Patent Application
20090213025
This present invention concerns telecommunication antennae, and in particular antennae of the ultra-wideband type (UWB).
This antenna type has long existed in the area of civil or military radars, but its attractiveness for general-purpose applications has only appeared in recent times.
By way of a non-limiting example, it is now known that such antennae open some very interesting possibilities in the area of high-speed multimedia applications, for a domestic or professional target.
There naturally exist other examples of applications for these antennae, but in any event, a known advantage of using such UWB technology, in relation to a conventional radio technology (of the narrow-band type with carrier for example), is to offer the possibility of very high transmission speeds.
Another known advantage of the UWB technology is that it is very robust in relation to problems of interference and fading of the signal in the case of multiple-path propagation.
Another known advantage of this UWB technology is that it has an extremely wide frequency spectrum.
As an example, a recent regulation of the FCC (Federal Communications Commission) allows the use of a frequency band of between 3.1 GHz and 10.6 GHz without a license.
As a fundamental component of such a communication system, many implementations of UWB antennae have already been proposed.
One is already familiar, for example, with a first large family of UWB antennae which are antennae of the dipole type (such as of the biconical, planar type with a square or triangular geometry) and of the single pole type (such as antennae of the single-pole, conical type, for example) [1-6].
It will be noted in this case, that in the case of antennae of the dipole type, solutions with radiating elements symmetrical or asymmetrical shape [4] have been proposed.
Although the antennae of this first family can provide good performance, one problem is that their dimensions are dependent on the working frequency of the antenna.
More precisely, the dimension of the radiating elements in particular is imposed by the lowest working frequency used in the application concerned.
Thus in the case of an antenna of the biconical dipole type, the dimension of each of the cones is equal to λ/4, where λ is the longest working wavelength in the application concerned.
As a consequence, knowing the working frequencies of the said application, a designer of such an antenna has very little room for manoeuvre in its implementation.
And the consequence can be that the antenna does not comply with a precise specification, in terms of compactness in particular.
One is also familiar with a second large family of UWB antennae.
This includes antennae of the horn configuration type [7-10].
We know in particular of antennae with radiating elements of the coaxial horn or transverse electromagnetic (TEM) horn type, for example.
Other variants in this second family of antennae are again based upon the use of radiating elements with shaped profiles, most often according to exponential laws, and excitation or feed systems based on baluns or cavities [9-10].
In the case of antennae in this family, the designer is able to manipulate a greater number of degrees of freedom than previously.
In particular, the constraint on the dimensions of the radiating elements as a function of the working frequencies is eased, offering the possibility of using radiating elements of smaller dimensions than that of the first family of antennae for example.
In spite of this, the design flexibility of the antennae in this second large family still remains incapable of satisfying very varied specification schedules, while also achieving a compact result.
As an example, in order to improve the performance of the antenna, in particular when its dimensions are small, use is made of a gradual matching element that allows one to achieve a coupling with a gentle transition between a feeder element and the radiating elements.
Now however, as a result of its principles of operation, it is this gradual matching element which occupies a significant space and which results inevitably in an antenna that is not compact.
Finally, a third large family of antennae is that of the shaped slot antennae.
One is familiar in particular with an antenna that includes radiating elements in a double-slot planar configuration [11].
Another antenna of this third large family has radiating elements in a configuration with two planar double slots positioned perpendicularly [12].
A drawback of these antennae is that they cannot be used to achieve a homogeneous radiation pattern in the azimuthal plane.
Moreover, if matching elements have also been proposed with these antennae, their dimensions are unfortunately imposed by the lowest working frequency in the specification.
In particular, the dimension must be equal to λ/4, which here again limits the compactness of the antenna.
Document FR 2 843 237 describes an antenna of the single pole type from the aforementioned first large family.
It can be seen however that the radiation pattern of this antenna varies as a function of the frequency, in particular in the azimuthal plane (OXY).
Furthermore, in particular, this antenna has the drawback of controlling the electromagnetic field only by means of a surface shaped like a chalice 1.
Furthermore, this antenna does not generate a signal in its central region in a localised manner, and does not have any matching element to favour localised coupling between the feeder means 4 and the said region.
Documents U.S. Pat. No. 2,532,551 and FR 2 573 576 concern an ultra-wideband antenna belonging to the aforementioned second large family. In fact this is a biconical horn antenna.
These have the previously mentioned drawbacks of this type of antenna, due in particular to an absence of localised coupling of the signal in the central region.
Document WO 02/056418 concerns a wideband electromagnetic probe.
Like document FR 2 843237, this antenna is not of the ultra-wideband type, and does not lend itself to such use either.
In particular, it has a single surface 100 to control the electromagnetic field, with this surface 250 being connected to a mass.
Moreover, it does not have any efficient and compact matching element to favour localised coupling between the coaxial drive 302 and the zone 400.
One aim of the invention is therefore to propose an improved antenna.
In particular, the invention has as its objective to propose a UWB antenna with omnidirectional radiation in the azimuthal plane and with the most constant possible value of gain with frequency in this plane.
Moreover, the antenna according to the invention advantageously has a simple geometry and provides great design flexibility in order to comply with very different specifications.
Furthermore, in particular by virtue of this great simplicity of implementation, it can also satisfy many other constraints, in particular such as high technological reproducibility, low cost and small size. The invention thus proposes an ultra-wideband antenna that is characterised in that it includes:
Preferred and non-limiting aspects of this antenna are as follows:
a. rectilinear,
b. concave in relation to the longitudinal axis,
c. convex in relation to the longitudinal axis;
a. rectilinear,
b. concave in relation to the plane orthogonal to the longitudinal axis and that contains the horizontal axis,
c. convex in relation to the plane orthogonal to the longitudinal axis and that contains the horizontal axis;
In addition, the invention also proposes a telecommunication system that is characterised in that it includes an ultra-wideband antenna that is designed with the aforementioned characteristics, either singly or in combination.
Thus, the appropriate combination of the various means presented above enables one to offer many degrees of freedom to a UWB antenna designer and to implement the latter in a simple manner so as to provide the advantages of the invention, in particular the ability to satisfy a varied specification schedule while still remaining compact.
Other aspects, aims and advantages of the invention will appear more clearly on reading the description that follows of the invention, with reference to the appended drawings, in which:
It will be noted right away that, in the following text, the term distal is meant in relation to the centre of the antenna.
Furthermore, in order to simplify reading, it is assumed that the longitudinal axis (Z) is aligned with a vertical axis, and therefore that the axis (X) represented in the figure is aligned with a horizontal axis.
Regarding
This antenna 1 includes two identical shaped surfaces 3, 4 placed opposite to each other in relation to a plane that is perpendicular to the longitudinal axis (Z) and that contains the horizontal axis (X).
A zone 2 is formed between these two shaped surfaces.
Zone 2 therefore generally displays an outline that is perfectly delimited by the two facing shaped surfaces 3, 4.
In this embodiment, the latter have a profile (C) in the form of a parabola that is open upwards and downwards respectively.
However, whatever the profile that is chosen, it is always arranged that its shape is such that an electromagnetic field existing in zone 2 has characteristics that allow a signal 5 supplied at the central region of this zone to propagate in an azimuthal direction, with a gain that is as constant as possible with frequency.
In other words, it is always arranged that the profiles and dimensions of these two surfaces 3, 4 are designed to control the electromagnetic field in zone 2 so that the antenna generally presents a gain that is as constant as possible over the selected frequency band, according to the direction or the azimuthal plane.
It will be noted that, according to the invention, by a gain that is as constant as possible is meant a gain whose variation remains below 1.5 dB over a passband that is at least greater than fmax/fmin=5.
As a consequence, according to the invention, the profile (C) of the shaped surfaces represents a degree of freedom (a parameter) in the design of the antenna.
This aspect will be described later in greater detail.
Returning to
Again more generally, the antenna, or at least the two shaped surfaces, possess a symmetry of revolution about the vertical axis (Z), which contributes in particular to achieving a high degree of uniformity in the radiation pattern of the antenna in the azimuthal plane.
The latter also includes a feeder means 6, typically a coaxial line, extending parallel to the vertical axis (Z) and capable of supplying a signal 5 to a central region of zone 2.
A part of this feeder means is incorporated into a vertical through orifice created more-or-less at the centre of the shaped surface 4.
In this way, the feeder means 6 can reach the central region of zone 2 from the exterior at the bottom of the antenna.
And as shown in particular in
Again more precisely, the feeder means 6 also traverses the central region of zone 2 to come into intimate contact with a local matching means 7 placed at the centre, under the shaped surface 3.
As a consequence, the matching means 7 is located more-or-less facing the through orifice.
As illustrated in
Such a matching means is used to locally favour a transition of the signal between the feeder means 6 and zone 2 while still remaining of small dimension.
It will simply be observed for the remainder of the description, that the stub 7 has a diameter and a height, respectively noted d and h.
It will also be remembered that there exists a configurable space, of length e, along the vertical axis between the bottom end of the stub 7 and the top end of the through orifice.
As mentioned previously, the feeder means, shown here by way of a non-limiting example, is a coaxial line 6 that includes a central core 6″ connected to the bottom end of the stub 7 and a peripheral conductor (screen) 6′ surrounding the central core 6″ and connected electrically to the shaped surface 4.
In this regard, it should be noted that the shaped surfaces 3, 4 are covered with a thin coat of conducting material and together form a radiating element.
Zone 2 will now be described in greater detail.
In this regard,
Here we have presented two antennae, whose zone 2 is entirely filled with a single block of material 10.
This single block 10 therefore lies about the vertical axis (Z) and from the central region up to the extremity of the antenna determined by the distal edge of the shaped surfaces 3, 4.
The surface of the single block 10 that is located in contact with the air in one side of the antenna constitutes a section (T) whose profile can serve as a degree of freedom (a parameter) in the design of the antenna.
It will also be observed in this variant, that the two shaped surfaces 3, 4 are respectively the upper and lower surfaces of the single block of material 10 so that there exists only a single physical part.
Thus, the essential of the volume of zone 2 is to some extent determined by the volume of the single block of material 10 itself.
It will also be noted that the matching means 7 and the single block of material 10 also constitute a single part.
In another variant, the two shaped surfaces 3, 4 are formed respectively by two separate elements 3′, 4′), that is by two independent physical parts.
Zone 2 can then be entirely filled with air, as illustrated in
In this case, means 10′ are provided in the said zone 2 in order to secure the two elements 3′, 4′ opposite to each other.
These securing means 10′ are can be spacers and/or rods for example, distributed about the vertical axis (Z) and whose extremities are fixed to elements 3′ and 4′.
Zone 2 can also be composed of air and of the single block of material 10.
A non-limiting example is provided in
Here, the single block of material 10 has two sections (T, T′) in contact with the air.
More precisely, it has an external section (T) constituting one external side of the antenna, and an internal section (T′) at least partially containing the central region of zone 2.
Thus, seen in horizontal section, the single block 10 corresponds to a ring placed around the vertical axis (Z).
The internal section (T′) contains air, but the invention also allows that it can contain another gas, preferably with dielectric properties.
Advantageously the single block 10 constitutes a support for the two separate elements 3′, 4′.
But, it is also possible to strengthen the stiffness of the antenna with securing means 10′ (not shown in
As can be seen from the above description, a designer therefore already has considerable flexibility in the design of a UWB antenna to a given set of specifications.
However the antenna according to the invention provides an even greater number of degrees of freedom (parameters).
As mentioned previously, one fundamental parameter of freedom consists of varying the profile (C, C) of the shaped surfaces 3, 4.
According to the invention, in longitudinal section, at least one portion of these profiles (C, C′) has a shape that is selected from the following:
a. rectilinear,
b. concave in relation to the plane orthogonal to the longitudinal axis (Z) and that contains the horizontal axis (X),
c. convex in relation to the plane orthogonal to the longitudinal axis (Z) and that contains the horizontal axis (X).
Thus, each of the two surfaces 3, 4 can consist a juxtaposition of several portions of surface, with these portions having a profile whose shape is different from one to the next.
Naturally it is not excluded that these two shaped surfaces can have a profile that, as a whole, has one of the shapes listed above.
This is also illustrated in a general manner by the appended figures.
For example,
In
And in contrast to the profile (C) of surface 4, in particular the profile (C′) of surface 3 includes an inflection point.
As illustrated in these last two figures and in
In the examples provided in
It is understood that the height in question corresponds to the distance projected on the vertical axis between one distal extremity of the profile and its centre located on the said vertical axis.
In addition, R and R′ refer to the radii of the respective surfaces 4, 3. Finally, S refers to the smallest distance that separates the two shaped surfaces 3, 4, or indeed the distance that separates these two surfaces at the centre of zone 2.
In the light of these definitions, the antenna of
(C′)≠(C), H′>H, R′<R
In the same spirit, the antenna of
(C′)≠(C), H′=H, R′=R
and that of
(C′)≠(C), H′>H, R′>R
Naturally, there exist other possible systems in which use is made of asymmetry in the profiles of the facing surfaces 3, 4, by varying at least one of parameters H, R and profile C.
Another parameter of freedom offered to the designer consists of varying the profile of the section or sections (T, T′) of the single block of material 10.
Like the profile (C, C′) of the shaped surfaces 4, 3, a portion at least of the profiles of the section or sections (T, T′) has, in longitudinal section, a shape selected from the following:
a. rectilinear,
b. concave in relation to the plane orthogonal to the longitudinal axis (Z), and that contains the horizontal axis (X),
c. convex in relation to the plane orthogonal to the longitudinal axis (Z), and that contains the horizontal axis (X).
Thus, a section can consist of a juxtaposition of several portions of section, with these portions of section having a profile whose shape is different from one to the next.
Naturally it is not excluded that these two shaped surfaces can have a profile that, as a whole, has one of the shapes from the above list. By way of a non-limiting example, and with reference to
Another parameter of freedom offered to the designer is the ability to have at least one conducting pattern 11 on a section of the single block 10 so as to contribute once more to controlling the characteristics of the electromagnetic field in zone 2, namely to controlling the radiation characteristics of the antenna such as, in particular, the appearance of the radiation patterns, the value of the directivity, or the polarisation.
In
Yet another parameter of freedom consists of varying the geometry of the stub 7 by modifying either its shape or its dimensions (d and/or h).
By way of a non-limiting example, the stub can have the shape of a trapezium in longitudinal section, with the smallest side being that at the bottom.
In fact, the antenna can be arranged to accept an electronic circuit not far from it 12, and to protect it from the electromagnetic field that it is radiating.
Preferably, the electronic circuit 12 is placed as close as possible to the antenna 1, which also results in optimisation of the signal-to-noise ratio.
As again illustrated in
When the antenna is viewed from below, this recess 13 corresponds in this non-limiting example to the recess formed by the concave shape of the profile (C) of the second shaped surface 4.
We will now present a process for the manufacture of an antenna according to the invention, such as the antenna of
This process is based firstly on the shaping of the single block of material 10.
It will be noted that the choice of the material also constitutes an additional parameter of freedom for the design of the antenna.
In general, it is proposed to use a dielectric material, preferably of the foam or plastic type, with electrical characteristics such that εr is relatively close to 1, and tg(δ) is of the lowest possible value (εr is the relative permittivity, and tg(δ) the dielectric loss tangent and preferably less than 10−3 in the invention).
The shaping of the single block 10 can be effected either by machining or by moulding of the desired part, from an appropriate choice of material.
Having completed the shaping, selective metal coating is performed on all profiled surfaces of the shaped surface 3, on which the matching stub 7 has been created, as well as on the shaped surface 4.
Only a circular resist at the connection with the coaxial line 6 is placed on the shaped surface 4. The said metal coating can be effected, for example, by the deposition of a conducting paint or by the electrochemical deposition of a metal.
In this regard, it will be noted that, for its part, the section (T) of the single block support 10 is not metal coated.
Finally, the coaxial line 6 can then be connected to the antenna.
In this case, electrical continuity, by brazing or by a conducting adhesive, must be provided firstly between the peripheral conductor 6′ positioned at the resist and the metal coating on the surface 4, and secondly, between the central conductor of the coaxial cable 6″ and the bottom part of the matching stub 7.
As will have been understood from the foregoing, the central core 6″ then traverses the single block of dielectric material 10 via a small hole of height e.
This manufacturing process has the advantage of being very easy to implement and a low cost.
Regarding technological reproducibility, having only a single part on which all of the elements making up the antenna are created enables a high degree of control to be exercised over the positioning of these elements, and particularly the spacing and alignment between the two shaped surfaces 3, 4.
We will now present some detailed examples of implementation of the invention, as well as performance results obtained from these examples.
Meeting the extreme edges of these two caps, the section (T) presented by the antenna then corresponds to a cylindrical section with radius R=26 mm and height 2H+S=29 mm.
For its part, the matching stub 7 has a cylindrical geometry with a height h=2.5 mm and a diameter of d=3.5 mm.
Concerning the feeder means 6, the retained solution is to use a standard Teflon coaxial cable, with a characteristic impedance of 50Ω.
The single block of dielectric material 10 is polymethacrylate imide foam with the electrical characteristics εr=1.11 and tg(δ)=7·10−4, these being measured at 5 GHz.
In this present case, this material 10 (a single block of foam, for example) has been machined by micro-milling to collectively create the assembly comprising the surfaces 3, 4, and the matching element 7 in a single part.
Regarding the selective metal coating of the conducting zones on the antenna, the latter was effected the material 10 by direct deposition of a silver-based metallic paint.
Concerning the operation of this antenna, a simulation exercise was conducted with the aid of an electromagnetic CAD application, working in the time domain.
Simulation of the reflection coefficient 39, presented in
In addition,
In this case, it is observed that the radiation of the antenna is indeed of the omnidirectional type in the azimuthal plane, with a slight dispersion of the value of the gain in this plane as a function of the frequency (0.6 dBi, −2.4 dBi, 1.1 dBi, 2.4 dBi and 1.7 dBi respectively for the previous frequency values).
Following an initial phase of simulation of the antenna performance, several prototypes were created and characterised by matching and by transmission, with the latter measurement being effected in the azimuthal plane and on the basis of a simple link performance between two antennae of the invention, separated by a distance D.
where λ is the wavelength, Pr the received power, G the gain of the antennae, and Pe the transmitted power.
From the general equation for link performance, it is then possible to deduce the experimental value of the gain of the antenna as a function of the frequency, in this azimuthal plane, and to compare it with that obtained from the theory.
The experimental results corresponding, to the matching and the value of the gain, confirm the performance simulated over a working band of 3.1 GHz-10.6 GHz.
Referring in particular to
For the value of the gain in the azimuthal plane as a function of the frequency, the measured curve 41 brings out ripple effects associated with the presence of multiple paths.
These exist for the reason that the characterisation was not conducted in an anechoic chamber.
The result obtained for the gain is therefore more qualitative than quantitative.
Nevertheless, it can be seen that, over the band of interest, namely 3.1 GHz-10.6 GHz, the measured values remain within the range [−2.5 dBi, 2.5 dBi], which is in agreement with the simulations.
A second detailed example of implementation of an antenna according to the invention is illustrated in
In this case it is a compact UWB antenna whose elements 3, 4 are asymmetrical in relation to the horizontal axis but that have a symmetry of revolution about the longitudinal axis (Z).
It can be observed that this coefficient 42 maintains a level of less than −10 dB over the whole band, namely 3.1 GHz-10.6 GHz. In addition,
It is again observed that the radiation from the antenna is still of the omnidirectional type in azimuth, associated with a slight variation in the value of the gain in this plane, as a function of the frequency (respectively 1.5 dBi, −0.4 dBi, −2.1 dBi, 0.5 dBi and 0.5 dBi for the frequencies mentioned earlier).
From the experimental viewpoint, the measurements effected on this antenna show that it is indeed matched, since the measured level of the reflection coefficient 43 is always less than −15 dB over all of the band 3.1 GHz-10.6 GHz (see
Regarding the value of the gain 44 in the azimuthal plane as a function of the frequency, the latter is again in agreement with the simulation, with a small variation over the range [−2 dBi, 2 dBi].
Finally, a third example of an antenna is described briefly below and illustrated in
The shaped surface 4 of this antenna comes in the form of a spherical cap, while the shaped top surface 3 has a profile shaped like an inverted bell flared out at the edges.
Experimental measurements on the matching and the gain in the azimuthal plane are provided in
It can be seen that the reflection coefficient 50 is always less than −12 dB over the whole working band of 3.1 GHz-10.6 GHz. This antenna is therefore matched quite satisfactorily, as in the case of the previous antennae.
Concerning the gain 51, it can be observed that this varies very little with the frequency, and that its value in fact remains permanently inside the range [−1.5; 1.5 dBi].
As a consequence, this third example of implementation enables one to offer satisfactory performance and in particular a very modest volume.
In fact, the volume of this antenna is just 37.7 cm3, while the volume occupied by the first implementation example described earlier is 61.6 cm3. In the case where one is seeking to reduce still further the volume occupied by the type of antenna chosen in this example, it will be noted that it was possible to create an antenna with a volume of 17.7 cm3, which is a reduction of 70% in relation to the first implementation example, while still achieving satisfactory performance, and in particular a reflection coefficient that is always less than −9 dB in the band considered, and a gain in the azimuthal plane that also displays small variations with the frequency over a range of [−2 dBi, 2 dBi].
It can also be seen that this antenna is advantageously compact. Naturally, the present invention is not limited in any way to the form of implementation described above and represented in the drawings.
In conclusion, the invention proposes an ultra-wideband antenna offering a very high degree of design flexibility and that can be used to satisfy very varied specifications.
Such an antenna can therefore be used both in military and civilian applications (for general or specialist use).
By way of a non-limiting example, one can envisage fitting one or more antennae of the invention in a variety of equipment such as in a computer, a fixed or mobile telephone, a printer, a television set, a CD-ROM drive, or more generally in any equipment where wireless communication is used.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0502922 | Mar 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/061035 | 3/24/2006 | WO | 00 | 12/15/2008 |
