strands can also be fixed and determined according to the needs.

According to a particular embodiment of the invention, the widths of said one or more strands and / or of said one or more slots of variable width vary regularly.

According to another more specific embodiment of the invention concerning a variable geometry antenna (according to the examples of FIGS. 11 to 14, the width of the slots and / or conductive sub-strands may follow a law belonging to the group comprising: linear laws, exponential laws, exponential double laws, step laws.

According to yet another approach, it can be provided that the widths of said one or more strands and / or of said one or more slits of variable width vary non-uniformly.

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 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

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

Figures La and Lb illustrate the principle of a printed quadrifilar antenna (HQI) respectively in a developed helical antenna position and helical antenna wound on a cylindrical support; Figures 2.a and 2.b illustrate an example of HQI bi-band antenna slotted respectively in a developed position and wound; FIG. 3 gives a description of the conventional current of an HQI antenna with a wavelength close to (3λ / 4); FIG. 4 illustrates a slotted tri-band HQI antenna, in its coiled antenna / developed antenna configuration; FIG. 5 shows 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; Figures 6 and 7 show the radiation patterns in the context of a two-band antenna with slots, in circular polarization, at different frequencies; Fig. 8 shows an illustration for a slit tri-band antenna of the evolution of the coefficient of reflection versus frequency curve compared to a conventional HQI antenna; FIG. 9 shows the radiation patterns of a tri-band antenna with circular polarization slots obtained relative to three resonant frequencies of the antenna; FIG. 10 finally proposes an illustration for a quad band antenna with slots of the evolution of the curve of Frequency reflection, compared to a conventional HQI antenna.

Figure 11 gives an example of a tri-band antenna, the sub-strands separated by slots extending the main strand in the form of a trident.

Figure 12 gives an example of a dual-band antenna, the sub-strands separated by slots extending the main strand in the form of a fork.

FIG. 13 gives an example of a two-band antenna whose sub-strands separated by slots are folded partially along the main strand.

Figure 14 gives an example of a dual-band antenna whose sub-strands separated slots are folded over almost their entire length along the main strand. More precisely, FIGS. 1a and 1b show a conventional quadriliform helix antenna, as already discussed in the preamble, which comprises four strands 1O ₁ to 1O ₄ of length 12 and width d (FIG. These radiating strands are printed on a dielectric substrate 11 of small thickness then wound on a cylindrical support 12 radially transparent (Figure Lb), 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 coupler structures -

90 ° and a hybrid ring, made in 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 corresponding markets. The bandwidth of this type of antenna is generally of the order of 6% to 8% for an ROS less than two. By way of example, the antenna configuration presented in FIGS. 13 and 14 makes it possible to reduce as much as possible the height of the antennas to be developed and integrated into the new generation mobile terminals. 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.

Thus, and as illustrated in FIGS. 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, of width Wa. One or more slots

22 are placed from the high end to the maximum current 23 of each strand (2O ₁ to 2O 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 be placed at a current belly 30 to allow multiband operation, as shown in Figure 3. The antenna is then wrapped around a transparent support to obtain the antenna in its operating state . Figures 2 and 3 show an example of HQI antenna with a slot 22 corresponding to a dual-band operation. More precisely, FIG. 3 gives a description of the conventional current of an HQI antenna with a wavelength close to (3λ / 4). For example, for a given resonance frequency, the entire antenna can radiate on a single sub-strand. FIG. 4 shows 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.

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

For example, FIGS. 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 .

Thus, FIG. 5 compares the evolution of the reflection coefficient measured at the input of one of the four conducting strands (2O ₁ to 2O ₄ , FIG. 2) as a function of frequency, the other strands being loaded under 50Ω, for a dual-band slot antenna according to the invention (solid line 51) and for a conventional HQI antenna according to the known techniques of the prior art (dashed line 52).

In a complementary manner, FIG. 6 gives an illustration of the radiation pattern of the two-band antenna of FIG. 2 and 3 in circular polarization, for a frequency of F1 = I.3GHz and FIG. 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 -1OdB on two bandwidths (53, 54) which reach 5% each, whereas with antennas

HQI according to the prior art it was possible to obtain a single adaptation of the HQI antenna less than -2OdB on a single bandwidth 55, for a center frequency of 1.3GHz.

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 to extend the possibilities of using such antennas and to adapt the communication systems they equip to several types of communication standards, which goes against the prejudices of the person skilled in the art.

In a variant of the aforementioned embodiment of the invention, FIG. 8 shows 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 having always a length of 0.83λ ₀ and a diameter of 0.18λ ₀ . Thus, for the three resonance frequencies considered Fl = I.22GHz, F2 = 1.48GHz and F3 = 1.68GHz, an adaptation of the HQI antenna of less than -1OdB on three bands (81, 82, 83) is observed. 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 illustrated in FIG. 9 for the frequency values F1, F2 and F3 of FIG. 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, FIG. 10 gives another example of adaptation of the HQI antenna according to the invention less than -1OdB on four bands (101, 102, 103 and 104) for four resonance frequencies considered Fl = I.22GHz , F2 = 1.48GHz, F3 = 1.68GHz and F4 = 1.81GHz, while maintaining good performance in terms of polarization values at the resonance 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 in FIGS. 11 to 14 which give examples of geometrical configuration of the sub-strands of the antenna. More precisely, FIGS. 11 and 12 respectively show examples of tri-band and dual-band antennas for which sub-strands (110) separated from one or more slots (111) extend the main strand (112) under the shape of a trident (Figure 11) or a fork (Figure 12). Such a configuration has the advantage of being able to apply in a technically relatively easy manner a current belly directly and optimally to the upper end (113) of the main strand (112). In addition, the antenna variants envisaged in FIGS. 13 and 14 have the main objective of enabling a substantial reduction in 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.

Thus, the dual-band antenna cited 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

A helical antenna comprising at least one helix formed of at least two radiating conductive strands, characterized in that at least one of said conductive strands is extended in at least two sub-strands separated by a slot of predetermined height and width. said slot being connected in at least one point to a current belly.

2. helical antenna according to claim 1, characterized in that said slot is of variable width.

3. helical antenna according to claim 1, characterized in that at least one of said conductive strands is extended in at least two sub-strands separated by at least two slots having different heights and / or widths.

4. helical antenna according to any one of claims 1 to 3, characterized in that said sub-strands are of variable width.

5. Helical antenna according to any one of claims 1 to 4, characterized in that said sub-strands adjacent to a slot have different heights, so as to allow the resonance of said antenna in at least two distinct frequency bands.

6. Helical antenna according to any one of claims 1 to 5, characterized in that said sub-strands adjacent to a slot are folded over a predetermined height to the side opposite the slot.

7. helical antenna according to any one of claims 1 to 6, characterized in that at least one of said propellers is a quadrifilar helix, comprising four conductive strands.

8. helical antenna according to any one of claims 1 to 7, characterized in that said conductive strands are printed on a flexible dielectric substrate.

9. helical antenna according to any one of claims 1 to 8, characterized in that said slots are placed between the upper end and a position where the current is highest on each of the conductive strands.

10. A helical antenna according to any one of claims 1 to 9, characterized in that the number of said slots separating each of said conductive strands is determined according to the number of distinct frequency ranges in which the antenna must be able to operate, so to provide multi-bandwidth operation.