The present invention relates to the field of antennas, notably that of polarization diversity antennas for telecommunications terminals.
Among the many steps for improving the signal-to-noise ratio in a mobile telecommunication system, resorting to transmission and/or reception diversity techniques is known. At the base station, antennas sufficiently distant from each other (by a distance larger than at least the half wavelength at the operating frequency) may for example be used, a network of antennas for forming beams pointing in distinct angular directions or antennas transmitting according to distinct polarizations may be used: depending on the case this is termed as spatial diversity, angular diversity or polarization diversity. Similarly, the same diversity techniques are in principle applicable to the mobile terminal. Either antennas sufficiently distant from each other will be used so that the received signals have been subject to non-correlated conditions of propagation, antennas having reception diagrams pointing in distinct angular directions or further antennas with distinct polarizations, for example according to linear polarizations orthogonal to each other, will be used.
Unfortunately, mobile terminals poorly lend themselves to the application of diversity techniques. Indeed, the small dimensions of the mobile terminals do not generally allow sufficient separation of the receiving antennas at the currently used operating frequencies (80 MHz-6 GHz). As a result, the signals received by the different antennas are correlated because of neighbouring conditions of propagation or because of coupling between antennas. The signals received may then have simultaneous fading and the mobile terminal does not fully benefit from the advantages of diversity.
A polarization diversity multi-antenna system for a mobile terminal was proposed in the article of N. Michishita et al. entitled <<A polarization diversity antenna by printed dipole and a patch with a hole>> published in Proc. of IEEE Antennas and Propagation Society International Symposium, Vol. No. 3, May 2001, pages 368-371. This system consists of a patch antenna and of a dipole antenna. The patch is perforated with a hole through which the dipole antenna printed on a substrate passes. This system is not planar and does not easily lend itself to integration into a mobile terminal.
A polarization diversity multi-antenna system for a base station was proposed in the article of N. Kuga et al. entitled <<A patch-slot composite antenna for VH-polarization diversity base stations>> published in Proc. of Asia-Pacific Microwave Conference, December 2000. It comprises two networks of interleaved antennas: a first network consisting of patch type elements with horizontal polarization and a second network consisting of patch type elements with vertical polarization. The elements of the first network are excited by slots cut out in the ground plane whereas the elements of the second network are excited by microstrip lines. Neither is this multi-antenna system compatible with integration into a mobile terminal.
The object of the present invention is to find a remedy to the aforementioned drawbacks, i.e. to propose a compact diversity multi-antenna system which may easily be integrated into a mobile terminal while only having low coupling between antennas.
The present invention is defined by a polarization diversity multi-antenna system comprising a first slot type antenna and a second patch type antenna, said first and second antennas sharing the same ground plane, the slot of the first antenna being laid out in said ground plane and the patch of the second antenna being at least partly plumb with said slot, said first and second antennas having a common operating frequency band, wherein:
Particular embodiments of the invention are defined in the dependent claims.
Other features and advantages of the invention will become apparent upon reading a description of a preferential embodiment of the invention, made with reference to the appended figures wherein:
The idea at the basis of the invention consists of associating on a same ground plane, a patch type antenna and a slot type antenna, the patch being at least partly plumb with the slot. The geometry and the orientation of the patch and of the slot are selected so that the patch type antenna and the slot type antenna may each transmit and/or receive according to a rectilinear polarization, the polarization directions associated with both antennas being orthogonal to each other. In a receiving mode, the signals received by the patch antenna and the slot antenna respectively, may be combined in order to provide reception diversity.
More specifically, the geometry and the orientation of the patch and the slot are selected so that the respective directions of established resonance in the patch and in the slot are substantially parallel. Conventionally, it is known that for a patch the distribution of the electric field along the direction of established resonance is sinusoidal and has two maxima at each end of the patch. Similarly, for a slot, the distribution of electric field along the direction of established resonance is sinusoidal and has two nulls at each end of the slot. In one case as in the other, the number of periods of the sinusoidal distribution depends on the order of the resonance. The electromagnetic field generated by the patch is conventionally denoted TMn0 where n gives the order of the resonance along the resonance direction x, the electric field being directed along this direction. Likewise, the electromagnetic field generated by the slot is conventionally denoted TEn′ where n′ gives the order of the resonance along the resonance direction x′, the electric field being orthogonal to x′ and parallel to the plane of the slot.
Surprisingly, it was seen that co-localization of the slot type antenna and of the patch type antenna according to the invention did not significantly change the characteristics of both antennas taken separately. In particular, the coupling level between the antennas is remarkably low. Further, impedance matching may be achieved independently for both of the antennas in a common operating frequency band.
Preferentially, the slot has a trapezoidal shape elongated along a longitudinal direction. It may however be of any symmetrical shape, for example rectangular or elliptical, or even non-symmetrical. Also, the metal patch 30 has an elongated elliptical shape along a longitudinal direction. It may however be of any symmetrical shape, for example rectangular or trapezoidal, or even non-symmetrical.
The directions of resonance of the slot and of the patch are denoted FF′ and PP′ respectively. As this was seen above, both of these axes are selected to be substantially parallel. These axes coincide here with the longitudinal axes of symmetry of the slot and of the patch, respectively.
The axes FF′ and PP′ may be shifted sideways with respect to each other in a plane parallel to the ground plane, or else contained in a same plane orthogonal to the ground plane, in which case the orthogonal projection of the axis PP′ on the ground plane advantageously coincides with the FF′ axis. In
The electric field generated by the slot type antenna has rectilinear polarization orthogonal to the middle plane. On the other hand the electric field generated by the patch type antenna has rectilinear polarization parallel to the PP′ axis. Reciprocally, the signal received by the slot type antenna is maximum when the electric field has rectilinear polarization orthogonal to the middle plane and the signal received by the patch type antenna is maximum when the electric field has polarization parallel to the PP′ axis.
Given that the patch being at least partly plumb with the slot, the orthogonal projection of the patch on the metal plane has a non-empty intersection with the latter. According to an alternative embodiment, the orthogonal projection of the patch on the ground plane entirely includes the shape of the slot. The slot type and patch type antennas are thereby co-localized and the multi-antenna system is particularly compact.
The slot type antenna may be excited by means of a coaxial cable or a coplanar line in a way known to the one skilled in the art. Alternatively, the slot may be excited by coupling with a microstrip line printed on the substrate on the side opposite to the ground plane.
The patch type antenna may be excited by means of a metal probe 35 as illustrated in
More generally, the patch type and slot type antennas may be excited by direct electric contact and/or by electromagnetic coupling.
The length of the slot along the FF′ axis is selected to be substantially equal to an integer multiple of half the guided wavelength, associated with the operating frequency. Also, the length of the patch along the PP′ axis is selected to be substantially equal to an integer multiple of the half of the guided wavelength, associated with the operating frequency. It is recalled that the guided wavelength slightly differs from the free propagation wavelength because of the presence of edge fields. It is equal to twice the fundamental resonance length in the guide. An analytic expression of the guided wavelength for a slot antenna will for example be found in the article of R. Garg et al. entitled <<Expressions for wavelength and impedance of a slotline>> published in the IEEE Trans. on Microwave Theory, August 1976, page 532. Also, the guided wavelength λg in a patch may generally be approximated by λg≈0.982 where λ is the free propagation wavelength in the constitutive medium of the guide (either air or dielectric).
The operating frequencies of the slot and patch antennas are advantageously selected to be identical. More generally, as this will be seen later on, it is possible to use the slot antenna and the patch antenna in a same band of operating frequencies without any significant coupling between both antennas. Typically, for a system intended to be used in a UMTS (Universal Mobile Telecommunication System) terminal, the operating frequency will be of the order of 2 GHz and the slot and patch lengths of the order of 6 to 7.5 cm. These lengths are compatible with the dimensions of a mobile terminal.
In order to further reduce the dimensions of the system, it is proposed according to a second embodiment, to use half a slot instead of an entire slot. More specifically, the slot is open on one side 21 over the whole of its width. This embodiment is illustrated in
It is also possible to reduce the length of the patch in the PP′ direction as indicated in
In order to still further reduce the dimensions of the aforementioned antennas, working at even smaller fractions of the guided wavelength (λg/8, λg/10, . . . ) and/or using materials with higher dielectric constants, allowing a reduction of λg and/or a loading of the antennas with discrete or distributed components (capacitors, inductors, . . . ) as known to one skilled in the art, may be contemplated.
In the second, third and fourth embodiments, excitation of the slot and of the patch may be achieved according to the same alternatives as discussed for the first embodiment.
The multi-antenna systems according to the invention may be combined in order to make up a composite system with higher gain and/or diversity order. In particular,
It is seen that in a frequency range around 2 GHz, the reflection coefficients |S11| and |S22| are both less than −10 dB, which expresses proper impedance matching of the system in a common frequency band. Additionally in this same frequency band, the coupling coefficients |S12| and |S21| are below −30 dB. With the low coupling level between both antennas, the polarization diversity may be utilized at best.
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06 53562 | Sep 2006 | FR | national |
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PCT/EP2007/059197 | 9/3/2007 | WO | 00 | 3/6/2009 |
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WO2008/028892 | 3/13/2008 | WO | A |
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