Diversity reception slotted flat-plate antenna

The invention relates to a planar antenna realised on a substrate comprising a slot of closed form dimensioned to operate at a given frequency in a short-circuit plane of at least one feed-line. The perimeter of the slot being designed such that p=kλs where k is a whole number greater than 1 and λs the guided wavelength in the slot, the antenna comprising at least one first feed-line placed in an open circuit zone of the slot and a second feed-line placed at a distance d=(2n+1)λs/4 from the first line, where n is an integer greater than or equal to zero.

This application claims the benefit, under 35 U.S.C. 365 of International Application PCT/FR2004/050357, filed Jul. 27, 2004, which was published in accordance with POT Article 21(2) on Feb. 10, 2005 in French and which claims the benefit of French patent application No. 03 09360, filed on Jul. 30, 2003.

The present invention relates to a planar antenna with diversity of radiation. It relates more particularly to an antenna that can be used in the field of wireless transmissions, particularly within the framework of transmissions in a closed or semi-enclosed environment such as domestic surroundings, gymnasiums, television studios, theatres or similar rooms.

BACKGROUND OF THE INVENTION

In the known high-speed wireless transmission systems, the signals transmitted by the transmitter reach the receiver by following a plurality of paths resulting from the many reflections of the signal on the walls, furniture or similar elements. When combined at the level of the receiver, the phase differences between the different rays having taken paths of different lengths gives rise to an interference figure that can cause fading or a significant degradation in the signal.

Now, the location of the fading changes over time according to the modifications in the environment such as the presence of new objects or the movement of people. The fading due to multipaths can lead to significant degradations both at the level of the quality of the signal received and at the level of the system performances. To overcome these fading phenomena, the technique most often used is a technique that implements spatial diversity.

This technique consists, among other things, of using a pair of antennas with wide spatial coverage connected by feed-lines to a switch. However, the use of this type of diversity requires a minimum spacing between the radiating elements to ensure that there is sufficient decorrelation of the channel response viewed from each radiating element. An inherent disadvantage to its implementation is the distance between the radiating elements that present a cost, particularly in terms of size and substrate.

Other solutions have been proposed to overcome this problem. Some of these solutions use diversity of radiation as described for example in the French patent A-2 828 584 in the name of the applicant.

BRIEF SUMMARY OF THE INVENTION

The present invention proposes a new planar type antenna with diversity of radiation.

Hence, the present invention relates to a planar antenna realised on a substrate comprising a slot of closed shape dimensioned to operate at a given frequency in a short-circuit plane of at least one feed-line. In this antenna, the perimeter of the slot is designed such that p=kλs where k is a integer greater than 1 and λs the guided wavelength in the slot. Moreover, it comprises at least a first feed-line placed in an open circuit zone of the slot and a second feed-line placed at a distance d=(2n+1) λs/4 from the first line, where n is an integer greater than or equal to zero.

According to a first embodiment, each feed-line terminates in an open circuit and is coupled to the slot according to a line/slot coupling such that the length of the line after the transition equals (2k′+1)λm/4 where λm is the guided wavelength under the line and k′ a positive or null integer. The line/slot coupling can also be realised in such a manner that the microstrip line terminates in a short-circuit located at 2k″λm/4 where λm is the guided wavelength under the line and k″ is a positive or null integer.

According to a second embodiment, each feed-line is coupled magnetically with the slot according to a tangential line/slot transition.

Moreover, the shape of the slot can be annular, square, rectangular, polygonal, or in the form of a clover leaf. If the slot is of a rectangular shape, the feed-lines can be equidistant from an axis of symmetry of the slot or one of the feed-lines is positioned according to an axis of symmetry of the slot.

DESCRIPTION OF PREFERRED EMBODIMENTS

To simplify the description, the same elements have the same references as the figures.

FIGS. 1 to 5relate to a first embodiment of the invention. As shown inFIG. 1, the planar antenna is constituted by an annular slot1realised on a substrate2by engraving on a ground plane that is not shown. The antenna operates on a higher order mode, more particularly on its first higher order mode. Therefore, the perimeter of the annular slot1is equal to 2λs, where λs is the guided wavelength in the slot. Generally, the perimeter of the slot is such that p=kλs where k>1.

As shown inFIG. 1, the excitation of the slot is achieved by using a feed-line3realised in microstrip technology. The line3crosses the slot so as to obtain a coupling between the microstrip line and the slot according to the method described by Knorr. Thus, the length Lm of the line3equals approximately (2k′+1) λm/4 where km is the guided wavelength under the line and k′ a positive or null integer, the most frequently Lm=λm/4. Moreover, as shown inFIG. 1, the distribution of the fields in the annular slot has maximum field zones (OC zones for Open Circuit) and minimum field zone (SC zones for Short-Circuit). The feed-line3crosses the annular slot1in an open circuit zone. Owing to the positioning of the feed-line and the perimeter of the annular slot, the distance between two OC zones or two SC zones is λs/2. This distribution of fields in the slot determines the radiation pattern of the antenna. The radiation is in the plane of the substrate, in contrast to the annular slot operating in its fundamental mode, for which the radiation is perpendicular to the substrate. According to one variant, the feed-line3terminates in a short-circuit. In this case, the length of the line (Lm) is chosen such that Lm=k″λm/4, where k″ is a positive or null integer.

In accordance with the invention, a second feed-line4realised in microstrip technology and crossing the slot according to the Knorr method is positioned at the level of a SC zone. The length of the feed-line4is determined according to the rules mentioned above. Thus, when the access is realised by line4, a second radiation pattern is obtained that is complementary to the first one. More specifically, the second line is located at +/−45° or +/−135° with respect to the first line, namely at a distance d such that d=(2n+1) λs/4. This relative position of the two accesses enables a good level of isolation to be obtained.

The dimensions taken for an embodiment compliant with that ofFIG. 1, which was simulated by using the IE3D software of the Zeland company, will be given below. On a Rogers R04003 substrate presenting a εr=3.38, a loss tangent Tan Δ=0.0022 and a height H=0.81 mm, was realised an antenna such as represented inFIG. 1. This antenna is constituted by an annular slot presenting an internal diameter Rint=13.4 mm and an external diameter Rext=13.8 mm, namely an average diameter Ravg=13.6 mm. The width of the slot equals Ws=0.4 mm. The feed-lines are realised using microstrip technology and have a width Wm=0.3 mm and length Lm=λm/4 such that Lm=Lm′=8.25 mm.

As shown inFIG. 1, the distance between the two accesses1′ and2′, when the slot is a circle, corresponds to ⅛thof the perimeter namely 2πraverage/8=10.68 mm. This corresponds to a quarter guided wavelength in the slot (λs/4=10.66 mm). At the level of accesses1′ and2′ for feeding the lines3,4, the impedance is 50 ohms.FIG. 2shows the results obtained concerning the isolation S and matching parameters according to the frequency. It is seen in this case that an isolation of around −20 dB is obtained.

Moreover, according to the radiation patterns shown inFIGS. 3aand3b, four lobes oriented according to directions Ox and Oy are distinguished when the access1′ is used, as shown inFIG. 3awhereas when access2′ is used, the lobes are turned by 45°, as shown inFIG. 3b. Therefore two complementary radiation patterns are obtained, as shown inFIG. 4which shows a cross-section in the plane θ=95° of the radiation patterns shown inFIGS. 3aand3b.

It should also be noted that with this antenna, the radiation is produced in the plane of the substrate, which enables a horizontal coverage to be obtained for a single stage use, for example.

In accordance with the present invention, the second access, namely the microstrip line4, can be placed at +/−135° (+/−3λs/4) in relation to the first access, namely the feed-line3. This enables an improvement of approximately 8 dB in the isolation level to be obtained, as shown inFIG. 5between the two curves S12(135° access) and S12(45° access).

A description will now be given, with reference toFIGS. 6 to 8, of another embodiment of an antenna in accordance with the present invention. In this case, as shown inFIG. 6, instead of having a circular shaped slot, a slot10of rectangular shape is used. The length of the rectangular shape is such that p=2λs=2(W+L) where W corresponds to the width of the rectangle and L to the length of the rectangle. More generally, p=kλs=2(W+L). In this case, as shown inFIG. 6, the rectangular shaped slot is fed by two feed-lines11and12realised using microstrip technology. The feed is produced by line/slot coupling according to the method described by Knorr and mentioned above.

In accordance with the invention, the first feed-line12is positioned on an axis of symmetry of the structure, namely the axis x, x′ whereas the second feed-line, namely line11is positioned at a distance d=(2n+1) λs/4 where n is an integer greater than or equal to zero. In these conditions, access to the feed-line11is not obtained by symmetry of the axis realised by the feed-line12. This asymmetry is located at the level of the impedance matching of the ports. Indeed, an imbalance occurs between the S11and S22impedance matching in terms of central frequency and impedance matching band.

In this case, the frequency can be recentered by modifying the quarter wave (Lm′Wm′) located between the access port and the line-slot transition as will be explained below.

With a rectangular shape as shown inFIG. 6, the radiation patterns as shown inFIG. 7afor feeding by line12or7bfor feeding by line11are obtained. It is observed that the patterns obtained are modified with respect to the pattern of a circular slot but remain complementary. Hence, through the shape of the slot, it is possible to control the radiation patterns.

The following describes a practical embodiment of an antenna as shown inFIG. 6. This antenna was simulated by using the IE3D software with the following dimensions in millimetres:

As shown in the curves ofFIG. 8a, it is seen that in this case, there are two peaks of impedance matching that are not centred on the same frequency. To obtain a centring of the two peaks, the quarter wavelength of the access1was modified such that Lm′=7.85 mm and Wm′=0.75 mm. In this case, the parameters S ofFIG. 8bwere obtained. The quarter wave of the access corresponding to line11not having been modified, the two impedance matching peaks are centred on the same frequency.

A third embodiment will be described below with reference toFIGS. 9 to 11. In this case, the antenna constituted by a slot with a closed shape is realised by a rectangular slot20with two accesses formed by the feed-lines21,22that are symmetrical in relation to the line x x′. With this symmetrical access structure, a balanced matching is obtained if the perimeter p of the rectangular slot is selected such that p=2λs=2(W+L) where W is the rectangle width and L its length, λs being the guided wavelength in the slot. As mentioned above, p can also be chosen such that p=kλs. Moreover, the distance between the access of the line22and the access of the line21is such that d=(2n+1) λs/4 where n is an integer greater than or equal to zero and the accesses formed by the lines21and22are equidistant from an axis of symmetry XX′ of the rectangular slot.

In this case, as shown inFIG. 10which gives the parameters S of the rectangular slot with symmetrical accesses, the two impedance matching peaks are exactly superimposed but the level of isolation is higher for the antenna constituted by a rectangular slot with an asymmetrical access as shown inFIG. 6.

The antenna structure ofFIG. 9gives different radiation patterns according to the access used, as shown by the pattern ofFIG. 11aand11b.

The embodiments shown above are related to planar antennas constituted by a slot of a closed, annular or rectangular shape. However, as shown inFIG. 12, other closed shapes can be used for the slot antenna, particularly an orthogonal shape30, a square40, a clover leaf shape50. One of the operating conditions is that the perimeter of the slot is an integer multiple k greater than or equal to 2 of the guided wavelength in the slot p=kλs and that the distance d between the accesses is such that d=2(n+1) λs/4 where n is an integer greater than or equal to zero.

In this case, a higher order mode of the slot is used, which enables complementary radiation patterns to be obtained. Particularly, the structures proposed radiate in the plane of the substrate, which is not the case with a slot antenna operating in its fundamental mode.

According to a variant of the present invention as shown inFIG. 13, the antenna-slot60that, in this embodiment, is constituted by a ring can be fed tangentially, as shown by the feed-lines61,62. In this case, the same design rules are used. The advantage of a tangential feed is to have feed-lines outside of the slot and to increase the bandwidth.

In accordance with the present invention and as shown inFIG. 14, if the closed shape slot antenna is constituted particularly by a rectangle or a square, it is possible to realise a structure enabling a reception/transmission operation with a good isolation and a diversity of the order of 2 for reception. The Rx/Tx isolation obtained is that given inFIG. 8in the case of a rectangular slot. The radiation pattern of the antenna fed by the access Tx corresponds to that ofFIG. 7aand that of the antenna fed by access Rx1corresponds to the pattern ofFIG. 7b. Likewise the pattern of the antenna fed by the access Rx2is symmetrical with respect to the axis Ox of the pattern represented inFIG. 7b. The distance between the two accesses Rx is λs/2 or more generally k′″λs/2 where k′″ is an integer greater than 0. Hence, the isolation is not intrinsically good between these two accesses. A switching device such as the SPDT circuit will be used at the level of the Rx access.

The use of this type of structure thus enables a good level of isolation to be obtained and a diversity of order2for reception with very low overall dimensions when an integrated switching device is used.

It is evident to those in the profession that modifications can be made to the structures described above without falling outside the scope of the claims attached. In particular, the feed-lines can be realised using techniques other than the coplanar technology or coaxial cables, the outer core of which is connected to the substrate.