Patent Grant
9755317
This application is a National Stage of International patent application PCT/EP2011/066563, filed on Sep. 23, 2011, which claims priority to foreign French patent application No. FR 1003900, filed on Oct. 1, 2010, the disclosures of which are incorporated by reference in their entirety.
The invention is applicable to the field of circular-polarized planar wire antennas for broadband transmitter or receiver devices. It relates to an antenna reflector for such an antenna, an antenna device comprising the reflector and the antenna, and a method for implementing the antenna reflector.
In the framework of certain applications, the antennas must have a wide band of operating frequencies, for example of the order of a decade, in other words a frequency band whose maximum frequency is equal to at least ten times the minimum frequency. Circular-polarized planar wire antennas, such as spiral antennas, belong to these wide frequency band antennas. A spiral antenna is generally composed of a dielectric substrate into which a radiating element is etched. The radiating element comprises at least two strands wound into a spiral whose inner ends are supplied with current. The electromagnetic radiation from the spiral antenna varies depending on the number of strands and the phase of the current in each strand. The width of the frequency band depends on the inner and outer diameters of the spiral.
From a theoretical point of view, a planar wire antenna possesses a plane of symmetry and therefore radiates into the whole of space, in particular in the two directions orthogonal to the plane of the antenna. For reasons of electromagnetic compatibility, the antennas must not interfere with the other systems situated nearby. Consequently, they are very often specified so as to radiate into a half-space. For this reason, the antenna is associated with a reflector which transforms the bidirectional radiation into a unidirectional radiation. From a practical point of view, this reflector also serves as a support allowing the antenna to be made more rigid and to be supplied with current.
According to a first solution, the reflector comprises an electrically conducting plane disposed at a distance from the antenna equal to a quarter of the mean wavelength of the radiation that it emits or that it receives. At such a distance, the electric field of the reflected backward radiation is then in phase with the electric field of the forward radiation. The main drawback of this solution is that the distance can only be adjusted in an optimal manner for a single wavelength. The electric field of the radiation emitted or received at wavelengths far from this mean wavelength therefore risk being affected, thus limiting the bandwidth of the antenna. Another drawback of this solution is that a quarter of a wavelength quickly corresponds to a large distance for low frequencies, which quickly leads to an overall relatively large thickness for the antenna. Furthermore, the electrically conducting plane allows the propagation of surface currents and reflection and scattering phenomena occur at the edge of the antenna, thus generating spurious radiation.
According to a second solution, the antenna reflector comprises a structure of the Artificial Magnetic Conductor (AMC) type disposed under the plane of the antenna on the side of the backward radiation. A conventional AMC structure comprises a dielectric substrate, electrically-conducting patterns disposed periodically on a first surface of the dielectric substrate and a uniform electrically-conducting plane forming a ground plane on a second surface of the dielectric substrate. Each conducting pattern can be connected to the ground plane via interconnection holes, generally referred to as “vias” in the literature. An AMC structure possesses the property of reflecting the electric field of the backward radiation in phase with the electric field of the forward radiation. It can therefore be positioned very close to the antenna and allows a reduction in the thickness of the antenna device comprising the antenna and the AMC structure. An AMC structure can also possess the property of prohibiting the propagation of electromagnetic waves in certain directions of the plane in which the conducting patterns are disposed, which prevents any spurious radiation from being generated. This is referred to as an electromagnetic band gap (EBG) structure. However, the properties of a structure of the EBG or AMC type are only manifest within a certain band of frequencies, referred to either as EBG band or as AMC band depending on the case in question. This band of frequencies, notably its central frequency and its low and high cutoff frequencies, depends on the shape and on the dimensions of the conducting patterns, and also on the thickness and on the relative permittivity of the dielectric substrate of the structure. In particular, for a relatively limited thickness of the dielectric substrate, in other words very small compared to the wavelength, whether either the EBG band or the AMC band are considered, the bandwidth is very narrow, in other words much less than an octave. Thus, the constraints relating to the thickness mean that the current antennas comprising a reflector with an EBG or AMC structure do not allow operation over a wide band of frequencies, greater than a decade.
One aim of the invention is notably to overcome the aforementioned drawbacks by providing an antenna reflector with a wide frequency band and having a reduced thickness based on a hybrid structure. This hybrid structure comprises both an electrically conducting plane of the type of the first solution and a structure of the AMC type based on the second solution. For this purpose, one subject of the invention is an antenna reflector locally exhibiting either electromagnetic properties of an electrical conductor, or electromagnetic properties akin to a magnetic conductor, depending on the radiation emitted or received locally by the antenna. More particularly, one subject of the invention is an antenna reflector onto which a circular-polarized planar wire antenna can be mounted that is capable of emitting electromagnetic radiation in two directions orthogonal to the plane of the antenna over a predetermined frequency band, the antenna reflector being characterized in that it comprises:
The reflector may comprise several reflection regions each designed to reflect, with a phase-shift included between two values on either side of the value of zero degrees, the electric field of the backward electromagnetic radiation whose frequency is included within a sub-band of frequencies. Each reflection region is then designed to face a region of the antenna able to emit electromagnetic radiation in the sub-band of frequencies in question, at a distance allowing the electric field of the backward electromagnetic radiation to be reflected substantially in phase with the electric field of the forward electromagnetic radiation.
Similarly, the reflector may comprise several reflection regions each designed to reflect, with a phase-shift close to 180 degrees, the electric field of the backward electromagnetic radiation whose frequency is included within a sub-band of frequencies. Each reflection region is then designed to face a region of the antenna able to emit electromagnetic radiation in the sub-band of frequencies in question, at a distance allowing the electric field of the backward electromagnetic radiation to be reflected substantially in phase with the electric field of the forward electromagnetic radiation.
According to one particular embodiment, the first sub-band of frequencies corresponds to the highest frequencies of the predetermined frequency band. The reflector can thus be placed at a distance from the antenna substantially equal to a quarter of the wavelength of the central frequency of this sub-band of frequencies, this being relatively close to the antenna.
Advantageously, the sub-bands of frequencies, taken as a whole, cover substantially the whole of the predetermined frequency band. The electric field of the backward electromagnetic radiation can thus be in phase with the electric field of the forward electromagnetic radiation over the whole frequency band of the antenna.
The reflector may comprise a substrate made of dielectric material and a ground plane formed on a first surface of the substrate, the first reflection region being formed on a second surface of the substrate by an electrically conducting pattern, the other reflection region or regions each being formed on the second surface of the substrate by a set of electrically-conducting patterns disposed in a non-conjoined manner.
According to a first embodiment, the first and second surfaces of the substrate are substantially plane and parallel to each other. According to a second embodiment, the second surface of the substrate has a conical shape.
The electrically-conducting patterns of the sets forming reflection regions designed to reflect the electric field of the backward electromagnetic radiation with a phase-shift included between two values on either side of the value of zero degrees can be electrically connected to the ground plane.
According to one particular embodiment, the two values of angle on either side of the value of zero degrees are substantially equal to −120 degrees and +120 degrees.
Another subject of the invention is an antenna device comprising a circular-polarized planar wire antenna capable of emitting electromagnetic radiation over a predetermined frequency band and an antenna reflector according to the invention.
Another subject of the invention is a method for implementing the antenna reflector according to the invention. The method comprises the following steps:
The method may comprise the following additional steps:
The invention notably offers the advantage of allowing a reflection coefficient to be maintained close to unity over a wide frequency band, nominally over the whole operating frequency band of the antenna.
The invention will be better understood and other advantages will become apparent upon reading the description that follows, presented with reference to the appended drawings in which:
A perfect electrical conductor, or PEC, is a structure with a surface having an infinite electrical conductivity. The electric field tangent to this surface is therefore always zero. An incident electric field encountering the surface is reflected in phase opposition, irrespective of its frequency. In the following part of the description, the electrical conductors will be considered as perfect electrical conductors. A perfect magnetic conductor, or PMC, is a structure comprising a surface on which the tangential magnetic field is always zero. A magnetic field incident on this surface is cancelled, whereas the incident electric field is reflected in phase. Structures exhibiting properties of perfect magnetic conductors cannot be implemented in practice. It is nevertheless possible to form structures exhibiting electromagnetic properties close to perfect within a certain frequency band and for a given polarization. It is considered that a surface exhibiting electromagnetic properties close to a perfect magnetic conductor within a given frequency band is a surface for which the phase of the reflection coefficient at the frequencies in question is included between two values around 0°. The phase of the reflection coefficient is for example included between −120 and 120 degrees. A surface exhibiting electromagnetic properties close to a perfect magnetic conductor within a given frequency band is generally designated as being a high-impedance surface for this frequency band.
In
The antenna reflector, being the subject of the invention, uses the operating properties of planar wire antennas. The radiating element of such an antenna, when it is excited, emits electromagnetic radiation from a localized region of operation, associated with the relative arrangement of the strands and with the phase offset of the current flowing in the various strands. This region of operation exhibits the particularity of varying as a function of the frequency according to a law specific to each type of planar wire antenna. In particular, for an Archimedes spiral antenna whose strands are powered in phase opposition, the region of operation from which electromagnetic radiation is emitted at a given frequency forms a ring whose mean diameter is substantially equal to the wavelength of the electromagnetic radiation divided by the number Pi (D=λ/π). The antenna reflector according to the invention, on which an antenna is designed to be mounted, thus comprises at least two reflection regions whose electromagnetic properties are adapted to the electromagnetic radiation emitted locally by the antenna. A first reflection region exhibits electromagnetic properties of an electrical conductor, notably in a first sub-band of frequencies ΔF1. This sub-band of frequencies ΔF1 corresponds for example to high frequencies of the operating frequency band ΔF within which the planar wire antenna emits. A second reflection region exhibits electromagnetic properties close to a perfect magnetic conductor in a second sub-band of frequencies ΔF2. This second sub-band of frequencies ΔF2 corresponds for example to lower frequencies than those of the first sub-band of frequencies ΔF1. The antenna reflector thus comprises reflection regions of two different types, namely at least one reflection region exhibiting electromagnetic properties of an electrical conductor, and at least one reflection region exhibiting electromagnetic properties close to a perfect magnetic conductor. The antenna reflector can also comprise additional regions exhibiting either electromagnetic properties of an electrical conductor (reflection regions of the first type), or electromagnetic properties close to a perfect magnetic conductor (reflection regions of the second type) in other sub-bands of frequencies. Advantageously, these various sub-bands of frequencies are determined in such a manner as to cover, with the first sub-band of frequencies ΔF1, the whole of the operating frequency band ΔF. According to one particular embodiment, the regions exhibiting electromagnetic properties of an electrical conductor are alternated with regions exhibiting electromagnetic properties close to a perfect magnetic conductor.
In the exemplary embodiment shown in
In a third step 103 of the method for implementing an antenna reflector 3 according to the invention, the minimum distance dBmin that can separate the spiral antenna 2 from a perfect magnetic conductor without altering the amplitude distribution of the electromagnetic radiation emitted by the spiral antenna 2, at least in one of the sub-bands of frequencies ΔF2 and ΔF3, is determined. The amplitude distribution is for example considered in the near field region. The distance in question may also be the height h. The step 103 can be carried out over a wide frequency band, for example over the whole of the operating frequency band ΔF. In practice, the idea is essentially to determine the minimum distance dBmin that needs to separate the spiral antenna 2 from the reflection regions 342A and 343A whose electromagnetic properties match those of a perfect magnetic conductor. The step 103 is therefore preferably carried out for the sub-bands of frequencies ΔF2 and ΔF3. Where appropriate, it is carried out for each of the sub-bands of frequencies in question outside of the frequency sub-band ΔF1. In a fourth step 104, the shape and the dimensions of the first reflection region 341A, exhibiting electromagnetic properties of an electrical conductor in the sub-band of frequencies ΔF1 (reflection region of the first type), are determined in such a manner that this reflection region 341A comes into the vicinity of the region of operation of the spiral antenna 2 in this sub-band of frequencies ΔF1. The step 104 essentially consists in determining the diameter of the conducting disk 36. In a fifth step 105, the shape and the dimensions of the reflection regions 342A and 343A, exhibiting electromagnetic properties close to a perfect magnetic conductor in the respective sub-bands of frequencies ΔF2 and ΔF3 (reflection regions of the second type), are also determined in such a manner that each reflection region 342A and 343A comes into the vicinity of the region of operation of the spiral antenna 2 in the respective sub-band of frequencies ΔF2 or ΔF3. The step 105 essentially consists in determining the inner and outer diameters of the reflection regions 342A and 343A together with the lengths of the arcs of circles radially bounding the conducting patterns 34. More generally, the step 105 consists in determining the location and the surface area of the conducting patterns 34 in such a manner that each set of conducting patterns forms a surface exhibiting electromagnetic properties close to a perfect magnetic conductor in a sub-band of frequencies. In the steps 104 and 105, it is considered that a reflection region comes into the vicinity of a region of operation of the spiral antenna 2 when it allows the electromagnetic radiation emitted by this region of operation to be reflected in the desired direction of radiation. It is to be noted that the steps of the method for implementing the antenna reflector 3 may be carried out in a different order, as long as the first step 101 is carried out prior to the steps 104 and 105.
The step 105 may, for example, be carried out by adapting conventional AMC structures. A conventional AMC structure comprises a dielectric substrate, a ground plane carried by a first surface of the dielectric substrate, and electrically-conducting patterns with a rectangular shape arranged according to a regular matrix and carried by a second surface of the dielectric substrate. The thickness of the dielectric substrate of the conventional AMC structure is preferably chosen to be equal to the thickness of the dielectric substrate 31 of the antenna reflector 3. An AMC structure exhibits electromagnetic properties close to a perfect magnetic conductor in a given sub-band of frequencies. In a first sub-step, for each sub-band of frequencies outside of the sub-band of frequencies ΔF1, the dimensions (length and width) of the conducting patterns of a conventional AMC structure are determined which allow a surface exhibiting properties close to a perfect magnetic conductor to be formed in the sub-band of frequencies in question. In the case of a spiral antenna, the surfaces of the conducting patterns forming the reflector become larger at greater distances from the center of the antenna reflector 3. In a second sub-step, for each of the sub-bands of frequencies in question, the conducting patterns of the conventional structures AMC are adapted to the corresponding region of operation of the spiral antenna 2, each adapted conducting pattern 34 conserving substantially the same surface area as that in the conventional AMC structure. In a spiral antenna, the conducting patterns 34 therefore take an overall annular shape, as shown in
The antenna reflector 3 obtained by the method according to the invention is designed to receive a spiral antenna 2 at a minimum distance for which neither the first reflection region 341A nor the reflection regions 342A and 343A alter the electromagnetic radiation. The minimum distance preferably corresponds to the maximum between the distances dEmin and dBmin determined in the steps 102 and 103. Given that the wavelengths of the electromagnetic radiation emitted in the first sub-band of frequencies ΔF1 are shorter than the wavelengths of the electromagnetic radiation emitted in the second sub-band of frequencies ΔF2, the electromagnetic radiations emitted in both the sub-band of frequencies ΔF1 and in the sub-band of frequencies ΔF2 can be in phase with the corresponding reflected electromagnetic radiations in the near field region. In order to maintain a reflection in phase over the whole operating frequency band ΔF of the spiral antenna 2, it is furthermore possible to vary the distance separating the spiral antenna 2 from the antenna reflector 3, or to use magneto-dielectric materials exhibiting various dielectric permittivities.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 03900 | Oct 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/066563 | 9/23/2011 | WO | 00 | 6/5/2013 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2012/041770 | 4/5/2012 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3358288 | Gerard Dubost | Dec 1967 | A |
| 3745585 | Barbano | Jul 1973 | A |
| 4608572 | Blakney | Aug 1986 | A |
| 5351063 | Kim | Sep 1994 | A |
| 5990849 | Salvail | Nov 1999 | A |
| 6067058 | Volman | May 2000 | A |
| 6219006 | Rudish | Apr 2001 | B1 |
| 20060220958 | Saegrov | Oct 2006 | A1 |
| 20090079637 | Fukui | Mar 2009 | A1 |
| 20100149060 | Lin et al. | Jun 2010 | A1 |
| 20100271267 | Roth | Oct 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 10034547 | Jul 2000 | DE |
| 10034547 | Jan 2002 | DE |
| 010034547 | Jan 2002 | DE |
| 10034547 | Jan 2002 | DE |
| 002922687 | Oct 2007 | FR |
| EP2365584 | Mar 2011 | FR |
| 2132022 | May 1982 | GB |
| 002132022 | Jun 1982 | GB |
| 2132022 | Jun 1984 | GB |
| 2253743 | Sep 1992 | GB |
| 357083901 | May 1982 | JP |
| 02007096868 | Sep 2005 | JP |
| 02008219125 | Feb 2007 | JP |
| 02011211420 | Mar 2010 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20130249762 A1 | Sep 2013 | US |
