Patent Grant
8044864
This application is a national phase of International Application No. PCT/EP2007/057351, entitled “ISOTROPIC ANTENNA AND ASSOCIATED MEASUREMENT SENSOR”, which was filed on Jul. 17, 2007, and which claims priority of French Patent Application No. 06 53071, filed Jul. 21, 2006.
The invention concerns an isotropic antenna able to transmit or receive an electromagnetic field over a large frequency spectrum. The invention also concerns a sensor for measuring measurable quantity which comprises an antenna according to the invention.
The invention is applicable to communicating objects which are small in size compared to the wavelengths used for communication. Typically, the objects concerned by the invention are terminals having dimensions in the vicinity of several centimeters operating on ISM (Industrial Scientific Medical), UHF (Ultra High Frequency), VHF (Very High Frequency), SHF (Super High Frequency), EHF (Extremely High Frequency) bands.
The antennae which equip such terminals have reduced dimensions relative to the operating wavelengths λ (dimensions typically smaller than 0.5λ). This specificity of the antennae defines a category of antennae commonly called “miniature antennae”.
The proposed antenna is an antenna which is applicable, among other things, to low-range, low-bandwidth and low consumption applications such as, for example:
The applications primarily concerned by the invention are applications for which the orientation of one or several apparatuses designed to transmit together is random and changing. The quality of the radio connection must, however, remain constant regardless of the orientation. One therefore is ideally seeking an antenna with substantially isotropic radiation characteristics. The proposed invention aims to resolve this problem.
Traditionally, the antennae used to date in the abovementioned applications are of the omnidirectional type, but one does, however, note that they still have directions in which the radiation is null. Transmission is therefore impossible in these directions.
A second aspect damaging the quality of transmission is the polarization mismatching of the waves transmitted or received by the antenna. When the polarization of the waves is linear, a tilt of the antennae relative to each other can lead to orthogonal directions of polarization. In such a case, the transmitted power becomes null.
The search for antenna structures having isotropic radiations began in the years from 1960 to 1970 for spatial applications. It continued into the 1990s. The problem which was then posed was the following: how to keep a constant radio connection with a satellite or a spatial probe whereof the orientation can vary in any manner during a transmission? All of the proposed solutions were antennae with large dimensions, i.e. the dimensions of which are equal to several times the operating wavelength. Their operating principle does not make it possible to miniaturize such antennae. For this reason and due to their unsuitable duty cycle, they cannot be transposed into the fields of application of the communicating objects of the invention.
With regard to miniature antennae, two examples of antenna structure from the prior art and their operating principles are presented below.
V2=V1ejπ/2
The radiation of a dipole is created by a distribution of current which is established, along the dipole, according to a half-wave resonance mode. The radiation produced is then maximum in the direction orthogonal to the dipole and is null in the direction of the dipole. Due to the arrangement in a cross of the two dipoles and their phase quadrature feed, the direction of maximal radiation of one corresponds to the direction of null radiation of the other. The assembly of the two dipoles therefore radiates in every direction. The radiation is thus quasi-isotropic in power. In fact, the characteristics of the radiation emitted are the following:
the typical bandwidth of the transmitted waves is substantially equal to 10% of the central frequency.
An IFA is made up of an electrically conductive plane 1 (ground plane), a wire or planar metallic piece 2, commonly called the “roof” of the antenna, most often arranged parallel to the ground plane (but which can also not be parallel to the ground plane), an electrically conductive connection 3 placed at a first end of the roof, in a first plane perpendicular to the ground plane and which short-circuits the roof and the ground plane, and an excitation means 4, for example a wire probe, placed in a second plane perpendicular to the ground plane and which is connected to a radiofrequency source RF which creates a difference in potential between the roof and the ground plane. The second end of the roof 2 is in open circuit. The ground plane 1 preferably has larger dimensions than the roof such that, from a geometric perspective, the projection of the roof over the ground plane is located entirely inside the ground plane.
The roof 2, the short-circuit 3 and the excitation means 4 form, seen in profile, an inverted F which is at the origin of the antenna's name (cf.
An antenna of this type is not isotropic. It has one direction which has a strong attenuation and this attenuation is more significant when the ground plane is large. The gap between the minimum and maximum power transmitted by the antenna varies from 9.5 dB to 28 dB. The value of 9.5 dB is obtained for a ground plane with small dimensions (i.e. l1=0.22 λg) and the value of 28 dB for a ground plane with large dimensions (i.e. l1=0.4 λg).
With regard to the polarization, it is close to a linear state over the entire radiation diagram, except for two reduced opening lobes for which the polarization is quasi-circular. The uniformity in circular polarization is therefore relatively poor. The bandwidth is typically equal to 1.25% of the central frequency.
The miniature antennae of the prior art have many drawbacks. The miniature antenna of the invention does not present these drawbacks.
Indeed the invention concerns an antenna which comprises four elementary IFA antennae, each elementary IFA antenna comprising a ground plane, a roof, a short-circuit between the ground plane and the roof and an excitation means, the four elementary IFA antennae being distributed around an axis in a first set of two IFA antennae having substantially equivalent far field elementary radiations and a second set of two IFA antennae having substantially equivalent far field elementary radiations, the two IFA antennae of the first set being aligned according to a first alignment axis substantially perpendicular to the axis and the two IFA antennae of the second set being aligned according to a second alignment axis substantially perpendicular to the axis, the first alignment axis and the second alignment axis crossing each other at a right angle at one point of the axis, the excitation means of the four elementary IFA antennae being fed by radiofrequency signals of like amplitude whereof the phases follow a law which is substantially progressive in quadrature by rotation around the axis (0°, 90°, 180°, 270°).
According to one additional characteristic of the invention, the two elementary IFA antennae of a same set of two antennae are identical and symmetrical relative to the axis.
According to another additional characteristic of the invention, the four elementary IFA antennae are all identical.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are distributed on a flat surface substantially perpendicular to the axis.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are substantially inscribed in a circle.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are substantially inscribed in an ellipsis.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are distributed on a substantially conical closed surface.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are distributed on a cylindrical surface whereof the generatrix is parallel to the axis.
According to still another additional characteristic of the invention, the cylindrical surface is a cylindrical surface whereof the directing curve draws a circle, or a square, or a rectangle.
According to still another additional characteristic of the invention, the roofs of the four elementary IFA antennae are formed by metallizations realized on a same substrate.
According to still another additional characteristic of the invention, the ground planes of the four elementary IFA antennae are formed by a same conductive layer.
According to still another additional characteristic of the invention, the antenna comprises means to switch the progressive law in quadrature between a first direction of rotation around the axis and a second direction of rotation around the axis, opposite the first direction.
The invention also concerns a sensor for measuring measurable quantity comprising means for measuring a measurable quantity and a transmitter provided with an antenna able to transmit the measurement of the measurable quantity in the form of a modulation of an electromagnetic wave emitted by the transmitter, wherein the antenna is an antenna according to the invention.
An antenna according to the invention is made up of an association of four elementary IFA antennae. Preferably, an antenna according to the invention comprises a single ground plane, four electrically conductive patterns placed above the ground plane and each forming an IFA antenna roof, four short-circuit connections and four excitation means.
The four elementary IFA antennae are grouped according to two sets of two antennae, the two IFA antennae of a same set being designed such that their far field elementary radiations are equivalent.
Two IFA antennae have equivalent far field elementary radiations when, being placed independently in the same marker with the same orientation, they radiate, in the useful frequency band, a wave of like amplitude and like phase in each direction of the space.
A simple means for obtaining two IFA antennae with equivalent elementary radiations consists of realizing identical antennae, i.e. having the same geometry (same shape and same dimensions).
It is, however, possible to realize two IFA antennae having different shapes or dimensions and having, despite everything, equivalent elementary radiations. Examples of such antennae will be described later, in reference to
The ground plane of an antenna of the invention is formed by a conductive element whereof the surface can allow, if necessary, stores of metallization and electronic components. The surface of the ground plane can be a flat surface which is circular, elliptical, square, rectangular in shape, a conical surface, a surface which closes on itself of the cylindrical, cubic or parallelepiped type, etc. In general, the surface which defines the ground plane has a symmetry relative to an axis. The surface of the ground plane has dimensions greater than or equal to the surface on which the electrically conductive patterns forming roofs are integrated such that, from a geometrical perspective, the projection, over the ground plane, of the surface in which the electrically conductive patterns forming roofs are integrated is located entirely inside the ground plane. The radiation of the antenna is more isotropic in power when the ground plane is small. This is why the ground plane will preferably be chosen with dimensions equal to the dimensions of the surface in which the electrically conductive patterns forming roofs are integrated. The ground plane will most often have larger dimensions when it has, for integration reasons, a circuit support function such as, for example, the RF circuit which feeds the elementary IFA antennae.
The RF circuit which feeds the four feed connections can indeed be realized on the upper or lower surface of the ground plane. The influence of its presence on the radiation of the antenna is negligible when it is correctly designed. Different possibilities for realizing the feed circuit are possible in the form of a parallel or serial network of microwave strips which may or may not include localized elements (coupling units, phase changers, etc.).
The patterns forming roofs can be wires or flat elements whereof the contours can have quite varied shapes: rectangular, trapezoidal, elliptical, folded in an arc or not, rounded ends or not, the general shape of a pattern and its dimensions greatly determining the radiation characteristics of the antenna, in particular its operating frequency. The patterns are arranged either parallel to the ground plane, or tilted by an angle relative thereto (the tilt angle of the patterns can, for example, be equal to 30° and can reach 45° or even more). The patterns can be realized on substrate using printed circuit techniques or machining of conductive pieces, for example metallic.
According to the preferred embodiment of the invention, the patterns are grouped into a first pair of identical patterns and a second pair of identical patterns. The patterns of one pair of identical patterns are aligned along an alignment axis perpendicular to the axis Oz of the antenna, the two alignment axes of the two pairs of patterns crossing at a right angle on the axis of the antenna. Also, the two conductive connections forming short-circuit between the ground plane and the ends of the conductive patterns of a pair of conductive patterns are arranged symmetrically relative to the axis Oz. The same is true for the two excitation means connected to the two conductive patterns of a same pair of conductive patterns.
The four excitation means feed the four IFA antennae with signals of substantially equal amplitudes, phase shifted according to a law which is progressive in phase quadrature such that, for antennae a1-a4 which follow each other around the axis Oz (in the clockwise direction or the counterclockwise direction), it comes:
Two IFA antennae aligned along an axis perpendicular to the axis of the antenna are strongly coupled (typically −3 to −4 dB). Their feeds are in opposite phase (180°) but, due to their opposite orientations, their resonances are phased. The coupling phenomenon is beneficial here because it advantageously allows a reduction of the length L of the roofs of the two IFA antennae which are across from each other compared to the case of a single isolated IFA having the same operating frequency. The dimension L can thus be less than λ/4. The set is thus smaller than the simple combination of dipoles in a cross, which is an advantage related to the invention.
Likewise, contrary to the combination of dipoles in a cross for which the coupling between dipoles is weak (<−40 dB), the coupling between two elementary IFA antennae of the invention whereof the roofs are perpendicular to each other is significant (−2 to −3 dB). The electrical field concentrated between the ground plane and the roof of the antenna is oriented in the normal direction relative to the ground plane. When two IFA antennae are arranged on the same ground plane, their field lines are oriented in the same direction perpendicular to the ground plane. Strong coupling then occurs between them. This coupling depends on the distance between the antennae and depends little on their orientations. For this reason, it is impossible to arrange two IFA antennae in a cross according to the operating principle of the dipoles in a cross. The strong coupling would not allow feeding of the IFA antennae independently in phase quadrature.
In the framework of the invention, coupling between the orthogonal pairs of IFA antennae is decreased due to the central space left between them. Coupling is thus typically brought to between −7 dB and −10 dB, which allows feeding with a 90° phase shift between adjacent IFA antennae. The space between the IFA antennae tends to increase the total dimensions of the set of antennae and therefore constitutes a limit for the miniaturization of the antenna. However, this is partially offset by the coupling phenomenon previously mentioned, thereby making it possible to decrease the length of each elementary IFA antenna.
From the perspective of electromagnetic performance, an isotropic antenna according to the invention advantageously has the following characteristics:
Other characteristics and advantages of the invention will appear upon reading one preferred embodiment done in reference to the attached figures, in which:
As a non-limiting example, a detailed description of an antenna corresponding to the seventh example of the preferred embodiment of the invention is given below.
The patterns forming roofs for IFA antennae are realized on an epoxy-glass substrate (εr=4.4; tgδ=0.018=loss tangent) of 0.38 mm thickness covered by a copper metallization with a thickness of 17 μm. The patterns forming roofs are realized by photolithography. The ground connections 3 are located at the outer ends of the patterns 2. The connections 3 are copper wires with a diameter of 0.6 mm whereof a first end is welded to the pattern 2 and the other end to the ground plane. The feed wires 4 are also copper wires with a diameter of 0.6 mm. The ends of the ground wires 3 and the feed wires 4 which are located from the side of the substrate S are distributed on a circle X.
The distance which separates, on a same pattern 2, the end of the ground wire 3 from the end of the feed wire 4 is substantially equal to 3.6 mm. The distance which separates the ground plane 1 from the substrate S is substantially equal to 4 mm. The diameter of the substrate S is substantially equal to 25 mm and the diameter of the ground plane is larger than the diameter of the substrate S, for example equal to 30 mm. As already mentioned above, other values of the diameter of the ground plane are possible once the condition of a diameter larger than or equal to the diameter of the substrate S is met.
The antenna described above has an operating frequency substantially equal to 2.5 GHz. In a known manner, the bandwidth and the exact frequency of impedance adaptation also depend on the feed network used.
The gap between the minimum and the maximum power transmitted by the antenna is typically 5.6 dB, which corresponds to good power isotropy. The polarization of transmitted waves is circular along the axis Oz and rectilinear in the plane of the patterns 2. The average of the axial ratio pattern is substantially 49%.
For comparison, the table below shows the typical gap performance between maximum and minimum of the directivity pattern and average on the axial ratio pattern for the antenna of the invention and two antennae of the prior art, namely the combination of dipoles in a cross and the IFA antenna alone.
The gap between the maximum and minimum of the directivity pattern makes it possible to quantify the power isotropy. The weaker the latter, ideally null, the better the power isotropy. The average of the axial ratio pattern enables quantification of the uniformity of polarization relative to the circular state. An average of 100% means that the antenna radiates with a perfectly circular polarization in every direction.
Another significant criterion enables comparison of the antennae to each other. This criterion is the coverage of the antennae. The coverage of an antenna is the proportion of orientation/tilt covered by the antenna according to the minimum power it receives when it is illuminated by an incident flat wave of unit power density. The coverage curves of the three abovementioned antennae (combination of dipoles in cross, IFA antenna alone and antenna according to the invention) are illustrated in
The curves C1, C2, C3 of
It emerges from these figures that the antenna according to the invention makes it possible to find all of the advantages of the combination of dipoles in a cross in the field of broad coverages despite its reduced size.
The sensor comprises a multilayer printed circuit CI made up of an insulating layer 5 on which are deposited, on one side, a conductive layer 6 which constitutes the ground plane and, on the other side, a substrate 7 on which different circuits x1, x2, x3 are integrated such as integrated circuits, battery, sensor, feed network RF, etc. The dimensions of the sensor are small, such that the antenna is its most voluminous component. The diameter D of the sensor is thus typically equal to λ/5 or λ/4. This dimension is to be brought closer to the diameter λ/2 of the half-wave dipoles in cross. The realization of the sensor in printed circuit technology advantageously allows mass production thereof at low costs.
The connection of electronic circuits and the antenna advantageously allows the realization of an independent sensor. The components and devices placed under the ground plane disrupt the radiation very little.
One example of use of the isotropic antenna of the invention will now be described, in the framework of a time division multiple access (TDMA) network, in reference to
The TDMA network is a star network for sensing motion which comprises a master node NM and a set of slave nodes N1-N14 which are in motion relative to the master node. At each slave node of the network, a sensor is placed which comprises an antenna according to the invention. The slave nodes are distributed as follows:
This star network, orchestrated by the master node, makes it possible to recover, at determined time intervals, the data delivered by the different sensors, the positions of which vary over time.
Each sensor located at a slave node is optimized in terms of size, integration and electrical consumption. It is made up of a physical measurement sensor and its packaging, a processing unit and a radio transmitter/receiver connected to an isotropic antenna according to the invention. Independent, it has an on-board energy source.
The sensor located at the master node is less subject to the size and consumption restrictions, but also has a radio transmitter/receiver and a processing unit. The antenna which equips the sensor located at the master node can be an isotropic antenna according to the invention or a dipolar antenna.
All of the interest of the antenna according to the invention in this context lies in its radiation pattern which covers the entire space, in its circular polarization state which optimizes radio transmission regardless of the tilt of the sensors and in its low bulk in terms of volume.
The antenna according to the invention which equips each sensor located at a slave node has an isotropic radiation in power in all directions and a circular polarization optimized such that there is no direction for which the transmission between a slave node and the master node would be interrupted. The antenna according to the invention equipping the slave nodes is circularly polarized, and the antenna equipping the master node is rectilinearly polarized. Thus, the transmission cannot be interrupted due to polarization mismatching.
The antenna according to the invention increases the overall dimensions of the sensors very little because its planar shape factor provided with a ground plane on one of these surfaces allows easy integration on the sensor. The antenna can be realized with the same printed technology as the rest of the circuit of the sensor. The functions of the sensor and the battery are integrated in a multi-layer under the ground plane of the antenna as previously mentioned.
A description of the operation of the TDMA protocol connecting the master node to the slave nodes will now be provided.
During a nominal cycle of the TDMA network, the master node transmits a timing synchronization word and information sent to the slave nodes, as well as a cyclic redundancy code (CRC). After this the slave nodes transmit, one after the other, their data to the master node as well as a CRC to detect communication errors. When all of the slave nodes have transmitted their data, they can become lethargic until the next cycle in order to increase their autonomy. During this period of time, management of the network can then be done: detection of new slave node, management of communication channels, parameterization of slave nodes.
Due to the isotropy of the antenna which equips them, the sensors of the invention advantageously make it possible to ensure a robust radiofrequency communication link at the position variations. Fewer errors are detected and the use of the retransmission procedure for information is much less necessary, which contributes to optimizing real-time flow and limiting the consumption of the sensors.
Different antennae variations can be realized in the framework of the invention, namely, for example, reconfigurable antennae, diversity antennae or antennae with coverage limited to half-spaces.
Reconfigurable antennae comprise means making it possible to switch phase states. A first phase state can then correspond to a phase progression 0°→90°→180°→270° between the different elementary antennae, while a second phase state corresponds to a phase progression 0°→−90°→−180°→−270° between these same elementary antennae. Phase switching advantageously makes it possible to turn waves with right circular polarization into waves with left circular polarization and vice versa.
In the framework of the invention, the diversity antennae are realized, when the coupling level between elementary TFA antennae allows, by feeding these via two or four independent paths.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06 53071 | Jul 2006 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/057351 | 7/17/2007 | WO | 00 | 1/16/2009 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2008/009667 | 1/24/2008 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6618016 | Hannan et al. | Sep 2003 | B1 |
| 7427955 | Choi et al. | Sep 2008 | B2 |
| 20060145926 | Choi et al. | Jul 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 2751471 | Jan 1998 | FR |
| 2005004283 | Jan 2005 | WO |
| 2008009667 | Jan 2008 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20090322631 A1 | Dec 2009 | US |
